Peter Williams - Fork Yokes

I was going through an old magazine yesterday and read this comment by Peter Williams :

" A racer's 'set-up' is different to a road bike, but I am embarrassed to say it was the experience of the new Commandos, which inadvertently taught me this.
Convinced that the steering geometry of the Commando Production Racer could be improved, John McLaren and I made two sets of forks, carefully assembled so that they were identical, except that one set had different yokes to give a reduced steering trail. We swapped the forks three times in a day's testing at Thruxton to provide back-to-back evaluation. My lap times were consistently one second faster with the modified fork. Times were similarly better on each short circuit, but at the Isle of Man it was half a minute.
When the works testers tried the modified geometry they all a greed that the handling improved, so I suggested to the factory that the production Nortons would benefit.
Not long after the Commandos went out of the showroom, reports rolled in from customers who had suffered 'tank-slappers' and crashed. It transpired that the incidents were all in wet weather, all after running over cat's eyes and all riders were relatively inexperienced. Good riders don't 'hang on' when the handle bars twitch, but with standard bars rather than clip-ons, a little twitch feels nasty.
The inexperienced riders were fighting the handlebars, which forced the front tyre to let go on the wet surface.'

Q.E.D. ? And sloppy isolastics would not help.

Be interesting to know the details of this new setup then, wouldn't it.
Do racers like the steering more stable for high speeds, or more nimble ??
Peter Williams in particular...

"experienced riders don't hang onto the bars".
Hmmmmm, can't wait for the next tank slapper then.

I would suggest Peter made two mistakes here.

The first, as he states, is not realising the difference between road bike and race bike use. Even a good and experienced rider cannot possibly push a road bike, on the road, to the same levels as a good racer can on a race track. The kind of difference Peter made with the yokes clearly made a big difference 'at the level he was pushing'. I would imagine those same differences would make a much less pronounced difference (perhaps none) in more 'normal' road use and speeds.

The second mistake is failing to recognise just how dammed good he was as a rider, and how massively different someone like him is to mere mortals like the rest of us. Its like he Princess and the pea story, the rest of us simply wouldn't notice the pea under the mattress, Peter is like a Princess, to whom the pea has a massive effect!

I believe that most road riders would just not notice the difference in what Peter is taking about. And most of the rest, who did notice it, would simply 'ride around' any difference without it having any impact on performance.

If I'm right, then the benefit to Peters 'racer yokes' on he road is naught.

But the potential downside, tank snappers etc, are huge.

So in this context, Norton definitely did the right thing in changing the yokes to a less radical set up.

All only IMHO of course.

I don't believe all of this superb rider stuff. If the bike is set up correctly, the handling inspires confidence and there is no need to 'push'. It all just happens. Bevel Ducatis used to have very stable steering geometry which gave them an advantage on some circuits such as Imola where it was possible to ride around the outside of the opposition even though the surface might have been bumpy. With the geometry that PW was using, it is normal to turn UNDER the opposition and get on the gas much earlier coming out of corners. I'm not kidding when I say that the TZ350 fork yokes which I fitted to my Seeley, made a MAJOR BENEFICIAL CHANGE to the handling. I don't usually shoot my mouth off about being a good rider, because definitely in this case - it is the bike doing the work.
As far as tank-slappers are concerned, they are nothing to be afraid of. For years I used a 7R front brake which was set up to be one-finger operation. All I had to do was lose concentration and lock the front wheel and I'd get the business. All you do is open your hands and if the tank-slapper becomes lock to lock, grab the tank until it straightens itself out. Then carefully grab the bars again. When it happens to you the first time, it scares the shit out of you. After that, it is not a problem.
Rohan, it is a matter of 'horses for courses'. A quick handling bike requires 10/10ths concentration, so might be a pain on a big fast circuit. I only use my Seeley on short fast circuits and in that environment, it is really great. A slow handling bike such as a bevel Ducati would be at a disadvantage. I have a friend who has an 860 , to which he has fitted short offset fork yokes. It is now much better around Winton Raceway, however the rear of the front wheel is too close to the front cylinder head.
With the Commando, it would be safer to have it slow handling, mainly because of the isolastics. However it would be much more fun if it was quick steering.
I've had this magazine article for a long time, probably before I ever sorted the handling of my own bike. However in reading it, I recognise what Peter Williams was doing to get his results.
It is interesting to note that Agostini fitted his own special fork yokes to the Evoluzione MV3 he was making and selling. If you've watched videos of him riding the MV3, he is either on the gas or off it - he never 'rolls' in corners. The quickest way round is when you use engine braking to just before the apex of the corner and then get straight back onto the gas while the bike is cranked right over, however with the right steering geometry, the angle of lean is not so great. It is a caster effect.

