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Calculate clamp force on bike handlebar

Calculate clamp force on bike handlebar

Calculate clamp force on bike handlebar

(OP)
Hi Folks

I am looking for some help in a calculation if possible.

I am looking at a bike design where the handlebars are clamped in place in the stem using only 1 bolt - see the picture below for an example.



http://i.ebayimg.com/00/s/MTE5OFgxNjAw/z/rdgAAOxyr...


There has been issues with the handle bars slipping forward when in use. I think that this is a manufacturing problem and not a design issue as this type of stem with one bolt is quite common.

However I would like to prove this mathematically. How do I calculate the force that the bolt and clamp have on the handlebars? I would like to compare a calculation of 1 bolt versus 2 and 4.

Any help greatly appreciated.

Thanks

RE: Calculate clamp force on bike handlebar

handle bars slipping forward ? yes, this is a common design (using one bolt to clamp the handle bars in place). in my experience the common "failure" is the handle bars slipping and rotating ('cause they're not clamped sufficiently) ... solution tighten the bolt more (what, more?). the design of the bracket should provide a round seat for the handle bars ... if they are sliding forward then the seat is too shallow ... the opening (that the bolt clamps across) should be less than 180deg.

another day in paradise, or is paradise one day closer ?

RE: Calculate clamp force on bike handlebar

designed by amateurs, tightened by wrestlers ?

like the old sketch "who decides the preload on a bolt ?"

another day in paradise, or is paradise one day closer ?

RE: Calculate clamp force on bike handlebar

Pinch bolt designs like this are difficult to analyze. I have seen the use of a triple integral to determine surface pressure and resulting forces. Usually joints like these are tested physically.

RE: Calculate clamp force on bike handlebar

Force generated by the bolt is (Torque)/.2/(diameter of bolt)

For multiple bolts multiply the force by the number of bolts. Generally more bolts are used so that a lower torque value can be used to get the same force.

Using the same torque settings 2 bolts deliver twice the clamping pressure of one bolt and 4 will deliver 4 times the pressure. Likewise 4 bolts at 1/4 the torque will give the same clamping pressure as one bolt at full torque. Also note that switching to a smaller diameter bolt will decrease your clamping pressure too for the same torque setting.

Clamping pressure is just the force divided by the area of the handle bars under the neck.

But your issue is how much torque can the handle bars take before they turn. This is a function of the friction between the handle bars and the neck, and the force applied. Also taking into account material selection and surface finishes. (Weaker materials will shear under lower loads, ridges provide added friction.)

Hope this helped. And good luck.

RE: Calculate clamp force on bike handlebar

i think your "forward" movement of the handle bars is them rotating in this clamp. i think your problem is more about the enforced contact displacmenet around the handle bar shaft than it is the bolt load. sure, the bolt load affcts the problem, but how do you get from bolt load to resisting torque ?

i'd look into the detail design of the clamp ... when the clamp is closed is the opening slightly less than the diameter of the bar ? is there a good amount of friction between the handle bars and the clamp ?

if you want to control the handle bar position, you could put flats on the handle bar and pads on the clamp, and trap it forever.

you could add a small rubber piece that'd compress between the handle bars and the clamp.

another day in paradise, or is paradise one day closer ?

RE: Calculate clamp force on bike handlebar

what does the poor 7 mm bolt/screw thread into?
How tight are you tightening the screw?
How is the screw protected from or accommodate bending? Is the 90 degree lower "lip" expected to both conform to the handlebar and align with the screw axis?

Is there a gap between the faces of the clamp when the screw is fully tightened?
Or, do the faces of the clamp butt when the screw is tightened

Are there gaps between the handlebar OD and the clamp when the screw is fully tightened?

It is not unheard of to knurl handlebars where they will be clamped.
http://www.porkchopbmx.com/images/HANDLEBAR-ART-KK...

RE: Calculate clamp force on bike handlebar

(OP)
Hi Tmoose

The issues you mention are the issues I feel are at the source of this problem. However I am trying to ascertain how I can calculate if 1 bolt is enough to hold the handle bar in place if all the things you mention are correct. I am being told that 1 bolt isnt enough and I simply dont believe this (if all other factors are as they should be).

To one of your points how would you protect a bolt from bending?

Thanks

RE: Calculate clamp force on bike handlebar

Can you tighten the one bolt enough to crush the tube or deform the clamp? If so then it is more than enough.

I'm not sure why you are looking at bolting instead of increasing friction.

