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Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

(OP)
At the beginning of covid I acquired a version of the bicycle shown below. It's a 1992 Miyata 1000LT. It was the best, production touring bike that one could buy in 1992 for any amount of money. It is legendary for being a sweet, smooth, "steel is real" ride and the legend could not be truer. When I ride the bike, I feel so little discomfort in my body that it's almost as though someone chopped off my head and mounted it to the handle bars. I feel nothing but joy and exertion.

Performance wise, the only non-awesome thing about the bike is the cantilever brakes. As cantilever brakes are prone to doing, hard front braking will induce a pulsating front end shudder that is very disconcerting. Before attempting to fix the problem, I would like to accurately diagnose it. Everywhere this question is asked out in the online Bike-o-Verse, you get the same "bow and arrow" explanation described in detail here: Link. Looking at that explanation as a structural engineer, I find it to be utterly implausible. Some suggested remedies can also be found at that link but I deem them to be questionable because, in my opinion, they are based on a flawed theory of causation.

So my first question here is this: what do you think of the "bow and arrow" explanation? Do you buy it? Do you share my skepticism? I have reasons for my doubt, of course, but I'm going to withhold them for a spell here in the hope of not contaminating the thought pool. I've reproduced part of the "bow and arrow" explanation below for reference.

I've got my own theory about the cause of the fork shudder which we'll get into in more detail on later. The short version is that I think that it's a function of torsional fork twist rather than the front to back fork flexing that the "bow and arrow" theory suggests.



Quote (Leonard Zinn)

Why does shudder occur?

Brake shudder is widespread because it’s built into the design of almost all ’cross bikes; it’s inherent to the design of a center-pull cantilever brake. To understand the reason why it happens and why reduced pad size, lots of toe-in, and a tight headset help take a look at the chart titled “Brake Shudder in cantilever brakes.”

As the brake is applied, the ground applies a force directed backward on the tire as shown, causing the fork to flex backward. Problem is, the brake cable is fixed at one end at the brake caliper and at the other end at the cable stop above the headset (as you can see in my case, at a cable hanger attached to a bolt on the stem face plate).

Think “bow and arrow” and imagine the fork between the cantilever bosses and the top of the headset is like the bow, and the cable is like the string. As the fork flexes back due to braking, the cable tightens like the string in the bow, because its two ends – the cable hanger and the brake calipers, have moved further apart. So even though you may have pulled the brake lever carefully enough to modulate it properly, as soon as the pad slows the wheel down, the fork flexes back and tightens the cable, which in turn pulls the pads harder against the rim. This in turn flexes the fork back further, which tightens the cable more, which pulls the pads harder against the rim, and so on.

Eventually, something has to give: Either the tire must slip on the ground, the rider must go over the handlebars, or the pads must break free from the rim. It is the latter that creates the shudder, the pads bind and release, bind and release, each time allowing the fork to flex back and forth and the tire to roll and stop, roll and stop. This is why the problem goes away in mud and wet sand, because the pad can break free smoothly. It is also why smaller pads with more toe-in help.

If the headset is loose, the problem is greater, because the length change between the brake posts and the cable stop atop the headset is greater as the fork moves back when the brake is applied.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

(OP)
Close up of the brakes. The second pic is the the Shimano CX50 system that I plan to install this summer. The CX50 is basically where evolution stopped with respect to cantis before V-brakes and discs took over. I speculate that the CX50's will improve the brake shudder if I make the straddle wire angle steep but, at present, that's a pretty tentative guess.


RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

Intriguing problem. The "bow and arrow" seems plausible in theory...but looking at that fork arrangement it seems to me it would have to be made of 10000ga aluminum to be flexible enough to make a significant difference.

I used to commute by bike, back before I moved to the exurbs, and I keep telling myself I'll get over to the local bike trail for lunch rides. But my bike was just a cheap Target-grade Schwinn. Never had a shudder problem that I noticed, but then again maybe that's because it's heavy as can be and probably a lot more stiff. If I squeeze the brakes hard I launch myself over the handlebars. Can you describe the phenomenon a bit so we understand better what's happening to the bike?

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

(OP)

Quote (phamENG)

Can you describe the phenomenon a bit so we understand better what's happening to the bike?