https://www.youtube.com/watch?v=V3BHLRQTBts

I once told an A-grade rider friend of mine about grabbing the tank when I get the tank-slapper. He said 'but then you are not in control'. That is not the fact of the matter. All you do is stay on the bike until it straightens itself out. How often have you seen a rider fall to the deck and his bike stays upright and motors on in a straight line, into a fence ? All you have to do is stay on-board and wait a bit before you try to get it back. If you grab the bars too early, you get launched. The other aspect is psychological - when this sort of shit happens, you get time dilation. There is usually plenty of time to regain control.
Don't ride without a steering damper. I had to laugh when Yves said he does not need one. - Not riding fast enough ?

acotrel said:

I don't believe all of this superb rider stuff. If the bike is set up correctly, the handling inspires confidence and there is no need to 'push'. It all just happens.

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Well, lets just agree to disagree right there.

MotoGP today is an example of the difference an excellent rider can make, the Honda at the start of the 2016 season was unrideable but Marquez was able to ride through it so he kept in the chase, he changed his previous 'win at all costs' to 'finish with the best points possible'. He rode so well that the season came back to him as the Honda was made ridable and he won the championship. He and the other 3 or 4 Aliens like Rossi and Lorenzo etc can still ride an unrideable bike and bring it home, the rest would fall off trying.

It would be interesting to see how much variation there is in rake and trail figures on bikes being pushed hard on a road surface like the IOM. I'm guessing the Dunlops, McGuiness's and other top rider's bikes are quite similar in rake and trail dimensions irrespective of make. The Classic TT might be a different subject though.

Fast Eddie said:

acotrel said:

I don't believe all of this superb rider stuff. If the bike is set up correctly, the handling inspires confidence and there is no need to 'push'. It all just happens.

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Well, lets just agree to disagree right there.

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As is most often the case, the truth is somewhere in between. Peter was an incredible rider, and very analytical, so of course I agree in part with what Fast Eddie is saying, and I know from experience that whatever you do on the road is not equivalent to what you do when racing.

In Peter's words he is describing incidents that occured to inexperienced riders, someone who may or may not become good, but certainly isn't at the moment.

And he is saying by contrast a better rider, lets just say here a more experienced rider, not a racing hero, learns that fighting the bike does not help, it generally makes things worse. Peter and his colleagues were instinctively riding based on knowledge and experienced they had gained, and it must be true to say that they would not react as the inexperienced rider would.

Peter is saying that the yoke modificaation they tested provided more responsive steering, from which an experienced rider could gain an advantage, and that that same responsive steering actually demands both more experience and a cool head to manage an untoward situation.

I have ridden and raced bikes that clearly don't handle well, none of them have thrown me to the floor. I have an opinion, no matter how wrong it might be, that, unless the machine has a sudden failure, no machine will just pitch you off, and that you should develop your knowledge of its own characteristics. It will depend on your ability, experience and knowledge just how quickly you can do the assessement.

Probably the worst handling bike I ever rode was a relatively new and unworn CB500T with huge silly handlebars, as fitted by Honda to this model. I test rode it on behalf of a customer at a shop I did some work for, after he complained of tank slappers at 90mph indicated. I confirmed it happened, but told him that is was possible to ride through it, if you relaxed and didn't fight it, however I suggested that he didn't; considering this was not only unpleasant but illegal, and since I had also confirmed that shutting off whilst going through the onset made it even worse, bottling out in the middle of was goiung to be the worst thing you could do.

I offered an opinion that smaller bars more suited to British roads and speeds might reduce the onset of the problem, but it would probably still happen, but now at a speed you had no power to drive through, my suggestion was stay under 85mph and stay safe.

I saw the bike about half an hour later with one bar end facing the floor, the other facing the sky.....and a few other marks having been hauled out of a local ditch.....

I am no riding god, but I probably had more experience that the owner, even at the tender age of around 22, and I knew not to fight the onset, and that maintaining power usually maintained stability, and I was able to manage the circumstance! I did feel some guilt that my comments obviously led him to try again, regardless of the fact I was trying to discourage him.

