Motorcycle front suspension design
Motorcycle front suspension design
(OP)
I am in the early stages of designing a race motorcycle front suspension. Looking at those available, all appear to have a bottom yoke substantially thicker than the top yoke, with up to four pinch bolts on each side, clamping the fork legs. A couple of simple force diagrams show that a stiffer bottom yoke imposes much higher loads on the fork leg, ultimately to the point where the top yoke takes no load and the full bending moment applied to the leg is taken out over the thickness of the bottom yoke.
(Deep breath)
Does anyone know of a reason for the popularity of this layout?
Pete.
(Deep breath)
Does anyone know of a reason for the popularity of this layout?
Pete.
RE: Motorcycle front suspension design
Quite simply it is very difficult to design a single yoke that is stiff/strong enough in torsion to react the vertical and longitudinal loads.
The bottom yoke needs to be bigger because it sees higher loads than the top yoke. I'd look at it the other way, and say that the top yoke can be made smaller !
Cheers
Greg Locock
RE: Motorcycle front suspension design
I certainly agree that the bottom yoke sees the higher loads, but I was working on the basis of the fork tubes being the weakest link in the design, and therefore most in need of stress reduction. The highest loads on the bottom yoke occur perpendicular to the axis in the leg, a direction in which the yoke is naturally fairly stiff. Vertical loads in the bottom yoke are only higher than in the top yoke if the bottom yoke is stiffer than the top (in bending). I don't have any issue with splitting loads between upper and lower (headstock) bearings, as the usual practise in motorcycles is to use lightly preloaded taper roller bearings, with both yokes attached reasonably rigidly to the stem. As I see it, the best load case for the leg would be acheived by mounting it in a spherical bearing at the bottom yoke.
It does occur to me that many of the bottom yokes I've looked at are massive immediately around the fork legs, but relatively slim towards the centre, with little torsional stiffness. Perhaps the large clamping area is simply to prevent distortion of the tube in an area where the inner leg has to slide within the outer?
Pete.
RE: Motorcycle front suspension design
However, having seen a few bent front forks over the years i think you are right, spreading the load is pretty important, and I suspect the pinch loads need to be carefully handled as well.
Cheers
Greg Locock
RE: Motorcycle front suspension design
You might find it interesting to do a google search for "Hossack", "BMW Telelever", "Difazio HCS", "Hub Centre Steering", "Cosentino Rotacular", "Honda ELF", "Bimota TESI", "Royce Creasey Voyager".
These are all either alternatives front suspension systems or particular bikes with them fitted.
Or try this link:
www.voidstar.com/bff/index.html
RE: Motorcycle front suspension design
I hope you got an email from someone telling you how there is a Motorcycle section for motorcycle related posts like I did when I posted a motorcycle question in here.
RE: Motorcycle front suspension design
In other branches of bike racing, like BEARS, BoTT, SoS there have been very successful bikes with Hossack type suspension and also Saxon-Motod (ancestor of BMW Telelever) suspension. Also don't forget the French ELF racers of the '80s. Admittedly they had variable results, but perhaps they were trying to do too many new things at once.
A few years ago, a privately entered GTS1000 Yamaha with a YZF750 motor placed high in class at the IoM TT.
If you never try anything new, how can you make mistakes to learn from?
RE: Motorcycle front suspension design
The only ones that use any of that stuff are people/companies that are doing it just to be different.
RE: Motorcycle front suspension design
We have, however, deviated from the original question of the relative stiffness of upper and lower yokes in a telescopic fork.
Pete.
RE: Motorcycle front suspension design
Even if the lower clamp has the necessary KILLER grip on the tubes, I suspect the lower clamp's grip on the stem would slip when subjected ~1000 lb-ft moment of heavy braking. Then some of it will resolve back into radial loads of opposing sign at the upper and lower bearings via the tubes.
Isn't the moment the same at the lower clamp whether fixed or simply supported? When I bent my 32 and 35 mm dia forks (with wimpy single bolt lower triple clamps) by panic-ing smashing into things, the fork tubes bent at the lower triple clamp, where the moment was highest, which would seem reasonable, even if the triple clamps were holding the tubes like simple supports.
One time I time I bent the triple clamp also, suggesting its single bolted grip was stiff enough to force it to transmit some moment.
I think the modern lower clamp's beefiness has 2 values.
1 is to better pass the axial nearly vertical loads into the lower head bearing without tipping or "working" the clamped joint. I think of the lower head bearing applying a force to a cantilevered beam "fixed" at the fork tube. Then, Just as you suspected, the multiple widespread bolts can provide the "fixing."
2 - Multiple clamp bolts in the lower clamp also help keep the tubes from twisting and shifting alignment. When viewed from the side, going from || to X. Maybe good for smooth fork action, and forcibly steering out of off-road ruts, among other things.
