Tony Foale Designs - Vyrus Suspension History

This article was written 16 years ago but little seems to have changed since then, except that BMW andYAMAHA have produced road models using systems mentioned herein, and the Britten has achievedracing success with a plastic "Hossack style" front end.

.© 1986 -- 2002 Tony Foale

Over the past few years we have heard much about a new generation of front suspension systems, usuallywith the promise that soon all motorcycles will be built this way or that. In particular, the Cortanzedesigned, Elf sponsored machines have been the favourite examples of most writers, convenientlyforgetting the earlier (c. 1968) systems from the British engineer, Jack Difazio, whom history may wellremember as the instigator of the present interest in alternative steering systems. However, as with mostengineering ideas in general and motorcycle ideas in particular, there is little that is really new. Interest in awide range of steering designs is as old as motorcycling itself. Although, only a small number of designshave ever been accepted for quantity production. The girder fork was the early favourite, being used bymost manufacturers at one time or another.

There were always dissenters though, the Neracar used a hub centre design and O. E. C. had anotherunusual system, both were known to give excellent results by contemporary standards. But by the early1950's, hydraulically damped telescopic forks were becoming well established, mainly due to theirimproved ride over the undamped or friction damped girders. Some manufacturers, however, were eventhen aware of the deficiencies of teles. and used some form of link forks, e.g. BMW went for the Britishdesigned Earles fork, even though they had been pioneers of teles. Ariel on the other hand used pressedmetal trailing link forks on their Leaders and Arrows.

So, is there really any significant problem with the forks that have adorned our bikes for so long; do thedesigners of the alternative proposals know something that eludes all others? Well, there are problems withnormal forks, and these begin to show up more as modern bikes get heavier and more powerful. All thenormally used types of front suspension/steering systems have a common feature, they all mount on andsteer through a steering head. So what, you may rightly ask?

The sketch shows how any lateral flex in the fork legs allows the tyre contact patch to move away fromthe steering axis. This really matters, because wobbles can be caused or greatly increased by thismisalignment, both on fairly smooth roads at a particular speed, and at any speed on a more bumpysurface. Consider what happens when the wheel glances a bump to one side, the forks deflect away fromthis bump and if still deflected when the next bump is hit, the force of this will produce a torque about thesteering axis that will cause the forks to steer to one side. We have all experienced this to some degree,sometimes it amounts to nothing more than a minor handle bar shake, but in far too many cases it candevelop into a frightening tank slapper with occasional fatal results. However, if the tyre contact patch washeld rigidly in line with the steering axis, then little torque would be applied to the steering, and thesuspension would be free to do its job of absorbing the bump forces, with the minimum of drama orawareness from the rider. There are many other problems that stem from the use of head stock mountedforks, but this potential for lateral flexibility is the single most important one.

Tony Foale Designs, article on motorcycle alternative front suspension s... http://www.tonyfoale.com/Articles/Steer/STEER.htm

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Another disadvantage is the large amount of leverage that can be exerted on the steering head itself. Thisresults in very large forces that have to be resisted by a strong and hence heavy frame. In addition,telescopic forks are well known for their propensity to nosedive under the influence of braking forces,encouraging the current solution of increasing the damping to slow the rate of dive. (This subject of divewill be considered in depth, next month.) Poor response to small bumps, such as expansion joints in theroad, which may be due to stiction in the sliders is another frequent complaint aimed at telescopics.

So now we know why the conventional setup can cause problems, but do any of the new (or old) fangledsystems really offer a viable and cost effective solution? For make no mistake, unless an alternative can beeconomically mass produced, there is no chance of seeing it fitted to standard roadsters, no matter whattechnical or safety benefits are on offer. I believe that some of these schemes have substantial potential,but whether this is translated into practice, depends on the standard of detail design and manufacture. It isno good comparing a set of top quality teles with a badly constructed hub-centre arrangement. There arethose magic words, again, "HUB-CENTRE", what do they mean and should all these different designs belabelled as such?