RE: Calculate clamp force on bike handlebar

(OP)
@ 1Gibson

This is a product being made in China so I dont have it physically here unfortunately! All of the above suggestions and what you have suggested are correct and steps I will be taking but I want to prove the design with numbers first if I can.

RE: Calculate clamp force on bike handlebar

i think your problem is geometry, unless you know that tightening the bolt clamps the handle bar before the faces of the clamp contact (which they probably shouldn't).

you could serate the inner face of the clamp, to improve it's "bite" onto the shaft. Of course, the "ultimate" fixing for the shaft in the clamp would be a cross pin.

what materials are we talking ? steel ? carbon ?? plastic ???

another day in paradise, or is paradise one day closer ?

RE: Calculate clamp force on bike handlebar

(OP)
We are talking stainless steel.

RE: Calculate clamp force on bike handlebar

ok, nice and ductile. how about the geometry questions ?

another day in paradise, or is paradise one day closer ?

RE: Calculate clamp force on bike handlebar

(OP)
Nope I am not sure on the geometry - in theory its all fine i.e. the drawings are all correct but they are producing may not be. The problem is they are blaming the design (the use of 1 bolt) and I believe it is a geometry problem during manufacturing/assembly so am trying to prove mathematically that 1 bolt is enough so I can press them on the other issues.

RE: Calculate clamp force on bike handlebar

ah, the plot sickens ...

you've got your design on hand, so you can verify that the clamp does (by dsign) clamp over the shaft.

The bolt (something like a 1/4") is plenty to bend the flanges of your clamp. you can stiffen these with radius/throat washers if needed.

It sounds like you need a Skype session with them to see how the clamp tightens, maybe it's deforming in an unexpected way?

Maybe wrapping some dense rubber around the shaft will improve the clamp-up ?

another day in paradise, or is paradise one day closer ?

RE: Calculate clamp force on bike handlebar

I think you could use some sort of simple keyed feature. Maybe drill a hole in the frame, and use something like this " O= " that goes in between the faces of the clamp (adjust clamp design if required) with the bolt going through the center.

Think of a small eye bolt, but it only needs a straight pin, no threads.

You might be able to use plastic. Cheaper than an extra bolt. A little more complicated to assemble, but is that really factory assembled? How do you ship the bikes?


Anyway, to answer your original question, calculate the pullout strength of that bolt into the clamp. Check to see if that will yield the clamp, or the tube. If it can, then one bolt can work, it's just a matter of determining the torque that keeps it from sliding. To do that, you need to know coefficient of friction.

Did the factory get tapping oil on the interface? That would certainly do it.

RE: Calculate clamp force on bike handlebar

There's actually a hinge joint (free to rotate)? A fixed joint requires bending of the flange arms. The hinge joint makes it much easier to calculate the clamp load and friction on the tube.

RE: Calculate clamp force on bike handlebar

(OP)
its a hinge joint yes.... can you tell me how to calculate the clamp load and friction on the tube?

RE: Calculate clamp force on bike handlebar

Quote (Gafferino)


Hi Dougt115

Thanks for the reply. Just to confirm your equation do you mean:

F = T/(0.2xD)

Dougt115 is quoting the standard screw torque equation T=KDF, where K is the friction factor, usually set to 0.2. My textbook (V.M.Faires) shows 0.15 if the joint is lubricated. This applies to standard bolted joints. Corypad is suggesting that your joint is very much more complicated than that. He wrote quite a few of the bolting FAQs in forum725: Welding, Bonding & Fastener engineering.

--
JHG

RE: Calculate clamp force on bike handlebar

(OP)
Yep sorry I had reworked the T=KDF Eqn to get F = T/KD above

RE: Calculate clamp force on bike handlebar

I gave the simple equation because it is simple and the task was to show the relationship between the number of bolts and the clamping force. This equation does it very clearly.

Adding an interface material can work but it will have to be harder than either of the other two materials. The shear forces in the material will turn any softer material into a pseudo lubricant and increase the slipping issue. (I did this as a kid to my dirt bike, quite a surprise when my handle bars rotated 90 degrees forward and left me on my face eating dirt. My friends thought it was funny, thankfully before video cameras were around.)

If an interface material, sleeve, is used you could consider adding bite by knurling the inside and outside of it. (Note, too much bite may weaken the parts by adding stress concentration points to the neck and handle bar.)

Second option is to increase the area where the neck grips the handle bars. This would increase the surface area and in turn increase the friction. You would need to increase the torque on the bolt to maintain the same forces on the surfaces.