If the forks were vertical (rake=0) with no offset in the triple clamps (o-o-o) the lower steering head bearing would take ONLY axial upward from weight, and side radial loads. THe upper bearing could take ONLY the occasional axial downward (dangling fork weight/inertia , as from wheelies, jumps, and potholes) and radial loads. Braking forces would resolve into opposing radial loads at the upper and lower bearings via the fork tubes.
RE: Motorcycle front suspension design
RE: Motorcycle front suspension design
I cannot shake off the feeling that there are forces other than enginnering driving the choice of front suspension design for many manufacturers.
My cuurent leaning is toward a girder fork layout, using a CF composite girder frame mounted on alloy wishbones with a single damper/spring unit coupled through a rising rate linkage. The girder design is essentially a 'proper' double wishbone suspension, with all the opportinities for dialling in combinations of desirable geometry.
The main stumbling block will be convincing the customer that they really want 1920's suspension!
Pete.
RE: Motorcycle front suspension design
RE: Motorcycle front suspension design
I am familiar with the work Tony Foale, having followed his writings in various publications since the 1980's.
Fortunately, my customer is a race team and has no requirement to sell to the public in the short term. (Perhaps public perceptions will be tempered by a few high profile wins on girder forks.)
Pete.
RE: Motorcycle front suspension design
It may not be obvious from the photos in Tony Foale's book but his updated girder front fork design is inferior to the Hossack layout, because a girder includes the mass of the spring (and linkages) in the steered mass, which increase the steering's moment of inertia.
The Hossack layout does not steer the spring at all.
If you still want to have a look at a range of novel chassis and suspension designs there's a lot of them on:
http://www.eurospares.com/
You may find Ollie Hoyt-McKaggen's take on Hossack/Fior particularly relevant.
ZR
RE: Motorcycle front suspension design
http://www.lozik.h1.ru/Auto9.htm
RE: Motorcycle front suspension design
In respect to Saxxon, Telelever, ELF et al front suspension technologies many of the riders found the suspension action disconcerting. The engineers are telling you it is better but 15 odd years of experience of the front end diving under braking and your instincts are telling you otherwise.
Cheers MB
RE: Motorcycle front suspension design
zedrider mentioned that my girder forks were inferior to the Hossack type.
I agree, but some explanation is in order. The Girder was a one off job for a customer who specified that type, and predates the Hossack by around 10 years. Initially I refused to make them but on reflection I decided that it would be an interesting exercise. They actually worked quite well.
I much prefer systems with a 4 bar linkage and without a conventional steering head, and to that end I also built some, very similar to Norman Hossack's, although they were only a stop gap until some special wheels were available which enabled the use of a single sided design with the linkage system mounted lower.
My web site ( www.tonyfoale.com ) contains many illustrations of different designs of motorcycle front suspension, for those of you interested. I believe that mentions of my book have been referring to the 1984 edition. This has recently (2002) been completely rewritten and has 5 times the text of the original and features many more examples of motorcycle suspension design.
RE: Motorcycle front suspension design
Thanks for your contribution. I have the old edition of your book, but I shall certainly now look for an updated copy. I am a fan of the girder fork layout, primarily because it offers several degrees of 'tuneability' which are not available from telescopic forks.
Pete.
RE: Motorcycle front suspension design
I now have access to a copy of your later book, by an unexpected route. The copy on my desk is that which you gave to Rob Williamson during your visit to my employers on 28th May! It is indeed a small world.
Pete.
RE: Motorcycle front suspension design
It's a pity that I didn't know that you were there. Apologise to Rob for my rather sudden departure, but I suddenly realized the time and had a plane to catch. In fact I just got there as it was boarding.
Tony Foale.
RE: Motorcycle front suspension design
On most motorcycles you can lift the front of the bike up on a stand so that the front fork swings freely. It is now easy to check for any play in the steering stem bearings. If any play is detected you remove the top triple clamp and tighten the adjusting nut. Since the wheel, forks rotors and so on are still attached you can get a good feeling if the bearing is too tight or to lose by seeing how freely the whole assembly will move.
On most sporting/racing motor cycles the clip on’s(handlebars) are below the top triple clamp. Thus when you remove the lightweight top triple clamp you haven’t disturbed any of the heavy components in the front suspension. So when you or the rider tests the bearing setting on the stand it is a very good prediction of how tight the steering will be on the track. This is also a fairly fast process which is good as it may take several laps for the bearing races to fully seat.
RE: Motorcycle front suspension design
All that you say is true and indeed very practical. However, the loads that I refer to are bending loads (in the fork legs), rather than the loads created by supporting the machine mass.
Pete.