It seems to me that there are two main classes of these alternatives, the first, like the Neracar, the Difazioand the Bimota Tesi which all have the main wheel support at, or very near to, the wheel centre. It is thesethat can reasonably be termed 'hub-centre', although the actual steering mechanism is not at the centre.The second class includes the Elf.e, the Hossack, my own design used on the Quantum, Parker and others.At the moment, there does not seem to be a satisfactory generic term to cover these systems. The best Ihave to offer is double link or double wishbone, but not all use a wishbone as such, more like a pivotedarm, some suggest the term, parallel link, but there is no need to have them parallel. Let's look at some ofthe systems in more detail and see how they do the job.

HUB-CENTRE

Generally a large diameter, steerable but non-rotating hub is mounted on a king-pin located within it.Another hub, of larger diameter, and forming part of the wheel, is mounted onto the first hub via large ballraces. The centre line of the king-pin defines the steering axis, and so the only flexure that can allow thetyre to deflect away from this axis, is in the wheel and the hubs themselves, but as we must also have thesecomponents with a conventional design, it can be seen that virtually all the other sources of compliance, inthe forks, have been eliminated at a stroke. It must be remembered though, that any wear or play in theking-pin bushes or bearings will permit the dreaded lateral displacement, play in bushes has a differenteffect from that due to flex, and is generally more detrimental to stability. This is what I meant by theimportance of detail design. Consider now, some actual examples and study their pros and cons.


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DIFAZIO

This is the design that most readily comes to mind when hub centre is mentioned. It incorporates thefeatures mentioned above, but the king-pin is in the shape of a cruciform and is free to rotate on the axle.To prevent free rotation (except the small amount needed for suspension movement) the inner hub isconnected upward to the steering links via an "A" frame on each side, the two being joined above the tyreby a crossmember. The brake calipers are mounted on these A frames and their torque reaction is taken bythe steering arms. The axle is held between the open ends of a forward facing swinging arm, which must bewide enough to give tyre clearance as the steering is turned. Suspension units are connected to this swingarm, and the suspension loads are carried through the king-pin bushes. Overall this is quite a clever designmechanically but not aesthetically, and remember that it was patented nearly twenty years ago. Asmentioned above the source of lateral compliance (relative to the steering axis) is reduced to that of thewheel, wheel bearings, and the king-pin bearings. Braking loads are spread between the bottom swing-armand the top steering links, and because of their distance apart and proximity (compared to a head-stock) tothe tyre contact patch, the loading is much less than that normally experienced.

Disadvantages also exist; as might be expected, we seldom get anything for nothing. As all the sidewaysrigidity is provided by the king-pin, this part is quite highly loaded when the wheel is subject to a lateralforce, e.g. hitting a bump whilst cornering. The potential strength of the side 'A' frames is not used torelieve this loading, which also tends to bend the axle into an S shape, and twist and bend the swing-arm.However this axle and swing-arm distortion does not allow the front tyre to misalign with the steering axis(because the axis moves with the wheel). Perhaps another problem is that it is a fairly complicated system,both in terms of the number of required components and also in terms of wheel changing ease.


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MEAD AND TOMKINSON photo

On some of their endurance racing bikes ( Nessie amongst them ), this team experimented with variationson the Difazio theme. One such design replaced the king-pin with a spherical joint that was free to pivot inall directions, to prevent the sideways movement thus made possible, a triangulated wishbone wasconnected to the middle of the cross piece joining the tops of the 'A' frames. A single suspension unit wasmounted on the top of the wishbone, relieving the hub pivot of suspension loads. Although the axle is stillsubjected to bending loads from braking, this system removes those due to lateral and suspension forces,and in addition, the possibility of swing-arm twist is eliminated. So far so good, but the potential formisalignment between the wheel and the steering axis is increased, flex in the 'A' frames can now add tothat. Despite this, I think that these modifications to the original have potential.