It feels very much like a higher frequency version of what you feel in a car when the antilock braking does its thing. As you can imagine, it's much less appealing on a two wheeled vehicle. Does that help?

Quote (phamENG)

...but looking at that fork arrangement it seems to me it would have to be made of 10000ga aluminum to be flexible enough to make a significant difference.

Exactly. Your spotting that suggests that you're already on track to arrive where my head is at.

Quote (phamENG)

Never had a shudder problem that I noticed, but then again maybe that's because it's heavy as can be and probably a lot more stiff.

An interesting feature of the cycling world's online discussions on this is that they cite things that anecdotally fix the problem and then, retroactively, use that as evidence that they were right about the cause of the problem to begin with. One such "fix" is the switch to V-brakes. I don't buy that because:

1) There are still a few reports out there of fork shudder with V-brakes.

2) V-brakes are linear pull and require more lever end cable pull which, I believe, means that they apply more mechanical advantage to the brake pads.

3) Most importantly in my opinion, by far, is that most V-brakes got installed on true mountain bikes and hybrids which, as you suggested, tend to have sturdier forks.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

I believe it is due the stick-slip, which originates from the difference between static and dynamic friction. The severity and frequency of the shudder is strongly a function of how rigidly fixed the the position of the brake shoes are. More like a violin string where the wheel rim is the "bow" and the brake shoe is the vibrating string. Loose fork bearings can make this shudder very severe.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

I don't like the bow and arrow theory.

Quote (Compositepro)

I believe it is due the stick-slip, which originates from the difference between static and dynamic friction. The severity and frequency of the shudder is strongly a function of how rigidly fixed the the position of the brake shoes are. More like a violin string where the wheel rim is the "bow" and the brake shoe is the vibrating string. Loose fork bearings can make this shudder very severe.
Agreed.

I believe the primary source of brake shudder is stick-slip type behaviour. The brake pad is vibrating back and forth along the tangential direction of the wheel rim. Toe-in/toe out of the brake pad can promote or suppress this primary source.

The rigidity of the entire brake system will accentuate or suppress this behaviour. I conjecture that the primary source of flexibility is torsional bending flexibility of the brake arm about the brake post and possibly a minor amount in the fork itself. In the case of cantilever brakes P-Delta effects of the tension on the brake cable accentuating the torsion of the brake arm. In a V-brake the P-delta effect would have a restoring affect on the torsion of the brake arm.

They are my first thoughts on this.

Second thoughts:

Quote (Kootk)

One such "fix" is the switch to V-brakes. I don't buy that because:
1) There are still a few reports out there of fork shudder with V-brakes.
Agreed. I wasn't immediately aware that cantilevers are more likely to vibrate but it sounds user experience suggests that they might be. My conjecture of P-Delta affects I think offers one explanation.

Quote (Kootk)

2) V-brakes are linear pull and require more lever end cable pull which, I believe, means that they apply more mechanical advantage to the brake pads.
It mostly evens out as the user is going to adjust their grip to reach the required force on the brake pads. Also cantilevers have user adjustable mechanical advantage by adjusting the length of the 'bow string'.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

Some quick googling to correct my ignorance, and I see my hybrid has V-Brakes, not cantilever.

Given your description of the shudder:

Quote (KootK)

It feels very much like a higher frequency version of what you feel in a car when the antilock braking does its thing.

I tend to agree with Compositepro. Since antilock braking is essentially a rapid cycling of application and release, stick-slip would likely cause similar response. The issue as I see it, is what causes the slip? You apply the brakes and have dynamic friction. If you reach a point where you stick (static friction), the friction force increases, right? Unless you start pedaling again or ease up on the pressure, it shouldn't un-stick. Unless it's coming from imperfections in the rim and minor out-of-roundness. That makes me wonder about your number 2...it seems the greater mechanical advantage offered by V-brakes would help to overcome that.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

I feel, as a long-ago cyclist, that this is just stick-slip. Setting toe-in so that as the load on the pad increases the rear of the pad sees less force than the front edge decreases self-amplification of the grip of the pad on the rim.

