[Technique] Frits Overmars : IUWA comments to Eric O.

Regarding IUWA: "I must admit that I still have not read everything you wrote about in in the forum.
If I understand it correctly, it is about the variations in angular movement of the tire, caused by suspension movements, that are supplemented upon the nominal rotation speed. I can see how these jerks can disrupt the tire grip.

As I wrote before, I do not see many problems at the front because the front wheel speed is always equal to the riding velocity; angular movements of the front brake caliper in relation to the wheel will only cause a variation in the relative velocity of brake pads and brake disc, but this will not result in brake torque variations, so I think there will be no jerks at the tire contact patch.

At the rear the chain ‘locks’ the wheel to the engine; variations in wheel rotation speed require that the crankshaft is accelerated/decelerated, and this certainly will cause variations in rear wheel torque and resultant jerks at the contact patch.

A long time ago (yes, that phrase sneaks into my stories more and more) I modified a bike with idle rollers above and below the swing-arm, so that the upper and lower chain runs were parallel. The only effect I could establish was that the rear suspension was less influenced by acceleration. I could not detect any effect on tire grip, but that may have been simply because I was not riding fast enough.

I understand you have been concentrating on designing a rear suspension geometry that eliminates IUWA. But would it not be possible to avoid a large part of IUWA by just using a good transmission shock absorber between engine and rear wheel? Such an absorber could accept up to 15° angular movement, which would considerably weaken the jerks. What do you think about this?"

TRADUCTION
Concernant les IUWA: Je dois admettre que je n'ai pas lu tout ce que tu as écrit sur le sujet, dans le forum. Si je comprend bien il s'agit des variations angulaires du pneu qui s'ajoutent à la rotation due a la vitesse nominale. Je peux voir de quelle façon cela peu casser l'adhérence du pneu.

Comme je l'ai dit je ne vois pas de problème à l'avant, la roue ayant toujours une vitesse proportionnelle à la vitesse, les mouvements angulaires causeront seulement une variation des vitesses relatives entre le disque et les plaquettes de frein qui ne causeront pas de variations de puissance de freinage, je pense que cela ne causera pas de secousses au niveau du contact pneu/sol.

Comme la chaine "lie" la roue au moteur, les variations de vitesse de rotation de la roue exige que le vilebrequin soit accéléré/dé-ccéléré, et cela causera des variations de couple et des secousses au niveau
du contact "pneu/sol".

il y a longtemps (oui, c'est une phrase qui se glisse de + en + souvent dans mes récits) j'ai modifié une moto avec des roulettes au dessus/en dessous de la chaine pour maintenir les brins //.  Le seul effet constaté est un moindre effet sur les suspensions, pas sur le grip, mais c'est peut-être que je ne conduisais pas assez vite.
J'ai compris, Eric, que tu t'es concentré sur des géométrie de suspensions éliminant les IUWA, mais ne serait-il pas possible d'en éliminer une bonne par en utilisant un bon amortisseur de transmission entre le moteur et la roue arrière, qui permettrait jusqu'à 15° de  mouvement angulaire, et diminuerait considérablement les secousses ? Que penses-tu de çà ?

If you permit me, I would make you observe that what an absorber can absorb in a direction have to be restituted in the other.
Doesn't it?

note: my knowledge concerning these phenomenons is clearly not the same as yours or Erics one.

Merci MANETON,
Je ne veux pas répondre directement à Frits (cela aurait l'air d'une conversation privée), mais tu viens de dire une chose avec laquelle je suis d'accord: Un amortisseur souple restitue la secousse dans un temps donné et ne peut donc être parfait que si les bosses ont une fréquence accordée avec la flexibilité de l'amortisseur.
D'ailleurs les roues de CBR sont dotées d'amortisseurs dans la roue, je crois qu'ils les gardent en moto2.
Autres chose que je pourrais ajouter sur le sujet "amortisseur", c'est que ma philosophie est plutôt de supprimer la raison d'une maladie (VIRUS), plutôt que de la soigner (ANALGÉSIQUES).

Bon, je vais faire la traduction d'un très bon texte de FRITS sur le comportement d'une moto: "RIDE".

MANETON a écrit:

If you permit me, I would make you observe that what an absorber can absorb in a direction have to be restituted in the other.
Doesn't it?
note: my knowledge concerning these phenomenons is clearly not the same as your or Eric's one.

My knowledge concerning IUWA is not so impressive either, Maneton. I had never heard of the phenomenon before I read Erics comments.
What an appropriate schock absorber should do, is to store the energy of the shock and then return it at a time when the system is more relaxed and to spread this shock over as much time as possible so the peak value will be considerably lower.

