front sprocket increase

[old-git]

hi , i have just put a larger front cog on the front .was 15 now 16 .
but it doesnt seem to be very different ,
do you think it would be worth trying two extra teeth , or is that going to far ?
____________________
live long and prosper .

[Scotsman37]

Here's Renthals explanation of what to do;

But what does that mean about gearing...

The stock gearing of your bike is likely to have been determined by choosing a compromise ratio based on what worked best for test riders in "average" conditions. As soon as the bike is taken out of average conditions - by engine tune, terrain, track design or rider style the stock gearing might no longer be the optimum solution - a different setup might get you round the track faster.

Maximum speed occurs when the driving force is exactly counterbalanced by the air and rolling resistances. At this point the acceleration has fallen to zero.

Setting up the gearing of any vehicle is a trade-off between acceleration and top speed.

Gearing a bike up to produce higher top speed with less acceleration is done using a larger countershaft (gearbox) sprocket or a smaller rear sprocket.

Gearing a bike down giving it more acceleration with lower top speed is done using a smaller countershaft (gearbox) sprocket or a larger rear sprocket.

The ratio chart shows the gearing ratios for different numbers of teeth on the gearbox and rear sprockets. The numbers given are the number of revolutions of the gearbox sprocket required to cause one complete revolution of the back wheel. These figures are calculated by dividing the number of teeth on the rear chainwheel by the number of teeth on the gearbox sprocket.

From the table it is clear that changing one tooth on the gearbox sprocket has a significantly larger effect on the gearing than changing one tooth on the rear sprocket. To make a small change in gearing it is therefore necessary to change the rear chainwheel size by one tooth, as changing the gearbox sprocket makes a far larger difference in gearing.


Renthal Motorcycle Final Drive Ratios Chart; https://www.renthal.com/site/resources/ratios.htm

[Scotsman37]

Watch out the dealer doesn't give a rear sprocket for track day bikes as like what happend to my pal because just after a couple of hundred miles his rear sprocket was literally loosing it's teeth and I do mean he lost 4 of them and the rest were being worn down !

[old-git]

Thanks for that.
____________________
live long and prosper .

[G]

Sounds like a dodgy sprocket.

The renthal ultra-light ones usually last pretty well.

I suspect the problem with your bike is the power. (Seem to remember it's a 4 stroke 125.)

You aren't really going to get a much higher top speed as I doubt your bike makes enough power to do so. Does your bike redline in top gear as it is?

What was your intention with chaning the gearing?

[old-git]

yeh top speed of my bike is 60mph and then it is screaming .
i haven't tried it flat out since the sprocket change .
but it seems more or less the same around town .
i think i would have to change the rear sprocket for anymore gear ratio's as there is not much room for a bigger cog at the front
____________________
live long and prosper .

[old-git]

[Scotsman37]

Changing sprockets for quicker or higher speeds is only really good for larger cc bikes as they are not restricted as they can afford to lose off the top end

[G]

If your gearing was 15-45, then you would have a change of ratio from 15:45 (0.333) to 16/45 (0.3555). That means an icrease of 6% or so. So 60mph would then be 64mph or so.
Where possible it's best to not go more than two out on gear sizes, to keep your chain length the same. For instance go down one or two on the back if you go up two on the front.

You may actually lose out on top speed, though the engine won't be revving quite as much.
If the engine needs all of it's power to do a certain speed, then up-gearing will mean that at that speed it isn't producing as much power. Thus it may not be able to go as fast.

[old-git]

many thanks ,that is just the info that i needed ..
i think i could go a couple of teeth less on the rear and that would do .
my bike is one of those that when you are in top gear ,you feel like you need to change up another one if you see what i mean.
thanks chaps .
.this is a quality forum
____________________
live long and prosper .

[G]

As I said, you may well decrease your possible top speed by doing this, so don't be too suprised if you lose 5-10mph or so.

[DM]

I remember going from a 12/38 sprocket combination to a 16/38. Before I was hitting a top speed in 5th of about 58 mph where the bike would run out of revs, but obviously could keep going much faster. After the change I was doing 55 mph in 3rd gear and could get up to around 70 mph before losing steam. I had one of those massive grins on my face. Before leaving the island to come here I had changed the front to a 15 and managed a top speed of 80 mph.