BIMOTA TESI photo1 photo2 photo3 photo4

Probably the most recent of the hub-centre designs, this has some features in common with the Defazio,although the king-pin is fixed to the axle and not free to rotate on it. Consequently the braking forces aretaken through the wheel spindle to short brackets, fixed to each end. These brackets are in turn located bytwo torque arms. Because these arms are relatively close to the swing-arm the forces are high compared tothe Difazio setup. The king-pin/axle is much more highly stressed, and cannot be rigidly clamped in theswing-arm, instead it must be free to rotate within it, to allow for vertical wheel movement. Hence, thespindle cannot add to the torsional stiffness of the swing-arm, which needs to be bigger as a result.Hydraulic steering, heralded in many press reports as a major innovation, is really just another source ofpotential problems, as they seem to have discovered. It is difficult to see any benefits which outweigh thedisadvantages. Anyone with experience of such closed circuit hydraulics would realize that provision forexpansion and piston rod displacement is necessary. It appears that Bimota did not, until reminded inrather dramatic fashion. The solution adopted was the provision of a pressurized gas chamber, separatedfrom the oil by a flexible diaphragm, similar to many suspension units. Unfortunately, the compliance thusintroduced may cause sponginess in the steering. Small wonder then, that some of those to have ridden it,have been less than optimistic about the feel of the machine. I have been told that it is definitely notconfidence inspiring. One could be forgiven for thinking, that Bimota would have been better off copyingDifazio's mechanicals, and using their Italian flair to transform the visuals. Press reports indicated that theTESI was designed as a college project, as such it is to be commended but it is far from a practicalproposition it it's first presentation.

( 1997 note ) The design of subsequent Bimota front ends has changed considerably since the first version,improving the areas of doubt mentioned above.

DOUBLE LINK

This type generally, is mechanically simpler, comprising basically an 'upright' (car terminology) to whichthe wheel and its bearings are attached. This upright is held in place by two forward facing pivoted arms orwishbones, the front ends of which allow for steering and suspension movement. Structurally, the variousdesigns within this family differ mainly by the locations on the upright to which the two pivoted arms areattached. Considering the lateral stiffness characteristics, we see that this is dependent only on the stiffnessof the upright and the wheel. Let's look at some actual systems.


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Elf. e (also similar to Dider Jillet). photo1 photo2

Here the upright is fairly short, and hence light-weight and rigid, but this necessitates that the two pivotedarms have their forward ends within the wheel circumference, which in turn means that they have to becurved outward for tyre clearance on full steering lock (to one side), the steering drag link must also bewidely spaced for the same reason. So, even though it would be hard to design a more efficient upright, thepivoted arms have somewhat less than the optimum shape. A fact aggravated by the need to carry thesuspension forces through one of them. This introduces torsional and vertical bending loads into that arm,which must then be large enough to handle them.

( 1997 addition ) Later versions of the Elf. went away from this design completely and used a systemsomewhat akin to a car style "McPhearson strut". Despite the fact that in the hands of Ron Haslam thissetup began to do well, I think that the original system had more promise. Measuring steering and linkgeometry from photos I am of the opinion that it was in this area that a rethink was needed.

HOSSACK / FIOR / FOALE Hossack photo

This is the antithesis of the above, within the double link family. Instead of a very small upright with largecurved arms, this setup has a larger upright which is located by very light and stiff wishbones. These aremounted above the tyre. Although the upright is constructed in the form of a fork, which allows for the useof normal wheels and brakes, there is no reason in principle why this could not be a single-sidedcomponent, to give the quick wheel changing of the Elf. e. But the stiffness to weight ratio is probablybetter with the fork.

Compared to the Elf. this design has both advantages and disadvantages. The steering joints are mountedhigher , the increased leverage causes greater loads in the spherical pivot bearings, and the potential forusing the engine structurally as the main frame (exploited by the Elf. ) is reduced, simply because of therequired physical location of the pivots.

