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Discussion Starter #1
After running my track RR with 180/55 208 & 209GP rear tyres I'm now looking at going to running the larger 190/60 tyres.

Obviously the larger circumference of the 190/60 alters gearing for circuits so I've done some rough calculations in Excel to estimate how I need to change final drive gearing to compensate. Also to run larger rear sprockets and maintain tyre to swinging arm (or suspension link) clearances a longer chain is needed, I'm running standard 112 links at the moment.

Has anyone got experience of doing the swap then altering gearing & chain length to minimise the overall effect to gearing?

From my rough numbers I usually run 14/44 with 180 rear at my local track and would have to go to 14/48 with the 190. So with 4 teeth extra on the rear sprocket the chain would need an extra 4 links to keep the wheel in the same place in the arm, maybe starting with 6 links to be absolutely safe?
 

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Your question forced me to go rummaging through old course notes for a definitive answer. But to no avail, my notes on gear design have misplaced themselves. But let me offer this observation:

By increasing the diameter of the rear sprocket (i.e. adding on teeth), you are altering the required length of the chain because of the increased arc length with which the chain makes contact with that sprocket. At the same time, you are decreasing the contact arc on the front sprocket (draw it on paper - it makes sense). However, you do not increase the overall contact arc by much on the rear sprocket, and it hardly affects the front sprocket. Furthermore, the length of free chain inbetween the front and rear sprocket will increase only slightly as the overall radius of the rear sprocket increases by a small magnitude.

The upshot of this is that the chain should (I reckon) only need 2, maybe 3 extra links to counter the effect of the increased contact arc on the rear sprocket.

But have I done this in practice? No... not yet.
 

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Discussion Starter #3
I've realised now I've been mistaken in how I've worked things out - as Raptor says going to a larger rear sprocket doesn't increase required chain length by the same amount because only part of the sprocket is in contact with the chain.

Back to the drawing board (excel sheet!)

:confused3
 

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I don't know what kind of tracks you run at, but I use 15/45 gearing that puts top speed around 145mph topped out in 6th, give or take a few mph. Your gearing would be like me having 4 more teeth in the rear and holy sh*t that would be some short gearing. Stock gearing is 16/43 unless the UK version had different transmission ratios.

If I were to put a 190 on the rear, I would start with the same gearing as the circumfrance of the tire shouldn't change gearing all that much. It would take the edge off slightly. Kinda the same effect of putting 18's on a car that came with 17's.

I know you are running dumlops, but still, the overall circumfrence of the tire is only different by maybe 5-7mm in profile height. Usually this conversation is more about what do I need to change the ride heights in the front and rear in order to compensate for the difference in profile heights but yet maintain the correct geometry.

By going to the 190 you are gaining 10mm in tire width. The overall effect to gearing should be slight. At the most I would add 1 tooth and that should put you exactly at where you are now. As far as chain length, you shouldn't need to add a link unless the chain you are using now is near new. If it is currently in the middle of the adustment or near the end you should have enough space. If you pull it in too close it will hit the shock linkage during acceleration due to tire expansion. But you already know this.

I would focus more on getting the suspension heights and settings you need to have the geometry right before you think or worry about changing the gearing.

Let me know if this helps at all.
 

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Discussion Starter #6
Thanks for the info guys.

Onedwn5up

UK spec RR's are geared 16/42, I reckon the internal ratios are identical. Good to see some of my figures were in the ballpark as my sheet estimated 15/45 giving 144 mph with standard rear tyre. I run 14/44 at Oulton Park in the UK but don't get into top gear much which would make it difficult to get obvious comparisons for gearing compard to somewhere with a decent straight where you could consistently tell max revs hit in top gear. Just the nature of the track, many BSB and BSS teams only run 5 gears when they race there. I know someone who went much lower and it made the bike a real handful to ride smoothly. Can link to some onboard video from last year if you want a look for yourself.

Was going to post up why I disagreed with your estimates about how much the tyre height affects gearing etc but just found another cock-up in my figures and correcting them gives maybe 2 teeth difference going 180 to 190. Of course all of this assumes the tryes actually measure up to the sizes and doesn't take into account any tyre growth at speed.

I can't work out geometry effects until I actually ride the bike, I just dont want to change tyres then find I've messed up on sprockets/chain length/tyre clearances.

Will have a look at NewRedRiders excel sheet and see how the numbers work out when using the proper calcs!
 

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Inever said my numbers are exact. They were just guestimates that should be somewhat within range.

The other thing is that stuff on paper sometimes doesn't work out the same on pavement. You need to sometimes just try stuff and see how it feels and works and go from there.

Also, tire growth is not something that is going to effect anything on an overall basis except for where you position the rear wheel in realtion to the swingarm linkage. The differences in profile height vs aspect ration usually end up negating each other as well.
 
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