Motorcycle front suspension design 1

[PTwizz]

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.

 

[GregLocock]

Our solar car started off with a bicycle style single yoke front fork system. I then had to spend a week designing a mini spaceframe and a top yoke to allow the loads to be transferred into the top bearing as well.

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

 

[PTwizz]

Greg,

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.

 

[GregLocock]

We had rigid forks, I went from the original single 22 or 25 mm tube (from memory) to a pair of half inch tubes, fully triangulated, down to the hub. ie, I have no design experience with telescopics.

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

 

[zedrider]

Conventional telescopic forks for motorcycles are fundamentally flawed in design - like an "A" frame without the horizontal cross-member.

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:

http://www.voidstar.com/bff/index.html

 

[spdingtkts]

If any of the alternative front suspension methods worked as good as the USD fork designs then they would be using them in MotoGP.

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.

 

[zedrider]

I personally think that the reason none of the current GP bikes use alternative front suspension is because none of the manufacturers wants to be first with new technology that doesn't have decades of development behind it.

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?

 

[spdingtkts]

The Japanese factories have experimented with alternative suspension designs.

The only ones that use any of that stuff are people/companies that are doing it just to be different.

 

[PTwizz]

I tend to agree with Zedrider re. the use of existing technology, especially as most MotoGP teams use suspension from a specialist supplier (Ohlins, WP etc.) who might have little intesrest in developing suspension systems which can be built as part of the chassis, requiring only the purchase of a damper.
We have, however, deviated from the original question of the relative stiffness of upper and lower yokes in a telescopic fork.

Pete.

 

[Tmoose]

>snip< 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.<<

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 &quot;working&quot; the clamped joint. I think of the lower head bearing applying a force to a cantilevered beam &quot;fixed&quot; at the fork tube. Then, Just as you suspected, the multiple widespread bolts can provide the &quot;fixing.&quot;
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.

 

[gregservice]

The popularity of the telescopic front fork design relates to its relative simplicity and cheapness to manufacture and lack of patents. The top yoke can be held on by bolts screwed into the fork tubes and there have been applications where the fork tubes have been welded to the bottom yoke. Generally pinch bolts are used so that the suspension units can be removed for repair/maintenance. Don't forget the importance of the wheel axle to overall structural integrity. If you want to see two wheeled suspension innovation, have a look at many of the scooter designs, much more innovative.

 

[PTwizz]

Thanks Greg,

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.

 

[gregservice]

Pete, have a good look at the Hossack/Fior design.

http://www.hossack-design.co.uk/

Tony Foale also did a more modern girder some years ago. As to motorcycle design, the innate conservatism of the purchaser is a major consideration.

 

[PTwizz]

Thanks Greg.

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.

 

[zedrider]

PTwizz,

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

 

[gregservice]

Found this site that has some research relating to your query.

http://www.lozik.h1.ru/Auto9.htm

 

[mburgess]

One major factor in the design of the lower clamp is to hold the upper fork tubes as parallel as possible. If the forces are shared between upper and lower clamps the the forkes can still twist. This way the frictional forces on the DU bushes inside the forkes are minimised. Also if you read a workshop manual the torque ratings on the pinch bolts of your triple clamps are quite low. This is to avoid deforming of the fork tubes. Imagine the frictional forces involved if the inner tube had to slide through the deformed outer tube. Thus a larger clamping area allows for less actual claming torque. You can see this in the designs. Older forks might use single m12 bolt for each leg where a modern racing clamp would use 4 x m6 bolts.
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

 

[TonyFoale]

This is a rather late reply but I've only just found out about and joined these forums.
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 (

http://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.

 

[PTwizz]

Tony,

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.

 

[PTwizz]

Tony,

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.