Longitudinal motorcycle engine - torque effects 1

[amorrison]

Lateral engine torque reactions are produced at the wheels.
Where are the reactions for a longitudinal engine ?
Rider sidewise lean?

 

[imcjoek]

Draw a FBD

 

[BrianPetersen]

In steady state conditions, the torque generated at the engine/transmission is opposed at the 90-degree gear drive in the final drive.

During acceleration conditions, the rider has to oppose the torque that it takes to accelerate the flywheel and crankshaft and everything else. When riding, the rider does have to lean slightly, although the effect is so small when riding that it's hardly noticeable.

Go to a BMW dealer, test-ride an R1200GS. Or even test-sit with the engine running. Give the throttle a big handful in neutral and tell us what you feel.

 

[Warpspeed]

Don't know about motorcycle engines, but I am told that when you blip a longitudinal V8 car engine in a bike, the block tries to rotate around the crank and flywheel mass. That could get interesting.........

 

[BrianPetersen]

The situation is exactly the same. 'Course, the BMW bike engines have nowhere near as much inertia, so the magnitude of the torque reaction is a lot smaller.

 

[MikeHalloran]

The torque reaction is not _that_ small.
You can see it while driving behind a BMW or MotoGuzzi or GoldWing or whatever.
Whenever the rider grabs a good handful of throttle, or closes the throttle quickly, the whole bike rolls around a longitudinal axis.
I assume the riders learn to ignore it.






Mike Halloran
Pembroke Pines, FL, USA

 

[thruthefence]

My "classic" 1275S Mini Cooper would change lanes under power
Kinda fun after you got used to it.

 

[pontiacjack]

I rode a new Moto Guzzi in 1967, a 700cc touring bike. Having gear drive in place of a chain was nice, but the torque reaction to angular acceleration of (mainly) the flywheel/clutch was annoying. I noticed that during every spirited up- or down-shift, the resulting track of the bike had a little 'S' curve in it. So, for about forty years, I put up with chain drive. Then, a few years ago I sat on a Guzzi V50III (500cc, 49 HP) and observed only slight reaction to engine acceleration in neutral. The V50 apparently has a flywheel/clutch with a severely-reduced polar moment of inertia. I've been quite happy riding it for many thousands of miles.

Incidentally, the mention in an above post of the torque reaction to the driveshaft torque has no bearing on this discussion, as it is totally resolved within the machine.

For whatever it's worth: to experience practically zero torque reaction with a longitudinal engine, find yourself an antique (fifties and earlier) BMW R25- traditional boxer layout with a 250cc single! [I borrowed one to ride for my European bike license test (a LONG time ago)- figure-eights were a piece of cake!]

 

[pontiacjack]

Yeah, okay... it's difficult for a single cylinder to be a "boxer". But ya' all know what I meant...

 

[Tmoose]

At low rpm and heavy throttle the torque variation from the individual firing strokes are very noticeable on an R60/7 BMW

 

[evelrod]

I rode a Goldwing for years. Honda has the torque reaction tuned out to a fair degree, mtr mounts, whatever. Correct in that after you ride for a while, engine tq is just not noticeable to any drastic degree, very low CG for a bike. I guess you just get used to it. The most difficult thing for me was adjusting to the width of the Honda.
BMW, very noticeable and I dislike the exhaust note.
Moto G...not too bad, I disliked the 'linked' braking system.

I race a 150hp Mini...You have no idea what 'torque steer' is until you put your foot down hard in a tight turn with limited slip diff...

Rod

 

[HRHtex]

A 150hp Mini might have a mind of its own but a normal road Mini should not change lanes etc. - possible cause is steering or suspension ball wear.

 

[Warpspeed]

I had a small Japanese FWD hatch with a twincharged engine.
Under acceleration it would rock from side to side, and weave under acceleration, classic torque steer.

Converting the same car to four wheel drive with a 60R/40F torque split fixed the problem.

Bikes are fun, but rain, not being able to carry anything bulky, and seeing the scenery upside down a few times converted me to four wheels.

 

[GregLocock]

If you want to see the same effect in a car just sit in a softly sprung V8 at idle in neutral and blip the throttle.

Cheers

Greg Locock


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[HRHtex]

The usual reason given to explain torque steering (in a FWD car) is that the drive shafts are of unequal lengths. This may be correct but I have to say that I cannot see why this would effect the torque applied to each wheel. Generally speaking, the length of a shaft does not effect the amount of torque transmitted.

One thing I do know (and have had experience of) is changing the scrub radius of a tyre - this certainly will change the torque steer effects.

 

[BrianPetersen]

Torque reactions in the steering of front-drive cars are obviously way off the original topic, but anyhow ...

The trouble with unequal half-shafts is that the centerline of the diff is not aligned with the centerline of the hubs - partly because ride height is never exact and partly by design, because universal joints don't like running absolutely straight - usually the diff is a little forward or aft of the hub centerline and a little higher than nominal ride height.

If the halfshafts are unequal lengths then the angle of the halfshafts is also unequal.

A universal joint that is not straight, exerts forces on its mountings.

Since the steering axis is not absolutely perpendicular to the drive shafts (it is always inclined inwards and there is always positive caster) there is a component of the reaction forces around the outer CV joint that is aligned with the steering, and because the angles are unequal left to right, it's different between left and right.

If the steering is not aimed straight ahead, then that introduces a whole bunch more reasons for torque reactions.

Regarding U-joints not wanting to run absolutely straight ... even in rear-drive cars with prop shafts, some measures are generally taken to ensure that the drive shaft is never completely straight. Sometimes the entire driveline is shifted slightly to one side (Mopars). Sometimes the input shaft to the axle is offset slightly to one side.

 

[Warpspeed]

Te effects of torque steer I was seeing on my own car were most likely due to positive caster and large scrub radius.

As we all know, positive caster adds stability in a straight line, and even more so during heavy braking. But positive caster also has the exact opposite effect when tractive effort is applied at the front wheels. The contact patch being behind the steering axis. If there is also a significant scrub radius (either way), this can lead to trouble if the front wheel wights change from side to side.

In low powered FWD cars it is not that serious, but with a lot of power, the tractive forces are much greater.

What I was getting was the car started to gently rock gently from side to side under acceleration, creating a slow cyclic body roll. The weight being transferred from side to side at the front, causing the car to weave.
The front was also much raised in height during this acceleration.

This weaving and rocking would be self reinforcing, gaining in amplitude. that, and the very light steering, becoming quite disconcerting.
Trying to correct for it only made it far worse.
Best to just hang on grimly and back off a bit....

I believe the cause was the combination of caster and excessive scrub radius, the lateral weight transfer and tractive effort combining to self steer the front wheels. This steering causing more body roll, which created more self steering.

The drive shafts on this particular car are of identical length, but I have never understood the often quoted theory that unequal driveshaft lengths can somehow steer the car.

I am working on the design of a four wheel drive hot rod right now, and plan to run zero front scrub, and only about three degrees of caster.
All quite interesting.

 

[MikeHalloran]

Weaving and rocking like that happens in RWDs with Posi.
I have the impression that Torsens are less quirky, or at least don't have that quirk.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

Thanks warpspeed for the info.

I only ever did suspension mods on RWD cars before and developed a taste for lots of castor. I had not considered the issue of lots of castor on FWD.

I am always inclined toward pretty much neutral or slightly negative scrub after having to wrestle with kick back on Toranas with 7" rim widths.

Regards
Pat
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[Warpspeed]

Pat, I much prefer rear wheel drive, but these days most smaller cars do not offer much choice. FWD works pretty well, until you start to push the limits.