How Suspension Reacts

[Hoon]

it doesn't matter whether its chain driven or shaft driven the transfer of energy has the same effect

No it doesn't. Chain pull is totally seperate force of its own and very different to a rotating driveshaft which is at a 90 degree plane.

Heres another explanation from one of my fav reference books Sportsbike Performance Handbook.

A Useful Semi-secret About Rear Suspension

Many riders and tuners don't know it, but when a chain-driven bike accelerates, chain forces try to extend its rear suspension. We are used to cars, which dip at the front under braking, and squat at the back under acceleration. We assume bikes squat the same way but they don't. The tension in the drive chain, acting at a slight angle to the plane of the swingarm, exerts a downward force that tends to extend the suspension. Chain force is greater in lower gears, and the lift it generates can equal or exceed the bike's normal tendency to squat under acceleration. Go and watch bikes coming off a first-gear turn, and see them lift or even top out their rear suspension as they hit second gear. Or watch a bike on a Dynojet dyno, rising at the rear as its power comes on.
The existence of this chain "tangent force" is the reason for all the current interest in adjustable swingarm pivot height for Superbikes. Chain lift force is determined by the angle between the taut upper chain run and the swingarm. The bigger the angle, the greater the lift force that will oppose squat. Raising the pivot height increases the lift force, and lowering it reduces it. Even changing rear sprockets can change this lift force somewhat, by changing the angle of the top run of the chain to the plane of the swingarm.

It then goes on about how to use this effect to go faster but you'll all have to buy the book if you want to know more!

[Brian d marge]

You can't assume that the sprocket is fixed to the swingarm - that allows torque to be transmitted into the swingarm which as you said is impossible.

Google makes things much easier to explain - no need to draw my own diagrams...
http://www.tootechracing.com/Engine%...Suspension.htm

Thats a good one ,,See that was exactly what I was going to say !!! NOT ,,I will sit down with that later ..and digest it .....
I know it happens its just my brain wont let me get it clear enough for a simpleton like me to explain it / and if I can explain it I understand it ,,,,

one hopes !!!

Stephen

[erik]

You can't assume that the sprocket is fixed to the swingarm - that allows torque to be transmitted into the swingarm which as you said is impossible.

Google makes things much easier to explain - no need to draw my own diagrams...
http://www.tootechracing.com/Engine%...Suspension.htm

The link doesn't work for me

Treating the sprocket as fixed to the swingarm is ok, as far as I can tell. All it does is stop you having to worry about what the road and wheel are doing.
When the wheel is turning, the tension in the chain has the same effect.
Say the rear sprocket was fixed to the swingarm, now because the chain passes slightly above the swingarm pivot, and because the front sprocket is in front of the pivot, if the swingarm rotates up, the distance between the top edge of the rear sprocket and the top edge of the front sprocket will reduce slightly, releasing the tension in the chain. This is what I mean about the chain tension making the swingarm want to rotate up.

A Useful Semi-secret About Rear Suspension

Many riders and tuners don't know it, but when a chain-driven bike accelerates, chain forces try to extend its rear suspension. We are used to cars, which dip at the front under braking, and squat at the back under acceleration. We assume bikes squat the same way but they don't. The tension in the drive chain, acting at a slight angle to the plane of the swingarm, exerts a downward force that tends to extend the suspension. Chain force is greater in lower gears, and the lift it generates can equal or exceed the bike's normal tendency to squat under acceleration. Go and watch bikes coming off a first-gear turn, and see them lift or even top out their rear suspension as they hit second gear. Or watch a bike on a Dynojet dyno, rising at the rear as its power comes on.
The existence of this chain "tangent force" is the reason for all the current interest in adjustable swingarm pivot height for Superbikes. Chain lift force is determined by the angle between the taut upper chain run and the swingarm. The bigger the angle, the greater the lift force that will oppose squat. Raising the pivot height increases the lift force, and lowering it reduces it. Even changing rear sprockets can change this lift force somewhat, by changing the angle of the top run of the chain to the plane of the swingarm.

If I may be so bold... I think he's got it wrong.