In common with the hub centre schemes the Elf. has limited steering lock, but on the other hand theHossack type is free from that constraint and as such is the only system, described here, that has any realpotential for off-road use.

( 1997 note ) Ollie McKan in America developed a system for off-road use, this seems to use a suspendedheadstock, allowing the main suspension loads to be fed directly into taper roller bearings. This type ofdesign has the added advantage that steering friction is reduced compared to steering through a spericalbearing subjected to suspension loads.

The New Zealand based Britten racing team has achieved considerable success with bikes fitted with thistype of front-end. The main upright / fork is made from a carbon fibre composite as are many othercomponents on the machine.

FOALE / PARKER photo1 photo2 photo3

Perhaps I should not comment on my own design, but I will try to be objective. This combines features ofboth the previous systems. The lower arm is curved and at hub height, like the Elf. , but the upper one is awishbone above the tyre, like the Hossack. WHY? Well, by increasing the distance between the upper andlower arms (compared to the Elf.), the loads fed into the pivot bearing are considerably reduced. Thesuspension unit can now be fitted to the top arm near the upright, thus eliminating torsional and verticalbending loads in the lower curved arm. This component can now be made thinner and lighter. The pricepaid for this, is that the upright must be longer, and hence heavier and/or less stiff. On balance it was myopinion that it was a better compromise, as it also allows the steering drag link to be above the wheelwhere it is closer to the handle-bars, thus simplifying linkages, and need not be mounted wide to clear thewheel on full lock. The Parker system differs from mine in that the steering mechanism does not employ adrag link, instead it uses a kind of telescoping torsion tube to pass the rider's messages on to the wheel.


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( 1997 note ) With the help of Parker this is the system adopted by Yamaha for use on their road bike.

SAXON / MOTODD photo

To close I would like to take a quick look at the Saxon / Motodd system. This has been left out of theprevious discussion, solely because it does not fit into either of the two categories considered, using justone pivoted link and a fixed pivot at the top(it is really an improved form of telescopic fork). Because onepivoted link is insufficient to fully locate the fork, this is further supported by two stantions, the fork beingfree to slide on these as with normal teles. Although the suspension function is handled externally by aseparate spring/damper unit. The steering axis is defined by the line drawn through the upper fixed pivotand the lower moving one, which is attached to the fork, thus the length unsupported is kept to a minimum.In some ways this design is an adaption of a "McPhearson strut" to bike use, although the overhang fromthe wishbone to the wheel is much greater than is usual in the car version.

I believe that one of the main design criteria, was that it must look similar to telescopic forks, in the belief(rightly or wrongly) that the market would respond better to such a design. Viewed in that light, I see it as aconsiderable potential improvement over normal forks, though not as great as that offered by the otherdesigns.

( 1997 note ) This is the basic system chosen by BMW for some of their current models.

SUMMARY

I have shown that the location of the wheel relative to the steering axis is not dependent on the stiffness ofthe pivoted arms, for either the hub centre variety or the double link type. But the rigidity of these arms isstill very important, because any flex allows the steering axis to move out of line compared to the rest ofthe bike, including the rear wheel. To put it another way the REAR wheel can move relative to the steeringaxis. This is less important than the front doing likewise, but is still to be avoided. However, thesealternative steering systems may in practice, also locate the rear wheel in a more positive manner. Thelower mounting points (compared to a normal headstock), for the connection of the front setup, are closerto the rear swing arm and hence the chassis may be stiffer, but in addition the loads fed in are smaller, thusfurther reducing the amount of flex.

In this article we have really only considered the stiffness characteristics of these newish suspensionsystems, there are other aspects to look at as well. Next month we can look at the anti-dive properties andhow that works. Steering geometry is also important and in a later article I'll describe experiments that Imade to evaluate various possibilities in this regard.


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Tony Foale Designs - Vyrus Suspension History

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