HI, do you have any left? what is center bore size? thanks

Hi. Yes I think they are still somewhere, what size bore do you want, the only one I can find is 76 teeth and 20mm bore. I know there are others but where I put them remains a mystery. I can bore it bigger if you want. You do realise they are really cheap from your local bearing supplier, I think these were about £5 or thereabouts from Industrial Bearings East Anglia. We are going to Rockingham I'll put it in the tool box.
Ashley

Are they a plate type or with a boss? could do with 91 really, and would need to be plate type as I would like to attach to a freewheel on bike.

Stu - I can give you a dxf design file that you can get cut out by your local laser cutter's if you get stuck.
As used in both Brian and Dougal yesterday....... bolts straight onto an ACS freewheel, (or a shimano with a little bit of filing...) It has 90 teeth.
The offer's open to anyone - absolutely no trouble for me to email anybody a small dxf file ;^)

hi bob, I have looked into this, but not having any luck finding anyone local, they either have a minimum order of £60+ or they are just not interested in a small 1 off job, if any one knows anywhere online that is not too expensive then I would love to know :)

Stu. All of mine are plate wheel type. But I can't find the 91's . I have one somewhere I will look again but they are not in any of the obvious places and if it wasn't pouring with rain it would be easier to search our trailer and our outside storage, You should aim for one sproket with even number and one with odd for a chain drive, reasons for which escape me but it sounds sensible. If I find a 91 you will need to bore it and lighten it by drilling parts of it away with a hole saw, the offer from BobC is a good one but I agree finding a laser cutter who will help economically is not that easy.
I have bolted one to a freewheel it took quite a bit of machining to make shouldered washers to fit the teeth on the freewheel and it's handy to have a rotary table to space the holes correctly, I used a transfer punch and it wasn't very accurate, whilst I was doing this I kept looking at the MIG welder and thinking it would be much easier to weld it but then of course you can't change the freewheel.
Use a Halo Clickster if you want a quality freewheel or there are some super quality BMX ones but they are £80 ish each (I can't remember the name but they are from USA) but all parts are replaceable failing that Dicta ones are OK and very cheap.
We use a ratio of 16 or 18 teetth to 80 teeth on one car (bike chain) and 16 to 76 on the other ( 8mm chain), having said this not sure we will actually finish a race yet with these ratios as we have only tested at Mallory, Rockingham will be our first real race.
Ashley

Just a thought on sprocket selection. IIRC, the best combination of sprockets is two prime numbers of teeth, as it reduces the opportunity for any given chain link to hit the same tooth on a regular basis thereby reduces wear. In practice, an odd one and an even one seems like a good compromise. Also IIRC your smaller sprocket should be >=18. Smaller ones reduce efficiency as there is more bending between the links and more rubbing on the sprocket but once you get past 18 the efficiency gained by going up another tooth gets progressively minute. I guess the gist is, 19 and another prime with 61, 67, 71, 73, 79, 83, 89, 97 all up for grabs. Of course, we don't have the luxury of being able to skip from 61 to 67 if we really want to to keep the motor in the sweet spot so we inevitably compromise and lose a fraction of a % chain wear to get better mileages.

Stu. OK I have found both a 76 and a 91 in a box at the back of our store. Both are 8mm pitch and have 20mm bore they are plate wheels without a boss and have lightening holes drilled. You are welcome to them, I am taking them home if you want them send me an email (ashley.slater(at)btinternet.com) and I will ship for cost of postage (probably £5)
Ashley