As I understand it:

The chain doesn't provide a lift force. The chain, like I described earlier, provides a torque that tends to rotate the swingarm up. The strength of the torque is determined by the tension in the chain and the distance the tight side of the chain passes above the swingarm pivot.
It is correct that raising the pivot point will make the rear of the bike stand up more on acceleration, but this is not because it increases the "lift force".
By raising the swingarm pivot, the distance between the tight side of the chain and the pivot is decreased, and this has the effect of reducing the turning moment that tries to rotate the swingarm up (turning moment is equal to the "chain tension" x "chain distance from pivot"). By reducing this turning moment, there is less resistance to the bike standing up on acceleration.
The reason why the bike stands up is because the innertia force resisting the acceleration passes below the swingarm pivot, hence the rear wheel tries to "walk under the bike". When the bike is on a dyno, the rear raises because the swingarm pivot is above the rear wheel axle, again making the rear wheel want to move under the bike.

Anyway, that's how I understand it.

I checked and it turns out they've got Sportbike Performance Handbook at the waitakere libraries, I'll have to get it out some time, it looks interesting.

[XP@]

Interesting question CaN

This guy has a few interesting ideas on the subject
http://www.tonyfoale.com/Articles/index.htm

From what I have been taught the rear does lift... why never been told that but how about this theory hot from the left field...

First pose the question what drives a bike? answer the back wheel.
So the back wheel is pushig the front. It is trying to catch up the front wheel, but is not succeeding because the frame is in the way. So something has to give, this is the swing arm.

If you go in to a corner with a steady throttle you will come out slower than when you entered the corner this is because friction has used mostly be the front wheel to help you turn and thus reduced your speed. To stop this happening we apply more throttle through and out of the corner, increasing the speed of the rear wheel. This makes the rear try and catch up the front and hey, presto the swingarm lifts.

I would suspect the best place to see this would be on a dirt bike setting off.

[Hoon]

The chain doesn't provide a lift force. The chain, like I described earlier, provides a torque that tends to rotate the swingarm up.

Are you assuming for a bike which has the rear sprocket higher than the front?? Then yes it would but sportsbikes rear sprockets (top edge) are lower than the front so it would pull down.

Treating the sprocket as fixed to the swingarm is ok, as far as I can tell.

This ignores another force of the chain pulling against the circumference of the rear sprocket. The chain has more contact with the upper half of the sprocket than it does with the lower half due to chain slack and centrifugal force (see pic). This net total results in another source of slight downward force.

The explanation in that book is by no means complete but it tells you all you really need to know in laymans terms i.e. what goes in (chain pull) and what comes out (downward force) without dwelling over what happens in the middle. The target reader is a rider/mechanic, not a physics student so has no real desire to analyse and breakdown each of the indivudual forces at work, only the net output.

[erik]

Are you assuming for a bike which has the rear sprocket higher than the front?? Then yes it would but sportsbikes rear sprockets (top edge) are lower than the front so it would pull down.

No, I'm not assuming the rear sprocket is higher than the front. The critical things that determine whether the swingarm tries to rotate up or down are the pulling force in the chain and the distance (measured at 90° to the chain) that the chain passes the swingarm pivot. Take a look at the attached drawing. In the top sketch, it's easy to see that the chain will pull the swingarm up. This is due to the turning moment about the swingarm pivot which is equal to the chain pull x radius R (measured tangentially from the chain to the pivot).
Next pic down, say the chain pull F is the same, but the radius is decreased. As the moment M = F x R, the moment is also smaller because R is smaller.
The third pic down is close to what a normal bike layout is. R is small, but still there, therefore there still exists a turning moment trying to rotate the swingarm up.

If the chain were to pass directly through the pivot point, R would be zero, and therefore the moment M would be zero.
The last pic shows that if the chain passed below the pivot, the moment would be reversed and try to turn the swingarm in the opposite direction.

This ignores another force of the chain pulling against the circumference of the rear sprocket. The chain has more contact with the upper half of the sprocket than it does with the lower half due to chain slack and centrifugal force (see pic). This net total results in another source of slight downward force.

I can't think how to prove it, but I'm pretty sure the only significant force the chain can apply to the sprocket is pulling on it tangentially (ie making the sprocket want to rotate about its centre). I might ask a lecturer about that.

The explanation in that book is by no means complete but it tells you all you really need to know in laymans terms i.e. what goes in (chain pull) and what comes out (downward force) without dwelling over what happens in the middle. The target reader is a rider/mechanic, not a physics student so has no real desire to analyse and breakdown each of the indivudual forces at work, only the net output.

