Effect of unsprung weight

**bogan** · 3rd September 2011, 16:47

Had some parts on the scales over the last week, curious as to how much sprung vs unsprung weight is on the rear as the spindle etc seems pretty heavy. Turned out to be 25kg unsprung, and 63.7 sprung (with full fluids), or 28.5% unsprung.

I understand the theory as to why lighter gives better traction and ride, but how does it go in practical applications. How much difference does reduction of unsprung weight make? How well can suspension systems cope with large amounts of unsprung weight?

**James Deuce** · 3rd September 2011, 16:58

Every kilo of unsprung weight you lose has the same effect as losing 8 kg of sprung weight. The bike steers better, brakes better, holds a line better, but you'll need to fiddle with the suspension a lot, changing springs and valving to suit. Suspension will work better with fine control in a wider variety of situations with the correct configuration and less mass to deal with.

Mass-produced bikes are built to a price and suspension is too. You get the most performance benefits from a value for money perspective from tailoring suspension to suit you and your bike followed by losing mass, again you and your bike.

**98tls** · 3rd September 2011, 17:15

When i changed the rims on my old girl the difference in weight between oem and new was

shit Ive forgotten,5 or 6kg from memory,possibly just a tad more,the difference when riding the thing is far greater than that,1st ride i was thinking na no way,somethings not right,everything was fine it just felt like a completely different bike.Long ago i read an article on rotating mass but so long ago Ive forgotten the details,a little off track..but.I look back on all the bikes Ive had and bought exhausts etc for in search of better performance,if i had my time again the 1st thing i would have done is ditch the oem rims instead of believing the rather questionable claims made by many exhaust manufacturers re hp, and weight savings.

**bogan** · 3rd September 2011, 18:00

Hmmm, good info!

Apparently I could save 2kg by eliminating the cush drive (non reverse-able operation though, unless I can find a second driven flange). But it also looks like sport demons are a heavy tyres, so maybe something lighter there would be the simplest option.

**Robert Taylor** · 3rd September 2011, 18:24

Its been answered very well already. More unsprung weight needs primarily firmer springing and damping commensurate with it. The same for heavy riders and load. But it can be counter-intuitive with rear rebound damping. A heavier rider / loading is more weight to return. So with firmer rear springing for a heavier rider / load you dont always change rebound damping, because there is more weight to return to the base ride position!

**malcy25** · 6th September 2011, 17:19

Originally Posted by bogan

Hmmm, good info!

Apparently I could save 2kg by eliminating the cush drive (non reverse-able operation though, unless I can find a second driven flange). But it also looks like sport demons are a heavy tyres, so maybe something lighter there would be the simplest option.

Almost everybike has a cushdrive style hub for a reason. It removes the harshness - I would not recomend not haviing it for the impact it will have on your whole drive train, chain life, gearbox life etc.....

**Robert Taylor** · 6th September 2011, 18:12

Originally Posted by malcy25

Almost everybike has a cushdrive style hub for a reason. It removes the harshness - I would not recomend not haviing it for the impact it will have on your whole drive train, chain life, gearbox life etc.....

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

**bogan** · 6th September 2011, 18:28

It's just a matter of being a smooth rider. Quite a few racers have removed the cush drive from RC-31s, and no ill effects, and they have an extra 30ish more ponies than me! However, the anti-chatter springs on the back of my clutch were worn out (almost certainly due to being run with a worn cush drive!), and while I rebuilt it as best I could, there is damage to some of the basket where they sit. So reckon I'll leave the cush alone this time (or until I upgrade to a slipper basket or something :P).

I also ordered a gel pad for my seat, as I have a rather firm low seat atm, so that might help the feel anyway.

**gatch** · 6th September 2011, 22:39

Originally Posted by bogan

Hmmm, good info!

Apparently I could save 2kg by eliminating the cush drive (non reverse-able operation though, unless I can find a second driven flange). But it also looks like sport demons are a heavy tyres, so maybe something lighter there would be the simplest option.

Sportmax a12h tires are light as hell. They supercede the gpra10. Lots of 400s with them on. Only problem is they don't last very long

**DEATH_INC.** · 7th September 2011, 07:24

The reason you need to reduce unsprung mass is fairly simple.It's both kinetic energy and in the case of wheels etc gyroscopic force.
The kinetic energy comes into play every time you hit a bump or undulation in the road/track. It takes more force to move the suspension, thus spiking the tyre loading, then once it is moving it takes more damping/spring to slow it down and stop it. Then of course it takes a lot more energy to return it to the track then of course it applies more of a 'spike' again. If you go over a hollow the same applies in reverse again.

We all understand the effect of gyroscopic force, basically once the wheels are spinning they take effort to turn (they want to stay spinning on their current axis) so the heavier they are, the more effort it takes to turn them. This also applies to leaning the bike, and picking it up again.
I put K3 wheels on my old SRAD, huge difference.

**DEATH_INC.** · 7th September 2011, 07:28

Originally Posted by bogan

Hmmm, good info!

Apparently I could save 2kg by eliminating the cush drive (non reverse-able operation though, unless I can find a second driven flange).

Do you have a 'how to' on this we could take a look at?

**bogan** · 7th September 2011, 10:20

Originally Posted by DEATH_INC.

Do you have a 'how to' on this we could take a look at?

Sorry dude, it's for single sided swingarms only, where there is an extra part called the driven flange which is normally part of the wheel. And the sprocket carrier is a bit bigger too because the cushes are internal to it rather than the wheel.

I can dig up the thread if anyone is still interested though?

**DEATH_INC.** · 8th September 2011, 19:07

Originally Posted by bogan

Sorry dude, it's for single sided swingarms only, where there is an extra part called the driven flange which is normally part of the wheel. And the sprocket carrier is a bit bigger too because the cushes are internal to it rather than the wheel.

I can dig up the thread if anyone is still interested though?

Lol, not planning on it myself, just wanna see what you're looking at doing...

**bogan** · 8th September 2011, 19:43

Originally Posted by DEATH_INC.

Lol, not planning on it myself, just wanna see what you're looking at doing...

http://hawkgtforum.com/forum/showthr...ht=cush+delete

Hack the flanges off the driven flange, and weld a bit of plate on to hold the sprocket. Perhaps turn the excess off the spindle if I was really sure to never go back. Will save this mod for my next bros build, perhaps a track bike, or superlight street.