However, I also know that when you are tired, cold, or wet, or any combination of these, your ability to relax and let things resolve themselves diminishes severeley, even the best don't manage it all of the time.

kommando said:

MotoGP today is an example of the difference an excellent rider can make, the Honda at the start of the 2016 season was unrideable but Marquez was able to ride through it so he kept in the chase, he changed his previous 'win at all costs' to 'finish with the best points possible'. He rode so well that the season came back to him as the Honda was made ridable and he won the championship. He and the other 3 or 4 Aliens like Rossi and Lorenzo etc can still ride an unrideable bike and bring it home, the rest would fall off trying.

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Casey Stoner rode that camel of a Duc to a series of wins and a World Champeensheep.
After getting that camel of a Honda to a series of wins, and ditto.
- no other rider even got that Duc on the podium ?
Including Rossi, it must be noted..
Lets see what Lorenzo can do next/this season...

We diverge, but there is definitely talent, and then there is talent.
No other rider in history has ever got 2 different makes of camel into winning form ?
In subsequent years it must be noted.

I think some of us have a different definition of a tankslapper. On the few occasions that I've had a real tankslapper there's absolutely no chance of accelerating through it. The bars are thrashing so wildly all you can do is hold on and pray. The worst one I had sheared off both lockstops, and when I arrived at the next corner I had no brakes because the whole front end had been flexing so much the disc brake pads had been pushed back into the caliper.
If it's possible to turn the throttle, it's a speed wobble not a tank slapper.

I don't have any experience racing motorcycles, but I was member of a dirt race car pit crew. Setting up any vehicle is science, not some voodoo. There are reasons why certain proportions work and reasons why certain things don't work. There are also reasons why some riders/drivers can't get good results with a certain set up, and other riders/drivers can.

There were races where my crew chief would be pissed off after the 2nd or 3rd lap, because he could see the driver didn't trust the set up and would lift off the throttle way too early entering the corners. Sometimes after the first race of the night he would berate the driver saying,

"The car will make the corner IF you drive it in hard, stomp the brake momentarily to "set the car", then gun it to shift the car's weight on to the back wheels and drive, BUT You're too chicken to trust the car, so you're just tooling around out there, playing it safe. We're not here to play it safe"

It was a reminder that having a high performance vehicle, still requires good technique to achieve that higher performance.

Needless to say If you drive it into a corner hard, with poor technique you are into the wall. When that happened from time to time, the driver usually blamed the car's set up, and the crew chief usually blamed the driver... :lol: I have seen good drivers go in to corners way too fast and use the throttle to keep the car off the wall. It's incredible to see, because the instinct once you lose your line is hit the brakes, but that only guarantees you're hitting the wall albiet going slower.... Like I said, it's crazy to see a skilled driver save, what would be a certain crash for 99% of the field of drivers.

'Peter is saying that the yoke modificaation they tested provided more responsive steering, from which an experienced rider could gain an advantage, and that that same responsive steering actually demands both more experience and a cool head to manage an untoward situation.'

I would not call it 'more responsive steering', it is actually a self-steering effect. The bike tightens it's line if gassed when cranked over. I believe that today, many MotoGP bikes do it. The problem is that if the bike tightens it's line too much, you get the hi-side - especially with bikes that have extremely high power available to the rider. I cannot relate to bikes that hi-side even if you don't move the twist-grip. ( I don't think that would ever happen with a Commando - even in the wet unless it was covered in oil ).
The tank-slapper is probably best controlled by simply opening your hands and letting it subside on it's own until you can grab the bars again. However that said, the last time I had one it gave me a dislocated collar bone which could easily have been my neck.
I suggest that what Marquez does in MotoGP is not relevant to this conversation - race strategy is something else. What I have mentioned is the way that the set-up of some bikes allow themselves to be ridden extremely fast and even encourage and inspire it. I suggest the JPN Monocoque is probably one of those. I once rode a 1961 Manx Norton which was like that - felt so good that you just had to give it more stick.
Do bikes have 'personality' ?

Actually I suggest that what Peter Williams was doing in changing the fork yokes to give less trail can be very treacherous. Anyone trying it should expect and be prepared for the crash. I only achieved it successfully with my own bike by accident - definitely NOT by design.
TZ350 fork yokes with 27degree rake and 92mm of trail on a Seeley with 18 inch wheels actually works. If you gas it when it is cranked over, it will turn to where you want it to go. -- Arse beats class every time !

pommie john said:

I think some of us have a different definition of a tankslapper. On the few occasions that I've had a real tankslapper there's absolutely no chance of accelerating through it. The bars are thrashing so wildly all you can do is hold on and pray. The worst one I had sheared off both lockstops, and when I arrived at the next corner I had no brakes because the whole front end had been flexing so much the disc brake pads had been pushed back into the caliper.
If it's possible to turn the throttle, it's a speed wobble not a tank slapper.

