Motorbike gearing

martinwitnam

I recently was unfortunate enough to snap the chain on my motorbike, that being the drive chain not a security chain.
I've been looking to change the chain and sprockets on the premise that on my other bikes I always change the cassette at the same time as the chain.
I've searched the inter-web and found that you can get different size sprockets. If I were to put on a larger front sprocket or a smaller back sprocket this would would lower the gear ratio as in lower engine revs for a particular speed. (Sorry if that sounds condescending but I didn't want any misunderstandings).
My question is wheteher this would improve fuel consumption or just over-stress the engine?

Keith1983

It would have the same effect as being in the wrong gear for a given speed. If I were you I'd replace like for like size wise. What bike is it out of curiosity?

carrock

apart from a motorbike having a gearbox, it's exactly the same as gearing on a bicycle- if you fit/use a smaller front sprocket it will make the engine/legs rev(olve) more for a given speed

If you fit/use a smaller rear sprocket, the engine/legs will revolve less for a given road speed, and so on.

And vice versa of course

martinwitnam

It's a 2002 Suzuki SV 650.
With a broken hugger, rear undertray and left seat panel.
Oh and no clutch!

martinwitnam

I understand the theory and obvious practice of gear ratios on a (pedal) bike. I've been riding a geared bike for roughly the past thirty years but I was interested in wheteher it might make a difference on a motorbike to fuel consumption, as the majority of miles that I do on a motorbike are done at motorway speed.
Because most of the miles I have done on the motorbike are done on motorways. I wouldn't use my motorbike to nip into town, I tend to use it to travel from Kent up to my dad's in the West Midlands.

markos1963

The only reason to change size of sprockets is if either of them are particulary small, this can lead to premature wear, Suzuki GS550s in the 70's were known for this, a very small gearbox sprocket with a large rear one. Any other reason to change isn't worth it, the performance gains are marginal especially on a low powered road bike such as yours.
What is more worrying is the fact you've let the chain get so worn it's broken, a bad enough thing to happen on a cycle let alone a 125mph motorbike!! I would advise you get it fixed and leave things well alone!

martinwitnam

I had only had the motorbike for four months, had lubed the chain every-other ride, had clocked about a thousand miles and had only done one wet ride.
The motorbike had only been back at the shop I bought it from the week before as it needed a new gear shifter. I would have thought that if there was a problem with the chain they would have noticed as the work would have been in that area!

Keith1983

Your bike isn't yellow by any chance is it? It sounds alot like my first bike!

As for fuel consumption that bike shouldn't be too bad on fuel, I think I'm right in saying they were on carbs? I would say a good service ensuring things like fuel and air filters were in good shape would be the most constructive thing you could do?

Pross

Surely putting a larger sprocket on the front or smaller on the back increases the ratio rather than lowering it? :?

MattC59

Pross wrote:

Surely putting a larger sprocket on the front or smaller on the back increases the ratio rather than lowering it? :?

Yep, that's what I thought.

In which case, you'll end up with a lower engine speed for a given road speed, which will reduce your fuel consumption at that speed. However, as you will have effectively lengthened each gear, your accelleration will be slower, and the load on your engine and clutch will be higher. Effectively, but not a pronounced as, starting in second gear, instead of first.

Personally, I'd stick with the sprocket sizes that you already have.

sagalout

Putting a larger rear / smaller front sprocket is a common mod to improve the acceleration of your bike at the cost of top speed - its an almost free power increase (well, sort of!)

My bike is geared for 180mph (GSXR1000) with a ridiculously tall first gear which redlines at about 106. For the track this is fine, but for the road its not really ideal. Having said that it still accelerates to 60 in sub 3 seconds and does 0 - 100 - 0 in about 10, so any change might make it hard to keep the wheel down!

In general it suits most bikes, but you need to allow for the speedo offset.

martinwitnam

Pross wrote:

Surely putting a larger sprocket on the front or smaller on the back increases the ratio rather than lowering it? :?

That's what I said in other words.
A front 15 connected to a rear 45 gives a ratio of 0.333333
A front 15 connected to a rear 44 (Later SV configuration) gives a ratio of 0.34
Ha ha that's a higher ratio!
But as I said originally, lower engine revs for a given speed.

sagalout

Ah sorry, I misread too. Essentially yes, to a point you could get better fuel economy in top gear, like an overdrive gear, but it depends on what torque the engine is putting out at the lower revs

MattC59

martinwitnam wrote:

Pross wrote:

Surely putting a larger sprocket on the front or smaller on the back increases the ratio rather than lowering it? :?

That's what I said in other words.
A front 15 connected to a rear 45 gives a ratio of 0.333333
A front 15 connected to a rear 44 (Later SV configuration) gives a ratio of 0.34
Ha ha that's a higher ratio!
But as I said originally, lower engine revs for a given speed.

No you didn't

martinwitnam wrote:

If I were to put on a larger front sprocket or a smaller back sprocket this would would lower the gear ratio

martinwitnam

The Haha in my last post was aimed at myself MattC59.

What I meant was that the ratio of engine revs to wheel revs would be lower.

MattC59

martinwitnam wrote:

The Haha in my last post was aimed at myself MattC59.

What I meant was that the ratio of engine revs to wheel revs would be lower.

I seeee.......... fair enough

JohnnyAllez

If you`ve got plenty power then changing the sprockets for lower gearing isn`t a problem, but on an SV 650, which isn`t the most powerful / torquey bike in the world, then you`ll (maybe) increase your MPG, at the expense of top end speed (which is still 70mph in this counrty ). Your MPG may not be improved though, as you tend to want to ride the bike the same as before, which usually means opening the throttle more to make the engine pull harder against the lowered gearing, so using more fuel in the process. If you potter around using the minimum revs possible then it will improve, but you`ll be changing down a lot more as the engine struggles.

Changed mine from 17 front / 40 rear (2.35:1) to 18 front / 36 rear (2:1) Decent drop in engine revs, but the bike has the torque to pull it no problem (FJ1200) so it doesn`t need to be worked harder to achieve the same speed, therefore improved MPG. Theoretical top speed of about 190MPH , but overgeared now so it will struggle to get anywhere near there unless down a big hill with a tailwind. Doing 140MPH is about it in top on the flat fully loaded, if I change down a gear it will accelerate no problem, just too much air resistance / drag etc for it to overcome when in top gear, but I can live with that Bike will easily do over 60MPG when touring.

Yossie

Essentially, gear it down on the front for acceleration, gear it up at the back for more top end.

In the real world on roads: gear a proper bike (ie anything that does 160 plus) down so it does about 120 and wheelies everywhere and you get huge air over humpback bridges (you don't really need anything more than 120 on the road imho).

Anything less than 160 is slow so just chuck it in the bin - can't polish a turd and all that (unless its a 2 stroke 250 or a 400 in which case you'll blitz everything through the corners so its double cool).

If you're going track daying on it, check the gearing for the circuit - Donnington gearing won't work with Knockhill which won't work with getting really huuuuuuge air over Cadwell.

Most bikes are over geared as they come out of the factory, so drop it down a tooth or two - all to do with manufacturer's top speed war.

HTH

Y

maddog 2

Yossie wrote:

Most bikes are over geared as they come out of the factory, so drop it down a tooth or two - all to do with manufacturer's top speed war.

+1

I ran a Blade/GSXR1000 for a few years and I always geared them down. 1 off the front and two up on the rear. Much more fun and much quicker under 140.

On a 650 twin the same concept applies, just not quite as obvious (as in it probably doesn't lift the front in 2nd off the throtle...)