Effect of unsprung weight

[warewolf]

Actually if you remove it the gearbox and other driveline compnents can be destroyed very quickly. Its also harder on your clutch.

Do you have any first hand experience with this? Not that I would think about removing it on a road bike.

My 640 Adventure has a cush drive. A potential replacement bike is an EXC, none have cush drives. My research online indicated it wasn't a problem. One chap even went so far as to say every gearbox failure he found was due to other reasons. For adventure bikes running some form of knobblie on-road, it was thought the tyre provided enough cush.

I can tell you - after running my EXC non-cush wheel in the 640 - that the connection between the throttle and the terrain is much, much more direct without a cush.

[Robert Taylor]

Do you have any first hand experience with this? Not that I would think about removing it on a road bike.

My 640 Adventure has a cush drive. A potential replacement bike is an EXC, none have cush drives. My research online indicated it wasn't a problem. One chap even went so far as to say every gearbox failure he found was due to other reasons. For adventure bikes running some form of knobblie on-road, it was thought the tyre provided enough cush.

I can tell you - after running my EXC non-cush wheel in the 640 - that the connection between the throttle and the terrain is much, much more direct without a cush.

Maybe it doesnt have a cush drive in the generally acepted sense but somehere in the drive train there will likely be some form of same.

[DEATH_INC.]

What I'm thinking is that maybe you could just run some small bushes in the 'plate' to give a small cush effect, similar to what my Marvics on the turbo have. Very small and light.

[bogan]

What I'm thinking is that maybe you could just run some small bushes in the 'plate' to give a small cush effect, similar to what my Marvics on the turbo have. Very small and light.

If you have bushes the chain tension will compress them all too though right? The Bros has anti-chatter springs on the back of the clutch which will take up a bit of the impulse loads.


Getting back to the suspension aspect of it. More mass = more inertia to deal with for the shock. Ground deals with it when it is accelerating upwards, shock has to deal with it when it is accelerating downwards. So from mid-stroke (potentially?) going up you want more compression damping to dissapate the inertia so the wheel doesn't get light at the top of the wheel travel. And from the top of the wheel travel to mid stroke (again, potentially?) you want little enough rebound damping that the wheel can return quick enough to maintain force on the road.
And add in the other factors of low enough compression damping so that every bump is not translated up to the sprung mass, and high enough rebound so that it doesn't extend too quickly and use the stored energy to push the rear of the bike up.

Not sure if that is right, so correct me if not! Will make my tuning heaps easier if I know what the symptoms are caused by.

[DEATH_INC.]

It's a catch-22 situation, as shocks work much more on velocity rather than position ( I'm sure RT will be along shortly to correct me...) so if you increase the compression to keep the wheel from 'hopping' from too much unsprung weight, it has a negative effect on tyre loading, so you reach a 'compromise' that works ok in both situations. And of course the same applies to rebound, you are fighting a loosing battle if the unsprung weight is too high...

[bogan]

It's a catch-22 situation, as shocks work much more on velocity rather than position ( I'm sure RT will be along shortly to correct me...) so if you increase the compression to keep the wheel from 'hopping' from too much unsprung weight, it has a negative effect on tyre loading. And of course the same applies to rebound, you are fighting a loosing battle if the unsprung weight is too high...

Yeh that's sorta what I was figuring, still will do what I can with what I've got, and continue to gaze longingly at this sort of thing

if anything will fix it, it'll be active damping!

[warewolf]

Maybe it doesnt have a cush drive in the generally acepted sense but somehere in the drive train there will likely be some form of same.

Hmmm wouldn't have thought so, but I shall investigate. I would have thought that competition dirt bikes would be light & simple and not have anything of that nature that isn't widely used in road bikes. I really would like to know the answer to that one.

[Ocean1]

Maybe it doesnt have a cush drive in the generally acepted sense but somehere in the drive train there will likely be some form of same.

