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sevaun
20-04-10, 09:04
Hi
having completed my first outing in the OMS at Harewood Spring National I find need to make one or two tweeks to improve performance.
The first of these is that I found 2nd gear to be a bit short and I was having to change up very briefly into 3rd and back down again for the approach into Country and Farmhouse. It would be better if I could hold second for longer on the short approaches to these corners as changing gear costs time.

Question 1 Do I just change the gearbox sprocket or the rear sprocket or both?
Question 2 Where's best to buy alternative sprockets?
Question 3 If I change the ratio will this compromise my overall speed/gearing


Thanks

Sevaun

team knifedge
20-04-10, 09:14
simplistically, increasing the number of teeth on the g'box sprocket will make the overall gearing longer...my guess is without an infinite number of sprockets you'll always be slightly compromised - I've stuck with 13tooth on the g'box of my Jedi for the last umpteen yrs (can;t remember the rear !) and it works ok in most places - sometimes holding the gear close to the rev limiter is better than an upshift followed by an downshift - bike shops supply std sprockets

Craig Powers
20-04-10, 09:22
Question 1 Do I just change the gearbox sprocket or the rear sprocket or both?
Question 2 Where's best to buy alternative sprockets?
Question 3 If I change the ratio will this compromise my overall speed/gearing


Hi Richard,

I would just go for changing the engine sprockets for now.
Try B&C Express.

I can give you a gearing chart in Excel so you play with the numbers. I may still have the one I did for Rev Barry who was using a Suzuki WP at the time.

Craig

Brib
20-04-10, 09:28
Question 1 Do I just change the gearbox sprocket or the rear sprocket or both?

Front may be easier, but one tooth more or less makes about 3 times the difference of one tooth on the rear sprocket.

Question 2 Where's best to buy alternative sprockets? search on the 'tinternet. What Craig said

Question 3 If I change the ratio will this compromise my overall speed/gearing.

Yes, it'll change it, may make it better or worse.

How many gears are you using? some venues I use them all and some only 4.

Is first gear a bit short too?

Craig Powers
20-04-10, 10:21
Misc advice re changing sprockets.

1. You may need 2 different chains, one with 2 links more depending on which engine sprocket you use and the chain adjustment method. The longer chain is also a spare; you can always shorten it. Or buy a long enough length to make, say, 3 chains.
2. You may need shims to move the diff housing back.
3. A chain splitting tool is useful. I have the Whale type that drifts out the pin with a hammer blow.
4. Mole Grips and a small nut are the essential tools for fitting a split link.
5. Hylomar (or similar) on the split link spring clip will help hold it in place whilst working and make it less likely to ping off when you remove it. Makes it easier to find the split link too because it's Blue!.
6. If you use split links, as I have to, buy several because they are getting hard to get hold of. Performance chain stockists try to sell endless chains and a reluctant to sell jointed chains.
7. Prove you can do a quick chain swap at home. Practice before you get to an event. Make sure you can get the engine sprocket off easily. Not always easy.
8. Always fit the proper (spring?) washer when fitting the engine sprocket nut. Always have a spare.
& finally...
9. Take Swarfega with you. My hands are always black at meetings.

Enjoy....

Craig Powers
20-04-10, 10:32
....and one more thought....obvious but I'll mention it anyway...

you may need to cut away part of the sprocket cover for ease of access and sprocket removal. Remove metal without removing the strength required for the clutch slave cylinder operation.

Craig

whimsy
20-04-10, 12:51
Just a lateral thought - if I remember rightly from a previous post you are using 21 inch rear tyres. If you swap to 22 inch you'll achieve the same effect of slightly taller gearing - but remember to reset your ride height! Worth thinking about if you had a change of tyres in prospect anyway but there are often reasonable used slicks on offer even if you weren't! As for the need to change your wets as well - ideally yes of course, but I have run mixed sizes in the past without problems because the cornering forces and speeds (down force) achieved in the wet mean the car is much more lightly loaded.

BMTR are usually very good at swapping tyres over (for a drink or two!) and this way means they need the swarfega not you!!

W

sevaun
20-04-10, 12:53
[QUOTE=Craig Powers;36285

I can give you a gearing chart in Excel so you play with the numbers. I may still have the one I did for Rev Barry who was using a Suzuki WP at the time.

Craig[/QUOTE]

Hi Craig the Excel chart would be great to have thanks and then I can see whether making the change for the sake of second gear will be either a good or a bad thing with regards to the other gears and how it pans out on track.
I'll send you my e mail via the PM mail system and maybe you could e mail tha spreadsheet please.
thanks yet again for your invaluable help.

Richard

Craig Powers
20-04-10, 12:59
Just a lateral thought - if I remember rightly from a previous post you are using 21 inch rear tyres. If you swap to 22 inch you'll achieve the same effect of slightly taller gearing - but remember to reset your ride height! Worth thinking about if you had a change of tyres in prospect anyway but there are often reasonable used slicks on offer even if you weren't! As for the need to change your wets as well - ideally yes of course, but I have run mixed sizes in the past without problems because the cornering forces and speeds (down force) achieved in the wet mean the car is much more lightly loaded.

BMTR are usually very good at swapping tyres over (for a drink or two!) and this way means they need the swarfega not you!!

W

I've no practical experience of changing wheel diameter from the design spec, but in theory changing the wheel diam and dropping the ride height will alter the geometry and hence roll centres behaviour.

Engine sprocket changing is quite easy and I manage it at the venue in about 15 mins. There's a bit of swearing and oily hands, but the audience seem to enjoy it.

sevaun
20-04-10, 13:06
Question 1

How many gears are you using? some venues I use them all and some only 4.

Is first gear a bit short too?

Hi Brib
At Harewood I'm using only 4. First seems ok and the only times I'm getting into third are:
Briefly before Country(tried holding 2nd, but on limiter for what seems like an eternity, but probably only 1 second)

Then 3rd up the short straight approaching Orchard. This is OK as I'm not revving out in third before I brake for the hairpin

Then 3rd again very briefly between the farm buildings approaching Farmhouse Bend. Like Country approach I tried holding 2nd but it seems to stay far too long on rev limiter

I'm getting into 4th up the final straight approaching Quarry.

As you can probably see if I could keep accelerating in 2nd on the run up to Country and Farmhouse then I guess I could probably make a full second up on my times

thanks for your input

Richard

team knifedge
20-04-10, 13:28
I think I'm geared for circa 112mph in 6th @ circa 11k rpm with the R1 engine.....when running the 600cc I geared it short and never used 1st - m'bike 1st gears are always short, giving a bit ratio step 1-2 and also you have to traverse neutral on the 1-2 shift

Craig Powers
20-04-10, 13:35
I can give you a gearing chart in Excel so you play with the numbers. I may still have the one I did for Rev Barry who was using a Suzuki WP at the time.

Craig
Excel gearing chart just sent by email.
Craig