I recently acquired a trash bike - literally tossed to the curb - with similar brakes and it had no shudder. Mainly because the pads are about as hard as nylon, which makes stopping enough of a challenge that I'll try for new pads to go along with the chain, which I broke. I wonder if using Evaporust was a problem for the chain as it was a rather brittle fracture, but the chain was as furry as a caterpillar with rust before so it was getting replaced one way or the other. With no chain, the brake situation has become resolved.

I do recall it is more likely on plain aluminum rims than on steel rims, but the grip is better on plain aluminum.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

Quote (phamENG)

The issue as I see it, is what causes the slip? You apply the brakes and have dynamic friction. If you reach a point where you stick (static friction), the friction force increases, right? Unless you start pedaling again or ease up on the pressure, it shouldn't un-stick.
Its a bit more complicated than that and maybe describing it as stick-slip is glossing over the finer details.

Globally there is clearly no full 'stick' as the wheel is continues to rotate and the pads are globally static. If there is proper stick you'll soon know it as you are launched into a lovely somersault which also gives you a brief moment of contemplating your life choices before pain interrupts such thoughts.

Locally I suspect there is minor stick slip behaviour or at least local fluctuation of effective force due to torsion. This is a common scenario in life. The violin bow across a violin string I think is a good example. The bow is always moving the string is on average static, though there is clearly fluctuations in the lateral force being applied. Other such scenarios occur such a dragging a rubber block across a surface or even dragging a 4 legged table across the floor. In the case of the table you likely do get observable stick slip behaviour, you can see and hear the legs vibrating. But globally the table is moving at a constant rate.

Quote (phamENG)

Unless it's coming from imperfections in the rim and minor out-of-roundness. That makes me wonder about your number 2...it seems the greater mechanical advantage offered by V-brakes would help to overcome that.
Out of roundness and out of trueness (axial imperfections) don't seem to significantly affect heavy brake shudder in my experience. I've seen badly out of true wheels brake just fine, the brake callipers do move back left and right following the rim, but they don't induce shudder as they a made to track the rim like this.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

Quote (human909)

launched into a lovely somersault where you which also launches you into a brief moment of contemplating your life choices.

Been there. It was into the woods, too. Still have use of my legs so I guess it wasn't too bad...

I agree my thoughts are a bit simplistic. As a novice cyclist I haven't really taken the time to consider parts of my bike in a rigorous engineering sense. I started typing that before your previous response - the torsional flexibility of the brake arm sounds good and the scale makes flexibility issues more plausible than the fork itself.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

Actually the more I think about it the more I think the table analogy is a good description of the stick slip behaviour. Humour me and forgive me if this explanation is obvious to you.

Consider a table being dragged at a constant rate over a rough surface at a force only slight above the global static friction. Every now and again one leg is going to catch a microscopic edge and move over into stick friction. It continues to stick and bend as the table continue moving until the force & energy builds up in the bending of the leg. Eventually this force exceeds the static friction and the leg slips and springs back. This can be happening on all 4 legs and can be quite noticeable on a table with slender legs because the reduced stiffness means a lower frequency, and higher amplitude & wavelength of slip.

So again to summarise the key ingredients you need is friction, imperfections and non rigidity.

In the case of a cantilever brake, I believe the primary non rigidity is the brake arm itself bending the arm and about the post. (I earlier described this as torsional, though I believe this was an incorrect description.)

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

No worries. It is pretty obvious, but more often than not stating it doesn't hurt. That's sort of where I was going with the out of roundness (though out of trueness is a better way of capturing my intent), but your statement is much more concise.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

Quote (phamENG)

No worries. It is pretty obvious, but more often than not stating it doesn't hurt. That's sort of where I was going with the out of roundness (though out of trueness is a better way of capturing my intent), but your statement is much more concise.
Thanks. My explanation was intended for all rather than you specifically. And like many things it was also intended for myself. I might think I understand something, but until I write it out or try to explain something I often find that I don't fully understand it and I learn things myself in the process. smile

Quote (phamENG)

As a novice cyclist I haven't really taken the time to consider parts of my bike in a rigorous engineering sense.
I'm an enthusiastic cyclist hence my enthusiastic posting. I've also thought a decent amount about the physics of cycling. I still wonder if Kootk has been stalking me as he had earlier singled me out for comments on a bicycle discusion. 3eyes

Understanding the physics of braking actually improved my cycling significantly when it dawned on me that maximum braking most non slippery surface is achieved at 100% front brake and 0% rear brake. On asphalt in a straight line I don't use my rear brake even in emergency braking scenarios.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

Stalking does fit his profile. Watch out for some guy in yellow crocks next time you're out for a ride.