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I agree with you big time here!

I've had many 'speed wobbles' as you put it, in race and road conditions, some were worrying, most were not, and I can relate to the theories of accelerating through, not gripping the bars, etc in this context quite easily.

But the only full on 'shock and awe' tank slapper I've had was many years ago on a GSXR 750 at something well over 130mph. It was a proper lock to lock slapper that bruised my inner knee's where they dented the tank!

It was so instnt, and so violent. I stayed on (and thus alive) by pure luck and sub conscious reaction. Skill and or theory didn't come into it at all!

Now, back to the rake / stability / agility topic... It seems from the examples given that a bike set up to turn quickly can (will?) therefore be unstable in some circumstances. It seems to me that this must be why most modern bikes have steering dampers; the rake gives the agility, the damper gives the stability.

Steve Harris told me several years ago that ALL motorcycles should have steering dampers. But some still believe the a 'correctly' designed chassis should not need a damper.

I think Steve was right in that the ideal set up for a motorcycle is one in which a damper is needed due to the geometry and the agility / stability mentioned above. As per modern sports bike norms.

Ok, so the truth may be that when I rode the CB500T in question I has a speed wobble and I rode it in such a way as to not turn the wobble into a tank slapper, the owner seems to have been less 'fortunate', so was it my style that got one result versus the other?, probably! It is hard to discuss powering through, I just had the throttle wide the first time I rode through it, I played with the throttle a bit after that, I didn't like it.

As for Gixers, I have had two. I had an occasional little wobble on the '96 SRAD, with Pirelli supercorsas, mainly with the front wheel off the tarmac exiting fast sweeps at Silverstone, but no tank slappers, used hard on track days with no steering damper fitted....These bikes bikes defined 'modern geometry'. Of course early in my ownership I had the forks and shock worked over by Maxton, may have helped!

The '85 Slab side was raced with no steering damper, and really no speed wobbles.....even if I had to do work to the bike to improve its set up (a lot) and therefore my corner exit speeds, I always felt able to control it....

What am I doing wrong guys?

I need to elaborate a bit so as not to inadvertently blame Suzuki here...

I had a GSXR750J when they first came out (the Slingshot model).

I was 21. King of the road. Unbeatable. Immortal. Etc.

Performance Bikes had an article written by Mick Grant on how to improve the forks, I think it involved dropping the forks through the yokes, altering all the settings and filling with a different amount of different grade fork oil.

Now, on these bikes there was no fork drain plug. Draining the oil meant removing the forks etc. as a 21 years old know-it-all in a hurry, this didn't happen.

So the forks were set up botched.

I had also 'improved' the rear shock settings (read botched).

And finally, I was on the road, not track, and hit a series of bad bumps on a very mild bend.

The bumps were kinda tame at legal speed limits. As was the bend.

At more than double that limit, the bumps came fast BANG-BANG-BANG in such a way as to really screw up the already screwed up fork action.

My 'improvements' to the stance of the bike probably didn't help much either.

So... No fault of Suzuki at all. Just what I guess we'd call 'youthful exuberance'.

Or stupidity.

Fast Eddie said:

Now, on these bikes there was no fork drain plug. Draining the oil meant removing the forks etc. .

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It's not actually necessary to remove the forks. What you do is remove the fork top nuts, immerse the bike in a swimming pool and the old oil will float to the top of the pool, then all you do is wait for them to dry out and re fill with the new grade of oil.

Yoke differences aside, going back to the original article, the weight distribution and aerodynamic differences between a PR and, say, a Roadster would arguably be far more significant, as would tyre choice and lack of white lines, tar overbanding and cat's eyes (road studs).

It's already been commented that high wide handlebars *could be* a factor, and when I got the T140's export bars swapped for flat bars and rearsets, the difference in the bike's handling was incredible - like the front end was suddenly connected to the road, and not skipping over it. I'm also very tall and believe the reduced sail effect was probably not insignificant...

Steve Harris told me several years ago that ALL motorcycles should have steering dampers. But some still believe the a 'correctly' designed chassis should not need a damper.

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have had a steering damper in place for some 30 years now as preventative medicine

don't know about all bikes but especially commandos