Question about gear/sprocket ratios

[JimboF650]

This weekend I'm having a new chain kit fitted to my small bike (MZ 125cc). The original spec chain always needed a lot of adjustment, so have got hold of a heavier duty chain (supplied by MZ to fit my bike).

The old sprockets are: 16/49, new sprockets: 13/40

My question is; will I loose any top end speed with these new ratios? (the bike only reaches 65-70 at a push, and as I commute on fast roads and motorway I need every bit of that speed.

Cheers guys

[Lord Sut]

I think that you've slightly lowered the gearing, so it depends on how it was coping before. If it was struggling to rev out then the new ratio's may allow a higher speed. If you were revving it's nuts off before then you're likely to loose a bit of speed. You should expect the front sprocket to wear 20% faster either way.
Of course, it's still early in the morning for me so that could be completely the wrong way around.

[JimboF650]

Cheers for that - after a bit of research into working out ratios... OLD:49÷16=3.06 NEW:40÷13=3.07, so very close. Simple really, silly me.

Now just wondering how the chain tension should be set (60-70mm on the old setup) will it be the same??, sure the mechanic will sort it for me.

[Lord Sut]

Yes, I'd have thought that the free play should be the same. The chain will be shorter but the movement between sprocket centres will stay the same.

[herman]

Just a thought and apoligies if I am trying to teach you to suck eggs but make sure the new small front spocket does not make the chain foul the swing arm or guide. B)

[JimboF650]

Would there not be more space????

It's come from MZ so hopefully should be ok. Fingers crossed

[Treadtrader]

As you say in your post, they've supplied a heavier duty chain and sprockets.
Probably a bigger chain with different link size, this would mean less teeth but physically similar size.

[herman]

Would there not be more space????

It's come from MZ so hopefully should be ok. Fingers crossed

I'm probably just paranoid ....but... if it is a smaller front sprocket and the swing arm pivots close to it then the chain could be forced over it rather than missing it easily. Some off road biased bikes have adjustable rear chain guides too in case a different rear sprocket is used.

[JimboF650]

Your right the smaller front sprocket could be an issue. I'm going to have a quick check just in case.
The MZ supplier did tell me I was the first person to order it, bit of a guinea pig.
Thanks for all your help guys.

[JimboF650]

As you say in your post, they've supplied a heavier duty chain and sprockets.
Probably a bigger chain with different link size, this would mean less teeth but physically similar size.

Took my little MZ for its service yesterday - new chain and sprockets all fitted now. As 'Treadtrader' mentioned the new sprockets had less teeth, but the diameter of them were the same as the old sprockets - so no change in how it runs