I came to the same conclusion about braking when I launched myself halfway into orbit. As a result, I rarely touch my front brake unless I need to slow down in a hurry. Even then, I always hit my rear brake hard first and then apply the front a little more calmly. Gives me peace of mind that it's taking the edge of the speed before I lock up the front wheel...

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

I think you misread my post. I effectively DONT use my rear brake. I can elaborate why if you wish.

Though your approach is definitely common and probably safer until you get adept with feathering the front brake. (Many people on bikes wear out their rear pads far faster than their front. I'm the opposite as I barely touch the rear brake.)

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

Nope - read it right. I was referring to the 100% effectiveness of the front brake. It scared me...

I've gotten a lot better about feathering it since then, but haven't changed things. I'd be happy to hear the elaboration. Might also help me understand the dynamics going on it KootK's problem a little better.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

Quote (phamENG)

I've gotten a lot better about feathering it since then
Yeah I got better at feathering since I forced myself only to use the front brake. When I first tested the approach out I did it in an empty car park at low speed and just practiced it. Now since it is almost the only brake I use my muscle memory is much improved.

Quote (phamENG)

I'd be happy to hear the elaboration. Might also help me understand the dynamics going on it KootK's problem a little better.
This elaborates on it well:
https://en.wikipedia.org/wiki/Bicycle_and_motorcyc...

As per the wiki. "For an upright bicycle on dry asphalt with excellent brakes, pitching will probably be the limiting factor.". Essentially overturning about the front wheel is the limiting factor in braking. Thus maximum braking is achieved at this limit point. At this limit point all weight is on the front wheel and zero weight is on the rear. Thus you can get ZERO effective braking from the wheel wheel at maximum braking.

Naturally you don't want to site at the tipping point of 100% theoretical braking. But with practice getting close is quite possible and if you are close you want VERY little (aka zero) force on your rear brake as you could readily initiate a skid there which will reduce your 'yaw' stability.

All bets are off on loose surfaces and your are back to due handed brake modulation. I do mountain bike and I'm probably 70-80% front 20%-30% rear...

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

maybe something like this? And oscillates between bites and slaps until user releases brake? Seems like the bowstring theory would require a huge amount of deflection of the axle relative to the brake mount relative to the head tube to make any sort of noticable difference in cable tension. Like, a catastrophic amount of deflection. I bet against the bowstring theory.

My 'deformed shape' dashed lines are just general. The deformation is probably mostly in the brake assembly joints and not so much the fork itself?


RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

Exactly that. Far clearer in pictures.

It also helps show why having the break pad "toe in" helps prevent shudder. (You want the pads front of the pad touching the rim before the rear as then you avoid the "bites" section of that diagram.)

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

Quote (koot)

One such "fix" is the switch to V-brakes. I don't buy that because:...

I suggest that v-brakes will disguise the problem based on where the cable pull line of action is relative to our....kern...of braking pressure? . I'd guess this behavior is just inherent in this type of design. The assembly seems about as stiff as me rotating my wrist about the axis of my forearm, holding a toilet plunger, trying to catch a passing car.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

A big component of the bow string and slip/ stick discussion is the flexibility of the fork steerer, which has not been considered in this thread.

The OP's bike uses a fairly long 1" dia. steerer which does not have a huge amount of bending stiffness compared to more modern bikes with a 1 1/8" steerer or more recently a tapered steerer with 1 1/2" at the fork crown to 1 1 1/8" at the top headset bearing.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

I think there are so many different inputs into this that it is difficult to get to any one explanation. Basically any play or flexing in any of the component parts or structure will generate juddering.