A perfect shock absorber should not have any resonance frequency, but nothing is perfect. If the shocks occur with this specific frequency, the effect may be quite unpleasant. I have seen something like this happen with the shock absorber beween an engine and the flywheel on a dynamic test bench. And if there are two shock absorbers in series, the effect can be even worse.
So back to the drawing board....

Tu as raison FRITS, un amortisseur de transmission est préférable à RIEN du tout, je ne conteste pas çà, bien sûr.
J'ai d'autres réponses à te faire concernant les IUWA avant, mais il serait bon que tu essaies de lire quelques uns des Topics sur le sujet. pour que je n'ai pas à tout ré-écrire sur le sujet. lol

As a matter of fact the "Linkage to the ground" (IUWA) is a science used in all Car R&D depts. by which you can evaluate most suspension reactions.

It's quite normal when you come to think about it: All forces being generated there (On the tire contact patch with the ground !!!

I feel quite embarrassed that I still have not read all the IUWA-information. I will do so ASAP, Eric, but it may be a while; now I am off to Germany, to continue the work at my FOS-scavenging system.

Frits Overmars a écrit:

now I am off to Germany, to continue the work at my FOS-scavenging system.

Eric Offenstadt a écrit:

Tu as raison FRITS, un amortisseur de transmission est préférable à RIEN du tout, je ne conteste pas çà, bien sûr.
J'ai d'autres réponses à te faire concernant les IUWA avant, mais il serait bon que tu essaies de lire quelques uns des Topics sur le sujet. pour que je n'ai pas à tout ré-écrire sur le sujet. lol

As a matter of fact the "Linkage to the ground" (IUWA) is a science used in all Car  R&D depts. by which you can evaluate most suspension reactions.

It's quite normal when you come to think about it:  All forces being generated there (On the tire contact patch with the ground !!!  

Hello Eric, and all happily reading + discussing.
I have been reviewing your beautiful GECO project and I can easily see what your suspension aims are concerning IUWA and the de-coupling of braking forces.
I am however struggling to find any detail on how and where the front shock absorbers themselves are mounted and actuated.
I also note that on the Geco website you describe the "current Britten/Fior/BMW Duo-Lever". Well John Britten made many trips to a friends house to study the girder forks of his Vincents and take measurements.
This is a very old post, but I thought you might find it interesting in relation to this topic :Innovative suspension
I haven't really seen anything as compelling in this area since the Tony Foale era of experiments, and so I wish you the best of luck in this endeavour.

nick gill a écrit:

.... I thought you might find it interesting in relation to this topic :Innovative suspension

Hi Nick, welcome to Pit-Lane.
Using the torque of a floating rear brake caliper to compress the rear suspension is a remarkable approach, but I don't think it is quite as innovative as the link title says. The 50 and 125 cc Garelli GP-bikes used a similar system in 1982.
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A la limite, on pourrait dire que toute moto qui attache le frein arrière au bras oscillant fasse pareil.

GrahamB a écrit:

A la limite, on pourrait dire que toute moto qui attache le frein arrière au bras oscillant fasse pareil.

Pareil, yes, but to what extent? The Garelli's rear brake torque arm is connected via a vertical rod to the rear subframe, just ahead of the upper shock mount. That is about the strongest influence you can get, apart from playing with the torque arm length. The picture below of the Garelli-50 may give a clearer view of the rod.
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The forces moment are more effective on the frame with the Garelli; all others "common" bikes have no such effect with the rear brake. I think it's a good way to stabilise the rear chattering when the tire spins and rebunds while hard front braking

Frits Overmars a écrit:

The Garelli's rear brake torque arm is connected via a vertical rod to the rear subframe, just ahead of the upper shock mount.

Could be a cable...

A la limite :-)
Yes, there is the question of leverage, but I find it amusing that we've gone from the budget, accidental pull-down design, to the full-floating neutral design, then the magnified pull-down and the concencus seems to have come back to "just bolt it to the swingarm".
There has to be an important issue of mechanical stability: a pull down system potentially increases braking potential prior to the rear lifting, but ceases to act abruptly at the point where the rear tyre loses traction... at which point the rider will need to modulate the front brake to avoid going over the front.

Even if the rider was able to balance on the critical point, would lowering the rear by a few cm compensate the need to maintain some load on the rear tyre? My suspicion is that it doesn't, so that in any case optimum breaking is achieved at the point where the rear tyre is just skimming the track... so that the rear brake is irrelevant for straight-line braking.

jmdonnat a écrit:

Frits Overmars a écrit:

The Garelli's rear brake torque arm is connected via a vertical rod to the rear subframe, just ahead of the upper shock mount.

Could be a cable...

As long as you don't brake rolling backwards....

