Side-effects of downsizing rear sprocket?

[rustyrobot]

I've booked my bike in to get the chain and sprockets replaced next week. My current chain is at it's final adjustment setting and has about twice the movement that it should have. I also have no idea how long it's been on there, and have been incredibly lax with cleaning and lube, so probably well due for a change.

The thing is that the bike shop could only get a rear sprocket a size smaller than the one than is suggested by the manufacturer, which they have said will give me a little more speed on the top end (which I could probably do with).

So I'm wondering what the flow-on/side-effects might be from this sprocket change? What will I lose to gain that top-end speed? What are the possible long-term effects?

[Gremlin]

250 aside, it will reduce acceleration response and increase top end. The opposite is naturally true if you go up teeth at the back.

This scenario is reversed for the front. To increase acceleration response you decrease the size of the front (measured in teeth).

However, for 250, its possible (knowing nothing about the spada) that yes, while technically, there will be more top end, the engine may not be able to supply sufficient power to make use of it. You will also have lower revs at any given speed in any gear, than currently, but 1 tooth on the back won't make a massive difference.

And as an aside, 1 tooth is worth 2.4 teeth on the back, when changing gearing.

[IdunBrokdItAgin]

I've booked my bike in to get the chain and sprockets replaced next week. My current chain is at it's final adjustment setting and has about twice the movement that it should have. I also have no idea how long it's been on there, and have been incredibly lax with cleaning and lube, so probably well due for a change.

The thing is that the bike shop could only get a rear sprocket a size smaller than the one than is suggested by the manufacturer, which they have said will give me a little more speed on the top end (which I could probably do with).

So I'm wondering what the flow-on/side-effects might be from this sprocket change? What will I lose to gain that top-end speed? What are the possible long-term effects?

Sounds like they mean one tooth less on the rear sprocket.

It'll mean that'll the negative effect will be reduced acceleration.

Dropping a tooth on the rear sprocket has less effect than increasing one on the front sprocket (as the rear has more teeth/ is larger - and dropping a tooth on the front has the reverse effect on top speed vs accel) so if it is just one rear tooth less I don't think it'll be that noticeable - maybe at first but then you should get used to it pretty quickly.

Confusing I know - but no biggie from the sounds of it. You can always change back to the original sized sprocket later if it doesn't suit you.

[pete376403]

It might go a bit faster, but it will accelerate slower. How many teeth reduction between old and new are they talking about? 1 tooth doesn't make that much difference

Forgetting about top speed, a smaller rear sprocket will drop the revs at your normal cruising drop a bit and fuel economy may improve.

If your chain is at it's adjustment limit now, is the shop removing links or fitting a new chain, otherwise it's going to be even looser than before.

[rustyrobot]

Ah thanks. All makes sense now. Should have been able to work that (+speed = -acceleration) from thinking about the dynamics of gear changes on my bicycle.

Yes, it was down 1 tooth on the rear I am pretty sure. Could do with improved fuel economy, but it's hardly a rocket off the lights as it is, so hoping the acceleration won't be too markedly reduced. That said, it IS a 22 year old 250, so I'm not expecting it to take off like a ZX-14.

Either way with a new chain it's got to be a hell of a lot safer than it was, and perhaps getting rid of that slack will help balance out some of the acceleration loss? (Or maybe I have that all wrong).

[IdunBrokdItAgin]

Ah thanks. All makes sense now. Should have been able to work that (+speed = -acceleration) from thinking about the dynamics of gear changes on my bicycle.

Yes, it was down 1 tooth on the rear I am pretty sure. Could do with improved fuel economy, but it's hardly a rocket off the lights as it is, so hoping the acceleration won't be too markedly reduced. That said, it IS a 22 year old 250, so I'm not expecting it to take off like a ZX-14.

Either way with a new chain it's got to be a hell of a lot safer than it was, and perhaps getting rid of that slack will help balance out some of the acceleration loss? (Or maybe I have that all wrong).

If your chain was already overly slack then you'll definitely feel a pick up in acceleration from having a new one fitted.
I think you're right in that that alone will more than counteract any feeling of loss in acceleration terms, from losing a tooth on the rear.

[gatch]

Bullshit they can't get a OEM sprocket.

Go to a different bike shop if you don't want it.

I had a spada and I went the opposite way, 1 smaller on the front. Was sitting on around 9k at 110, but made it a little more fun around town..

[Conquiztador]

One tooth at the back will not make much of a difference really. Lets say that in first gear you have earlier been doing 50k/h at 7000 revs. Now you would be doing 53k's at 7000 revs.

You can always compensate by fitting a low profile tyre at the back. But then you will need to adjust up the suspension to be at same height...

And I always buy chains in a box, then cut and join to get the right lenght. I would assume that any bike shop would do the same.

An interesting thing when playing around with chains on racing bikes: By changing front and rear sprockets we can make the chain go slower and so there is less weight to move around.
Here: If we assume that 15/64 gives us same speed at same revs as 10/50 you can see that the chain would be travelling faster by using 15 at the front at the same revs (5 more links per revolution of front sprocket). Therefore requiring more energy to move it around. Also, the 10/50 requires less chain reducing the total weight of the bike. OK, as there is road friction, pushing the bike forward, a 80kg rider etc, the gain is very small. But everything helps... ;-)

[tri boy]

Bullshit they can't get a OEM sprocket.



Importers do ocassionally run out of stock you know.
A stock order is probably underway, but lead time can be a few weeks at least.
And yes, sometimes they can do one off orders, but that is a pain in the arse for small cost items.
One tooth less will make naff difference.
Go with whats in stock, before the chain jumps and sticks a nice hole through the flywheel cover. MHO