Myself I've usually found that reducing any play on the brake arms always helps as does getting new pads or roughening up the ones you've got. Rim type brake blocks need to be soft enough to have a good CoF, yet hard enough that they don't wear away too much in use. They often seem to get "glazed" and a very hard surface develops which generates more of the stick slip phenomena.

Have you investigated a retrofit to disc brakes? They are just superb and might make your bike even better....



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

A fork with a 1" steerer and fork blades designed for rim brakes is not suitable for the greater eccentric loading imposed by a disc brake. Replacement disc brake forks for 1" steerer are pretty rare and, together with the replacement equipment required to run a disc brake, are cost prohibitive for an old touring bike.

A fork crown cable stop for cantilever brakes removes steerer flex as a contributor to front brake chatter.
https://www.bikehugger.com/posts/cyclocross-tidbit...

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

(OP)

Quote (LittleInch)

Have you investigated a retrofit to disc brakes? They are just superb and might make your bike even better....

I have, that's actually where I started when I got the bike. When I was a child, I was flying down a hill when the front cantilever brake straddle wire on my bike hopped the cable guide. The straddle wire then fell on top of the knobby tire, caught, and brought the bike to an almost instantaneous stop. I flew over the handlebars and collided mouth first into the tailgate of a parked F150. The fake front tooth that I earned that day still causes me difficulty and cost me $$$.

So... I'm not a fan of cantilever brakes. To an almost pathological degree really. I inspect them before every ride and probably spend way too much time inspecting them while I ride. Part of the move to the CX50's is that the straddle wire setup should nearly eliminate the possibility of my experiencing that same nightmare a second time.

Switching to disc brakes would be awesome. However, it would be impossible at the rear because the frame mounting hardware for that is non existent. Additionally, I've only got 130 mm between the dropouts there which is probably not enough. I could potentially go to disks at the front but it would require me replacing the fork with one having a 7/8" steerer tube and, preferably, a quill stem setup. That's a pretty rare setup that would be tough to come by. Additionally:

1) The asymmetry of disk on the front and rim brakes on the back would bother me immensely.

2) The best thing about the bike is it's compliant steel frame. I'm hesitant to mess with that, even at the fork.

3) I've come to have a begrudging respect for cantilever brakes. And I believe that I can make them safe with an improved setup and proper maintenance. When I was a kid neither I nor my parents was really paying any attention to the maintenance of my bikes.

Surprisingly, even switching to V-brakes is not an option without a fork replacement. The cantilever brake post spacing on the vintage bikes is very narrow by modern standards. 2.5" vs a more common 4"+. That doesn't sound like much but it results the the V-brake arms being flared out at 45 degrees and the angles of everything being all wrong with respect to leverage and shoe contact angle. The folks on the bike forums suck at physics on average but in this they gave me good advice: the brakes need to stay cantilevers.

The pic below is somebody else's attempt at the V-brake. Even at that, you need a very particular model to get it done at all.



RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

Can't say that I have ever run into the shudder on any of my bikes but they all have 1 1/8" steerer tubes.I did have a lot of squealing problems on my 1994 bike. I fixed that by replacing the brake arms as mine were very worn out. I tried to put V's on instead but ran into the same issue you had - the post spacing was off. They worked in the front, however, so I have V in the front and canti in the back. My mountain bike has disk brakes and it is a game changer.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

(OP)

Quote (phamENG)

As a novice cyclist I haven't really taken the time to consider parts of my bike in a rigorous engineering sense.

If anything, that makes your input here more valuable to me. Like anything, bike mechanics is saddled with it's own, established dogma that can be an impediment to fresh thinking.

Quote (phamENG)

Unless it's coming from imperfections in the rim and minor out-of-roundness.

Thanks for that suggestion. I hadn't considered the wheel being out of true as contributing to the problem. While I'm not convinced that it is the problem, I'm also unable to rule it out logically. Happily, that's something that I can test easily. When spring finally arrives here, I'll swap front wheels with my precision road bike and see if that makes a difference. Two things mitigate this possibility in my opinion:

1) I true my own wheels on a semi-pro truing stand. My front wheel is within 2mm of true everywhere and closer to 1mm most places.

2) One of the perceived advantages of the cantilever brake setups was that the straddle wire can slide left to right, allowing one side of the braking system to come into contact with the rim before the other side does with out it being a problem. It's sort of self equibrillating. That said, at a very high wheel RPM, that may not happen fast enough to avoid a "grab" problem.

