Board logo

ZX12r BEC midi rear sprocket.
rodoe - 11/3/17 at 05:44 AM

Hoping someone might tell me what sprocket sets they're running on their zx12r chain drive car. Obviously would need your wheel diameter also. I'm building this open wheeler as a track day car so top speed more important than quick take offs hillclimb style.
I'll be running Formula ford tyres at 21" diameter and have no idea where to start. Any help greatly appreciated.


Ian-B - 11/3/17 at 06:52 PM

Been a long time since ran a bike sport and RGB car with a ZX12, looking at my old calculations typically 16:50 should get you somewhere near with around 130mph with the tyre size you have indicated, this speed is quite easily obtained with closed wheel car on the circuits with longer straights, maybe slightly lower gearing if open wheel with a little more drag. At these ratios with a 560kg car the 1st gear acceleration is still traction limited with road tyres. If you want to work out the ratio's your self the gear ratio on the ZX12 to the drive sprocket is;
1st 1:3.88
2nd 1:2.91
3rd 1:2.30
4th 1:2.00
5th 1:1.80
6th 1:1.63
Hope this helps

rodoe - 11/3/17 at 08:37 PM

Thanks Ian, awesome help. Also found a calculator on Westgarage for zx12r. Can I ask, I also remember reading there is a minimum length you should run the chain or they get too hot for circuit days. Do you know about this or is my memory as bad as I think it is?

daniel mason - 11/3/17 at 08:42 PM

What car is it?

rodoe - 12/3/17 at 09:56 AM

Hi Daniel, it's a real mongrel. The original plan was a Terrapin in a Brabham bt21 style but plans had to change. My budget is $5k and I have a 2 year deadline as my company will close shop then.
Picked up a chassis of unknown origin that came with Formula Vee front end which speeds things up somewhat.

Ian-B - 12/3/17 at 12:30 PM

With our setup we would either run 78 or 80 link chain dependent on the ratio (resulting in around 340mm between centres). Never experienced overheating issues or chain failures (except constant stretching when we tried an alternative to DID chain), would change the chain at the end of every season. I am certain that the most important factor to avoid overheating is to ensure good alignment of the chain, not just statically but also under load, the load under high acceleration exceeds 10kN (about a tonne) so it is well worth paying attention to ensuring that your differential mount and rear engine mounts are suitably stiff.