Hmmm wouldn't have thought so, but I shall investigate. I would have thought that competition dirt bikes would be light & simple and not have anything of that nature that isn't widely used in road bikes. I really would like to know the answer to that one.

There isn't one on an EXC. And without doing any actual research I'd be surprised if you'd find one on any full-time dirt bike.

[vifferman]

A simplistic answer is this: you can really feel the extra unsprung weight. I went from a VFR750 to a VTR1000 to a VFR800. The first thing I noticed was that I could feel the rear end more over bumps, due to the extra mass of the rear hub/swingarm. I didn't really notice what the rear end of the FahrtSturm was doing (despite the crappy shock).

[Spearfish]

If your a bit down about your unsprung weight on your bike and a bit short of the billion bucks to save a gram or five, go for a test ride on a scooter especially one that has the whole drive train, engine and the exhaust muffler and wheel bolted rigidly together as one piece and pivoting around a big central bolt.

Your unsprung weight might not seem so bad in comparison.

[Robert Taylor]

There isn't one on an EXC. And without doing any actual research I'd be surprised if you'd find one on any full-time dirt bike.

In the back of the clutch drum there is usually damper rubbers or springs, that is part of the driveline albeit very upstream. Most dirt bikes Ive ever worked on have them.

[Robert Taylor]

If your a bit down about your unsprung weight on your bike and a bit short of the billion bucks to save a gram or five, go for a test ride on a scooter especially one that has the whole drive train, engine and the exhaust muffler and wheel bolted rigidly together as one piece and pivoting around a big central bolt.

Your unsprung weight might not seem so bad in comparison.

Yes and damping is woeful, especially on the nastiest Chinese scooters.

[Robert Taylor]

If you have bushes the chain tension will compress them all too though right? The Bros has anti-chatter springs on the back of the clutch which will take up a bit of the impulse loads.


Getting back to the suspension aspect of it. More mass = more inertia to deal with for the shock. Ground deals with it when it is accelerating upwards, shock has to deal with it when it is accelerating downwards. So from mid-stroke (potentially?) going up you want more compression damping to dissapate the inertia so the wheel doesn't get light at the top of the wheel travel. And from the top of the wheel travel to mid stroke (again, potentially?) you want little enough rebound damping that the wheel can return quick enough to maintain force on the road.
And add in the other factors of low enough compression damping so that every bump is not translated up to the sprung mass, and high enough rebound so that it doesn't extend too quickly and use the stored energy to push the rear of the bike up.

Not sure if that is right, so correct me if not! Will make my tuning heaps easier if I know what the symptoms are caused by.

In fact if there is too little compression damping in the rear end the bike can actually feel harsh. That is because the rear end will blow through its stroke too readily ( on bumps or under acceleration ) and arrive at the more accelerative part of the linkage ratio, where the shock shaft is moved further and therefore faster for any given unit distance of rear axle movement. As Death correctly states damping is velocity dependent. Just like if you push on a fire door with a closing damper, the harder / faster you push the greater the damping force that is generated. To twist it all around damping with a linkage bike is also position dependent, the further it gets into the link the greater the damping force that is generated.
So, within sensible limits stiffer can work out to be softer because you are keeping away from the more acceleartive part of the link.
This is in fact a problem with many or most oem suspension units and is one of the very first things we address if the bike is going to be used for track days / road race. And its not about playing with the external compression clicker, its only a bypass in a circuit accounting for only 13% of total damping force. Its all about recalibrating the internal shim stack and shaft bypass jet.

[Spearfish]

Yes and damping is woeful, especially on the nastiest Chinese scooters.

Just for interest, have you ever worked on upgrading big scoot suspension, not so much the Chinese slightly modified bicycle pump shocks but, say a Burgman or Tmax?

[Robert Taylor]

Just for interest, have you ever worked on upgrading big scoot suspension, not so much the Chinese slightly modified bicycle pump shocks but, say a Burgman or Tmax?

Burgman yes and the potential for improvement is huge at both ends.