Quote (phamENG)

That makes me wonder about your number 2...it seems the greater mechanical advantage offered by V-brakes would help to overcome that.

In a way, I'm thinking along similar lines. I agree with compositepro, human909, and dold that this absolutely is a stick and slip problem. But the larger questions is why it's a stick and slip problem for this particular setup. I view the braking mechanism as similar to concrete shear friction in that, along with the shear resistance, there is always a propensity to kick the joint apart. Here, that means throwing the pads off of the rim momentarily. I speculate that the greater mechanical advantage of V-brakes tends to keep the brake pad in proper contact with the rim further into the braking load history. It's like a kick-ass clamping for in shear friction.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

Quote (phamENG)

Stalking does fit his profile. Watch out for some guy in yellow crocks next time you're out for a ride.

Whoa whoa whoa, you think he's going out in his dress crocs stalking? Come on, you should know that's when he wears the camo coloured crocs.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

(OP)

Quote (phamENG)

Stalking does fit his profile. Watch out for some guy in yellow crocks next time you're out for a ride.

You're right, it does fit my profile. On this forum, I mostly only get to know folks by way of their ideas. At some point, when I've found enough of someone's ideas interesting, I start to want to know more about their background in order to better understand their ideas. Where are they from, what kind of work do they do, how old are they, do they have kids... I find that, over time, I can decipher a fair bit by just paying attention.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

human909 and dold seem to have a very plausible theory in my opinion. I don't buy the "bow and arrow" theory at all.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

(OP)

Quote (LittleWheels)

A big component of the bow string and slip/ stick discussion is the flexibility of the fork steerer, which has not been considered in this thread.

Quote (XR250)

Can't say that I have ever run into the shudder on any of my bikes but they all have 1 1/8" steerer tubes.

While I'm grateful for the suggestions on the steerer tube, I'm pretty set against that as a potential source of the problem. Here's why:

1) I suspect that the stiffness of my 1" steel steerer tube is actually greater than that of your average 1.125" aluminum steerer tube once the difference in elastic moduli is considered. The thing is basically a plumbing pipe of old.

2) More than anything, I can say with considerable confidence that caliper brakes do not produce the shudder problem. A picture of one of my high end vintage calipers is shown below. As shown in the sketch, I believe that caliper brakes and cantilever brakes both exert identical forces to the steerer tube. But only the cantilever brakes exhibit the shudder. That leads me to believe that the steerer tube is not the difference.

It was in examining the caliper brake below that I came to my "aha" moment on this. The same forces shown in red that would torque the forks on a cantilever setup are also present in the caliper setup. The big difference, I feel, is that the caliper setup rectifies the torsion within the brake assembly itself rather than passing it through the fork legs. That's how I've come to believe that torsion occurring between the brake pads and the fork is the culprit.



RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

(OP)

Quote (phamENG)

I'd be happy to hear the elaboration.

It's bascially the situation shown below with braking applied only at the rear. It's an unimpressive endo of sorts where your back wheel never leaves the ground. The reduction in contact pressure between the back wheel and the ground means that any serious braking will result in the back wheel breaking into a skid and all of the braking power coming from the front wheel.

Quote (phamENG)

Might also help me understand the dynamics going on it KootK's problem a little better.

It certainly helps me understand it better. I think that a logical question to ask here is this: "why is shudder always a front brake phenomenon and never a rear brake phenomenon?". The proponents of the bow theory would say that it is because the fork is flexy and the rear brake is mounted to a stiff, triangulated frame. I don't buy that because I don't buy the bow theory. Rather, I feel that shudder doesn't happen with rear braking simply because the physics of it make it such that you never really do any serious rear breaking. You know, at least not unless you get shot out of a cannon up a 45 degree slope and try to stop in a hurry.

In fact, if I had the ovaries required to ride my bike backwards at 40 km/h, I speculate that I could make my rear brake produce more shudder than the front brake does. That, because the seat stay tubes that the rear brakes are attached to are of a significantly smaller diameter than are the front fork legs. Thus, the possess much less torsional stiffness.


RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

(OP)

Quote (phamENG)

I was referring to the 100% effectiveness of the front brake. It scared me...

I may be able to offer some meaningful help with regard to your cycling technique.

Firstly, in a lot of situations, rear braking is safer than front braking. That basically includes serious cornering or any time that you're on a surface where you might break traction at the front wheel (ice, rain, leaves, gravel). The reason is simple enough: a front wheel that is skidding sucks pretty badly at steering. And steering can be pretty important. So there's logic in rear braking. The real weakness with rear braking is that it simply won't get the job done in a high speed emergency situation when you need to stop quickly. That, because of the endo weight transfer thing that we discussed above.

Secondly, I've read some interesting stuff suggesting that our common perception of what happens when we front brake and fly over the handle bars is deeply flawed. What everybody perceives is that they hit the front brake hard and then flip over the front wheel with their bodies and their bikes traveling together as a rigid body of sorts. Apparently, what really happens is this:

1) You hit the front brake hard and your bike slows fast. Faster than your body.

2) Because your arms are poorly braced, your body flies over the handle bars separately from your bike.

3) Your bike only flips over because you drag it along with you. This is especially the case if your shoes are clipped in.

4) You and your bike wind up in a heap of pain and shame so quickly that it's almost impossible for you to accurately parse out what really happened.

The moral of the story is that, from a physics perspective, it is nearly impossible for you to flip over the front wheel, along with your bike, as a combined rigid body. You'll invariably start skidding the front wheel before that happens. So, from a technique perspective, what you need to do is to simply brace your arms as stiffly as you can before you hit the front brake. This encourages you and your bike to travel as a rigid body.

I practice this technique a few times at the beginning of each cycling season just to get reacquainted with it. I'll do it in an empty parking lot and can indeed lock up my front caliper brakes without flipping over. It's kind of fun really.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

With rim brakes (side pull, center pull, or cants) I have always been able to fully lock the fronts.
Having a feel for that is one key to good bike handling.
I almost never use the rear brake (one of my bikes doesn't have a rear brake).
I have one bike that likes to shudder a lot when braking.
It has a very flexible fork which only makes it worse.
And it is a very steep frame (not touring at all).
I replace the pads at the beginning of each season.
Wash the rim sidewalls with solvent (after dismounting the tires).
And make sure to use a bit more toe-in on the pads than is typical.
Brake flex is part of this, and that can't be changed.
I have tried three different sets of brakes on this particular frame, and they all do it to some extent.
This is only an issue on front brakes because the brake loads on the rear are not enough to cause this to happen.
If you to-out the rear pads you can cause it to happen a little bit.
But usually when they bite it is enough to lock the rear.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

You can def. flip a mountain bike over with disk brakes as a rigid body (squishy fork, sticky tires and a high CG). For maximum braking, they teach us to drop out heels and our CG as much as possible and put your weight back. You can then use both brakes more effectively. Having a dropper seatpost that is already down helps even more.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

(OP)
Yeah, mountain bikers are always very resistant to the concept. I suspect that there are many complicating factors on typical mountain bike ride where one flips over:

1) Maybe downhill bumping up and down.

2) Front wheel may catch in a rut, against a root, or something that changes the physics a bit.

3) Riding stiff armed is pretty much anathema to comfort on a mountain bike.

My hardtail is a Marlin 6 with hydraulic breaks and some sticky, Schwalbe Big Apples on the rims (basically giant road tires). I can lock that up on pavement with just the front brake without going over. The stiff arming basically encourages the weight shift to the rear.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

My instincts align with a lot of what's already been said. I had a pretty solid laugh though as I scrolled to the bottom and started seeing diagrams getting thrown out.

But now I have other opinions!

(a) Disc breaks are wonderful. I thought it was probably overblown until I started using them.

(b) Yes, general wisdom is that front braking isn't a thing you should do while cornering to avoid skids. However, it's generally an important part of the whole stopping plan in any other situation.