I'll agree that he's got the relationship between accelerating and the rear of the bike raising correct, and also the effect of raising the swingarm pivot, that raising it will make the rear of the bike raise more on acceleration.

I guess that if you don't mind having an incorrect understanding of the mechanics behind it, then it's enough to just know that accelerating makes the rear stand up and that raising the pivot will increase this effect.

Still it would be better if he had the mechanics correct, even if he didn't go into detail proving them.

For me, I'm studying mechanical engineering and learning to work on my bike, so understanding this kind of stuff as well as I can is one of my goals in life.

[Hoon]

No, I'm not assuming the rear sprocket is higher than the front. The critical things that determine whether the swingarm tries to rotate up or down are the pulling force in the chain and the distance....

The reason I ask is that I'm not disputing your logic which is sound so I suspect we may be talking about two different models here? My interpretation of how a swingarm works is closer to your 4th pic where the chain is pulling at a more downward angle to that of the swingarm due to the offset of the pivot and front sprocket i.e. the centreline of the two sprockets is below the pivot of the swingarm. Does this change things in any way?

[TwoSeven]

I usually find that when there is a thing that can be observed/measured in engineering/physics there ends up being a sinle word or phrase thats used to describe the phenomina. In this case we are talking about suspension extension, so I guess there should be a unique engineering term to try and describe it.

From my perspective I use design to try and improve how to ride in that understanding how something works allows me to develop and test riding theories for improved performance. In this respect, understanding how the machines traction and steering geometry changes under acceleration allows me to use rebound and throttle control for example to control accerlation more precisely (starts and corner exits), as well as how to use the front setup in tandem with the rear to control corner entry and mid corner control.

The other aspect is I can work out ways on how to improve shite bits of the bike that the original designers have done for cheapness purposes.

[erik]

The reason I ask is that I'm not disputing your logic which is sound so I suspect we may be talking about two different models here? My interpretation of how a swingarm works is closer to your 4th pic where the chain is pulling at a more downward angle to that of the swingarm due to the offset of the pivot and front sprocket i.e. the centreline of the two sprockets is below the pivot of the swingarm. Does this change things in any way?

I need a smilie with a cheesy grin here.

I think my fault was in imagining the sprocket fixed to the swingarm.

Take a look at the two attached pics. The first one is based on your dimensions, the second one on a photo of a zx10.

I think I should have considered the fact that the chain and sprocket are moving (as was suggested). Assuming that the chain is in constant tension which pulls tangentially from one sprocket to the other, when the swingarm moves down, the tangential distance between the sprockets decreases (as shown in the pictures). Therefore, the chain tension will try to make the swingarm move down to relieve the tension.

In my original idea, if the rear sprocket was fixed, moving the swingarm down would increase the tension, which is opposite to the above.

Assuming I've got the reasoning right this time, I stand corrected.

[edit]
maybe the third sketch is an easier way to see what's happening.

[cowpoos]

its not worth arguing about...because the facts have been stated....rear will lift under accereration....the only exception is when the axle is 5 degrees + higher than swingarm pivot....its to do with tractive force's...the chain is simply trying to shorten its self....and the wheel and rear sproket are causing a tractive resistance....so the the front sproket is trying to pull....the rear + the wheel is resisting...so the side effect force is the chain gets pulled incredibly tight....which starts to pull the swing arm down...causes bike to rise...if the rear wheel starts to spin alot of the force that causes this effect is lost in the spin....so there is the potentail to get a better drive if a small amount of spins is induced....

[Motu]

Oh dear,it's so simple it's hard eh? And the sportsbike expert got it wrong of course - my Pajero lifts the rear under acceleration,and not all cars nose dive under braking,like the Peugeot 306 I drove last week - it's all about geometry and how they set them up...each case is seperate - you need to be Rolf Harris to paint with a 6in brush....

[erik]

its not worth arguing about...because the facts have been stated....rear will lift under accereration....

Well, for me it was worth arguing/discussing because it's helped me understand things better.
If we didn't argue and instead just accepted the facts without really understanding them, it'd be like religion...

[cowpoos]

Well, for me it was worth arguing/discussing because it's helped me understand things better.
If we didn't argue and instead just accepted the facts without really understanding them, it'd be like religion...

true brother very true.....

my bad