RE: Calculate clamp force on bike handlebar

what does the load in the bolt tell you about the torque on the shaft ? Yes, due to the bolt load there is some clamp up applied to the shaft, some resistance to torque applied via the handle bars; but how to relate these ? btw, the T used in the P equation isn't the "torque applied via the handle bars".

The bolt preload (well, really its deflection) applies some pressure to the shaft, which courtesy of friction between the shaft and the clamp, resists torque applied by the handle bar. no?

another day in paradise, or is paradise one day closer ?

RE: Calculate clamp force on bike handlebar

How much torque is the handlebar allowed to put on the joint before the handlebar moves?
How much load can the bolt take before it fails?
What is the diameter of the handlebar?
What is the free diameter of the stem?
What is the friction coefficient between the handlebar material and the stem material?

I think the limit for torque is mu-D-2*bolt tension. The factor of 2 is because the bolt tension is balanced by the load in the hinge.

RE: Calculate clamp force on bike handlebar

1gibson is on the right track.

You have a clamping force vs. rotational friction issue. If you increase the clamping force, the friction increases. If you increase the roughness of the handlebar section under the clamping section, it will also increase the friction...same result.

RE: Calculate clamp force on bike handlebar

Need details of the hinge.

RE: Calculate clamp force on bike handlebar

Hi Gafferino,

Do you have an actual assembly to evaluate in person?

If so:
- Is there a gap between the faces of the clamp when the screw is fully tightened?
- Or, do the faces of the clamp butt when the screw is tightened?
- Very important, Are there gaps between the handlebar OD and the clamp when the screw is fully tightened?

regards,

Dan T

RE: Calculate clamp force on bike handlebar

Has there been a tolerance study done on the parts?

If your company can afford to spend the big bucks for Chinese fabrication, surely they can afford the tolerance study.

Also, when calculating the clamping force ask yourself which dimensional tolerance to apply for a worst-case calculation. And think about how completely unknown the coefficients that you are using really are. The only reliable way to analyze this is to conduct experiments with real parts that cover the gamut of dimensional stack-ups, fastener tolerances and applied torque tolerance.

RE: Calculate clamp force on bike handlebar

Ultimately what matters is the friction moment existing at the clamped interface. It does not really matter how many bolts you use. What matters is the stiffness of the mating parts and how much clamping force is produced at the interface. If the handlebar tube is thin and lacks radial stiffness at the location it is clamped, you will never be able to tighten the clamp screws enough to prevent the handlebar from slipping.

RE: Calculate clamp force on bike handlebar

Hi Gafferino

Well the problems been well defined by the members here and I agree with most of what's been said but can I ask what was the original torque or preload figure given for tightening the screw?
If there was no information given for the screw tightness then, the people making these components have a point, even if you prove that a certain bolt preload which assembly can withstand and shows the handle bars can't slip, if this information wasn't on a drawing or in a instruction manual then the design department has failed to give all the information for a satisfactory assembly.
Also the problem is more complicated than just running numbers because friction is extremely variable, so even when you obtain the right figure for the clamp how are you going to define it so it works consistently?
The only way I know to get a consistent joint is by practical testing and in addition studying tolerance build up, surface finishes etc.

I'm not trying to criticise here but pointing out when you obtain the clamping force and say to the manufacturer "you need to tighten to this figure" then the response might be "why were we not given this information at the start"

RE: Calculate clamp force on bike handlebar

the problem is the design and manufacture are separated by several timezones, continents, and cultures !

if designed properly, the bolt should be able to hold the handle bar shaft as tight as you'd want ... it is a pretty typical design. so part of the question is "is it well designed?" eg tolerance studies, clearances, etc.

the other part of the question is "is it well manufactured?" (of course they're saying it is !)

if you're being told "no matter how much we tighten the bolt, it doesn't hold the handle bars" then either
1) it isn't made according to your design, or
2) the design is flawed in some unexpected manner (maybe a flange is bending, without tightening around the shaft?).

i'd've thought that a skype session would help understand the problem. if not, have them send you the parts to verify assembly.

another day in paradise, or is paradise one day closer ?

RE: Calculate clamp force on bike handlebar

3) They got grease or oil on the surfaces. Maybe retained in the tapped hole even after attempted cleaning, and pushed through to the surfaces as the bolt is installed.

RE: Calculate clamp force on bike handlebar

looks like OP has yet to log on since Friday

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