GrahamB a écrit:

.... the rider will need to modulate the front brake to avoid going over the front.... optimum breaking is achieved at the point where the rear tyre is just skimming the track... so that the rear brake is irrelevant for straight-line braking.

I'm with you all the way, Graham. Keeping the rear low will allow a bit more deceleration before the rider goes over the handlebars. But keeping the rear low should not be induced by the rear brake, but by the front brake. I'm working at it....

It's logical, the lower the weight, the higher capability to front brake hard before starting a stoppie, no need to let down the leg (last fashion riding) to help this phase .

By the way , the big twins made this kind of reaction when throttleling off, the chain acts like the rear brake and this linkage is at the lower side of the swing arm... this is not helping in the corners , but the forces are different there, the rear arm angle is small enought to oppose a reaction, it 's another compromise. 

Thanks Frits for the warm welcome and the juicy Garelli photo !
I agree with GrahamB that chasing this logic may not yeild faster results, but It could make something more marketable ! [controversial  ]

I messed around with a front fork of a vespa (a caliper hanger tethered to the shock absorber stub at the end of a trailing link) , and I allowed the caliper hanger to move independently of the trailing link and shock, and gave it an anti-dive rod linkage. I could lengthen and shorten this rod to affect the weight transfer but all modifications seemed to result in loss of feeling and/or front wheel lock. The best results for me were to have the suspension neutral ( link to rod ratio 1:1 ) without the influence of torque.

Sorry to bore you clever guys with my silly story !

Nick, as you may have noticed in the above pictures, the Garellis utilized both anti-dive front and pro-squat rear ends.
I still believe the pro-squat has a raison d'être (this is a french forum after all ) but the usefulness of anti-dive disappeared when the character of most circuits changed during the 80's. I wrote a little dissertation called RIDE about it at the time, and years later Eric Offenstadt urged me to post it on Pit-Lane. He even provided for a translation

Eric Offenstadt a écrit:

Bon, je vais faire la traduction d'un très bon texte de FRITS sur le comportement d'une moto: "RIDE".

I'd love to give you the link but I am terrible at finding things back at any forum so I will leave the searching to you.

Eric Offenstadt a écrit:

Bon, je vais faire la traduction d'un très bon texte de FRITS sur le comportement d'une moto: "RIDE".

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Thanks Frits and Eric! I enjoyed reading that.
I've had a little bit to do with a locally designed classic racer built in the 80's around a Suzuki GS1100. It has a lot of pro-squat in the geometry, and by todays standards a high C.O.G.
In its shakedown runs, the young rider wheelied all over the track and I wasn't sure if it was exuberance or mechanical or both. Come race day professional racer and IOMTT winner Cameron Donald wiped 3 seconds off the class lap record with the bike, and he wasn't battling the wheelie nearly as much. I believe he carried so much more corner speed that the bike was effectively no longer ever particularly upright on the short circuit and that eliminated the problem for us that day. Cameron was worth every penny too.

nick gill a écrit:

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Woo! Nice article Frits.

One aspect you didn't mention in your list of 3 reasons for soft suspension: the harder the suspension, the more energy will be stored in the tyres during a change of load (be it bump, braking, start of high-side etc). And tyres are rather poorly damped...

This subject has possibly veered away to a more general one on suspension geometry... but I rather like anarchy :)

GrahamB a écrit:

This subject has possibly veered away to a more general one on suspension geometry... but I rather like anarchy :)

Every topic that I get involved in seems to go astray. But then you often encounter the most interesting ideas whilst looking for something else.
And if you really want to blow the cobwebs off your mind, try comparing the input and output of Google Translate  .

Frits Overmars a écrit:

And if you really want to blow the cobwebs off your mind, try comparing the input and output of Google Translate  .

Funny you say that Frits. I feel like I'm following you around this forum a bit, but one reason is because you are the best excuse to communicate in english. I should have paid more attention in french lessons

 
Cameron seemed a nice guy too, from the little I spoke to him. In 97 I bought my first TZ from his big brother Darren, who was selling his 93 model to finance a shot at the Australian 250 title on a 96 model owned by Dietmar Schultz, a local tuner. While Darren and Dietmar ran out of money, Cameron was just starting to make a reputation in 600s.

To come vaguely back to the topic, Darren had an interesting set up on the 250. He believed the standard paddock logic that running the rear soft would increase traction via more weight transfer, so despite his 76kg he kept the standard rear spring. Then to try to stop the bike running wide in fast corners, he pulled the forks way up through the triples. The manual says 16mm max, he had them at 25.
First time I raced the bike I found out why the max was 16... when I ran off track and bottomed out in hole, the front guard smashed itself on the lower triple and the tyre hit the radiator.

I wonder if Darren might have done as well as Cameron if he'd had the same methodical approach...