(c) I would amend Koot's braking technique suggestion of just bracing your arms as hard as you can. On top of that, you should lift your butt of the saddle and shift back as far as you're comfortable. Being off the saddle gives yourself the ability to play with your center of gravity and the angle of force applied to the pedals. You'll feel more stable, you'll be able to take a bit more of the braking force into your legs, your center of gravity will shift down and back. People have told me that the primary benefit of this is the shifted center of gravity. Personally, I think it's more about the fact that you're getting the angle of your legs closer to the angle of the resultant force. I picture it as standing on the pedals as that resultant moves.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

I really like the diagram dold put together.

Thanks for the cycling tips. I'll have to put them to the test next week. Now, if you see me posting between 11am and 1pm Eastern time next week - yell at me for not going riding. This whole "wake up, work, eat, work, eat, hang out at home (or work some more), sleep, repeat" thing is taking a serious toll.

Had a prof that was an avid mountain biker. He did the rigid body over the handle bars thing once. Disappeared from the university for a couple weeks, showed back up in a neck brace. Went head first into the side of a mountain. Doc said a quarter degree entrance angle up or down and he would have been dead. Instead, he hit straight on and creating a textbook compression split of one of the vertebrae in his neck. By himself, he rode the remaining 10 miles of the coarse down the mountain to finish the race and went to the hospital...

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

(OP)

Quote (TLHS)

I think it's more about the fact that you're getting the angle of your legs closer to the angle of the resultant force.

Yes, and I would argue that is another version of bracing yourself to your bike to encourage rigid body motion of the two of you together. Just with your legs instead of your arms.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

(OP)

Quote (phamENG)

I really like the diagram dold put together.

Me too, especially because it saves me the effort of putting a similar diagram together myself.

I agree that the stick and slip is the front end of the problem but I feel that the lager question is why that's a particular issue with the cantilever brakes. Dold's suggestion that it's about the cable pull line of action doesn't speak to me for a couple of reasons:

1) Different models of cantilever brakes and v-brakes actually reverse the positions of the pads vs the cables.

2) I feel as though the main arms of both brake types are pretty solidly moment connected to the braze on mounting post. I don't see the cable offset forcing a rotation through that.

When you look at the diagram below, I feel that it is the transition from "clamped" to "bites" that is of most importance. And, just as dold showed it, that would seem to be a study in torsional load versus torsional stiffness. To the extent that a system lacks torsional stiffness, that gets you to the bite faster. With regard to potential flexibility, here's what I see for options.

1) Lateral flex in the brake assembly. I don't feel that this is a major contributor because:

a) I've held them in my hand and they are pretty sturdy.

b) The flex that there is feels, to me, mostly like the take up of slop. And that happens very early on in the breaking process and would be common to all of the non-disc brake styles including calipers which never produce shudder.

2) Flex in the plates of the little braze on boxes to which the cantilever post is welded. On older bikes like mine, the boxes are three sided as shown below rather than a larger diameter tube which would be better. The walls on my bike's "box" look to be about half as thick as the ones shown.

3) Flex in the wall of the fork which is very thin.

4) Twist in the fork leg which is of a fairly small diameter and thickness.

In contrast, caliper brakes eliminate all of those things but #1. And caliper brakes can deliver a very strong braking force without brake shudder.



RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

If you don't already own them, it's time for a Fright Harbor trip to get some inexpensive dial indicators and indicator stands to measure twist in the pad holders when the brake is applied.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

(OP)
Coming back to the truthiness of the "bow" hypothesis, this is the structural model that I see. I feel that it makes it rather obvious that fork flex could not be the cause.

RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

(OP)
Similar to this link, the commonly proposed solution is to add one of these cable hanger things that brings your housing closer to the brake. As I see it, that only modifies my previous model as shown below. As such I don't see how it would accomplish anything meaningful. It might actually make things worse by adding extra housing length and softening the transmission of braking force from the lever to the pads. Me no understando.



RE: Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

If there is a stick-slip the fork will tend to amplify the effect. As the pads stick the force shoots up, bending the fork/steering tube in addition to the deflection of the pad. When the pad then slips, the force drops off and the fork/steering tube snaps back. Depending on the mass of the rider and the spring rate of the fork/steering tube it might set the natural frequency into an uncomfortable/noticeable range. To be clear this is the deformation from the force the axle applies to the fork tips, not what the brake mounts apply to the fork.

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