Pivot position

[jato]

Brushing up on the detail of swingarm pivot position and found an article with this in it in relation to accelerating out of a turn " a certain amount of squat loads the rear suspension and improves traction, too much can unload the front wheel " Bollocks i reckon... the weight transfer from accelerating increases traction, and squat will decrease that transfer. (totally mindful that too much anti-squat geometry will over-rule your nicely set up rear shock). as for unloading the front bollocks again as a lower (lots of squat) bike wont transfer as much weight off the front. (again totally mindful that lots of squat will play havoc with the geometry of the front also).
The concern is the author of the article has misunderstood physics so anything else in the article could be worthless.
Has anyone found a good useable range of the pivot in relation to countershaft and rear axle? I've been measuring lots of sportbikes but keen to hear real world observations

[Grumph]

If I was to build a frame for a high power engine now, I'd make the pivot adjustable. Not much, maybe 10mm above and below the point where the sprocket centers and pivot pin are in a line with rider aboard.
Mike Sinclair in his book says that once they had a frame with an adjustable pivot, they started to learn a lot. It was a Roberts team initiative, not Yamaha.

[F5 Dave]

You need to find a copy of Bradley's the racing motorcycle I'd say. Sadly I've read it but it was borrowed 2 decades ago and I recall not much useful.

[jato]

Tony Foale's book would be nice but so far a used copy in spanish at 179 usd or 450 usd! for a new english one...

[Grumph]

You need to find a copy of Bradley's the racing motorcycle I'd say. Sadly I've read it but it was borrowed 2 decades ago and I recall not much useful.

Volume one is the one with the geometry basics. I don't have it.

I thought all Foale's stuff was available on line ? Husa, your cue.

[jato]

I've found Foale's E book for 49 euro - still a bit eyewatering. I can copy zxr/gsxr geometry and give thought to an adjustable pivot, although that may conflict with the proposed construction (thinwall tube trellis) .

[F5 Dave]

Golly, I've only found vol 1 on Ebay circa $600.

I'll ask Simon if he still has his copy.

[Grumph]

I've found Foale's E book for 49 euro - still a bit eyewatering. I can copy zxr/gsxr geometry and give thought to an adjustable pivot, although that may conflict with the proposed construction (thinwall tube trellis) .

Presumably you'd locate the pivot in machined housings with the frame tubes welded to them. Round housings are the easiest - but if you can CNC square ones, or rectangular - replaceable inserts become very easy.

[jato]

Yes - a pair of round bosses with all the desired gussets etc . rectangular pockets are an option but rigidity with lightweight will need some thought. Foale's E book coverts to just under 100 nz so not too bad in the scheme of things...

[husaberg]

https://www.cycleworld.com/how-does-suspension-squat-affect-motorcycle-handling/

https://www.suspact.com/Anti_Squat.html

https://suspensionsecrets.co.uk/anti-squat-geometry/

https://www.datamc.org/downloads/motorcycle-anti-squat-calculator/


https://www.youtube.com/watch?v=-Xf-bhkdwQk

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

https://www.youtube.com/watch?v=cC_kArG3Hy0

you can affect it with different sprockets as well.
I am pretty sure i posted something about it from performance bikes back in the 1990's with the ZXR900.

[F5 Dave]

Actually that zero one co have a few videos on wheelbase, trail etc as well. It's hard not to sound OnceOverLightly but they did a sound job of not being confusing.

https://www.zerooneracing.com/

Hmm, 750 GBP for the software 1 year licence. Time to start a give a little page for jato.

[jato]

Looks like the Foale e book will be worth getting. no charity needed ( cow #118 has been on thin ice lately - i can turn her into software if necessary). i see someone once said lots of squat during acceleration takes weight off the front and now everyone is saying that. a bike exiting a corner with a lot of squat will have much more rake and trail (and handle accordingly) but physics reliably tells us there will be less weight transfer on a lowered bike.

[Pursang]

Looks like the Foale e book will be worth getting. no charity needed ( cow #118 has been on thin ice lately - i can turn her into software if necessary). i see someone once said lots of squat during acceleration takes weight off the front and now everyone is saying that. a bike exiting a corner with a lot of squat will have much more rake and trail (and handle accordingly) but physics reliably tells us there will be less weight transfer on a lowered bike.

A squatting bike isn't 'lowered' until the squat has already occurred.

Squatting under acceleration looks like weight transfer to the rear but, in the first instance, it is just the engine pulling the chain against the rear tyre traction. The rear axle pushes the swingarm. The swingarm pushes the bike through the swingarm pivot. In squat, the swingarm is pushing forward and down by rotating over the pivot. This compresses the rear spring(s) and lifts the rear wheel off the track (up into the bike) also reducing rear suspension travel & rebound responsiveness. The geometry of the bike changes (more rake & trail). All contributing to Understeer out of corners. Any loss of rear wheel traction allows the rear shock to extend again, pushing wheel down and possibly finding traction again. (Pogoing can be the result)

Anti squat, under acceleration, rotates the swingarm under the pivot, lifting the rear of the frame and forcing the rear wheel down onto the road, extending rear spring, stiffening compression and reducing rebound. Rake and trail are reduced (All contributing to Oversteer)

There will be some weight transfer due to the line of force through the pivot and it's relationship to the centre of mass of bike and rider. Within the normal range of bike/rider combinations these variations will not be the most significant effects (though, ultimately, if one wheel is off the ground All the weight is on the other) These should be managed by the rider through body position and throttle.
Squat induced handling is much more about geometry, suspension and force vectors.

Swing arm pivot point is best located for it's relationship to the engine sprocket (as close as possible) and to allow chain clearance.

Shock lengths are the easier ways to adjust swingarm angle and squat!

[Pursang]

I did this illustration many years ago for a flat track website.
It reappears all over the web.
https://www.apriliaforum.com/forums/attachment.php?attachmentid=258912&d=1406808987

[Grumph]

The direction of crank rotation has a bearing on this too. Forward rotating cranks transfer weight rearwards nicely for drive. Backwards rotating cranks don't.
Surtees reckoned bikes with backward rotating cranks should be as short in the wheelbase as possible.
I'd go along with this plus I've found running quite a lot of swingarm droop gives back the lost drive.

[jato]

Crank rotation...i've done a few experiments and what i found is gyroscopic precession of a forward spinning crank will help you save a front end slide , but not an off throttle back end slide.
a fairly heavy crank like on a multi twostroke that is forward spinning will transfer quite a bit of weight (and therefore traction) to the rear if it starts to spin but a backward spinning crank removes traction when you need it the most. Motogp is a different kettle of fish with there rearward spinning cranks for a few reasons -imagine you are designing one of those engines, you'll need an irregular firing interval for traction/feel so you'll need a balance shaft... to package it nice and compact the thing to do would be to run the crank backwards so the balance shaft runs forwards and the use the balance shaft to drive the clutch - i believe this is why those engines run backwards.not to mention fourstroke cranks are puny and with their irregular firing interval the (forward spinning) balance shaft will be negating a good percentage of the cranks negative effects. and of course the motogp bikes are electronically controlled so much that a computer will chip in to counter wheelspin (within limits i guess as occasionally someone manages a highside) if anyone knows a recent or current motogp engineer that is no-longer needing to be coy on technical detail it would be nice to have clarification...