View Full Version : MM per rev

25-02-2004, 08:31 AM
Sorry to cover old ground. Can someone advise how the mm per rev calculation is calculated? Thanks

Mark Stiles
25-02-2004, 09:31 AM
pi x wheel diameter, divide by spur, multiply by pinion.

25-02-2004, 09:51 AM
nice on mark, cheers!

Mark Stiles
25-02-2004, 10:50 AM
what motor are you running?

26-02-2004, 08:30 AM
can you guys advise us on the standard diameter of front and rear tyres?

26-02-2004, 08:57 AM
There isn't an ideal size. I think most people turn their new tyres to be about 46mm front and 52mm rear. Most cars will take tyres down to about 41mm front and 46mm rear.

If you look at the 'quick' drivers at major meetings, I reckon their tyres are usually about 44mm front and 48mm rear. Some of the 'quick' drivers might like to add their thoughts to this.

I also find that you need to maintain a difference between front and rear tyre diameters of about 3 to 4 mm. I'm not sure why, but my car feels better balanced, and tends to understeer if I go less, and become twitchy if I go more, than this. HTH :)

26-02-2004, 09:54 AM
Hi mark I am running a 18x4, 4 cell. Also is the standard spur a 100T 64dp and what pinions to use? Obviously gearing is unique to the circuit, any ideas on a starting mm/per rev?

Mark Stiles
26-02-2004, 11:26 AM
On an 18x4, well over 40 mm/rev! Most people use a 12 or less. I ran a 10x1 at wickham! A high turn motor will make the car easier to drive, as it will probably be slower. However, after some time (especially if you're racing on a large track, with additive) you might be wanting more power. See how the 18 goes first though!

I run 48dp gears, so am not sure about approximate sizes for 64dp.

26-02-2004, 12:10 PM
cheers mark, I have not run the car before so a bit of a novice, and alot to learn in a short time! tonight at LRRC will be a steep learning curve as I have 1 set of tyres, unknown shore? no spare T bar,no one else runs a Switchblade, and no idea of the gearing.

just the way I like it!!!

26-02-2004, 01:44 PM
well pinion size would be (MPR/(Di x Pi)) x Spur

so if your tyres are 46mm and you want a MPR of 40 then you need to use a 27 pinion with a 100 spur. so you work it out to uyour tyres, unless they are 46mm

i use a 10x3 at 37mpr with 46mm tyres and thats fast so with a 18 then as mark said somthing about 42mpr etc


Mark Stiles
26-02-2004, 02:04 PM
I used to run a 12x2 at 40. I would say nearer 45/46mm/rev for an 18x4.

26-02-2004, 09:37 PM
In reply to Pete's comment as a so called 'quick' driver, I tend to start with large tyres and let them wear down, hence getting longer life and saving money (you could say I'm stingy).

Starting at 54mm should be OK, with fronts around 49mm.

I find the rears wear quicker than the fronts and you need to maintain the 3-4mm difference.

With large tyres you are better off when the grip is low but you can grip roll when the grip comes up.

For the record I ran 53mm rears at Wickham to win on Sunday with a Reedy 10 double geared on 40mpr


27-02-2004, 05:10 PM
Paul - you ARE a quick driver; no so-called about it!! ;D

28-02-2004, 01:08 AM
guys the 18x4 was dog slow, thought I was going backwards, gonna stick a 12 in it next time and try and get round without crashing.
It seems the faster you go the more grip you generate!

Mark Stiles
28-02-2004, 02:40 PM
1/12th is all about being smooth and accurate. If you're on and off the throttle all the time then you won't be as fast as if you push the stick (or pull the trigger!) gently! ;)

Stick a 10 in it and be done with it! ;) ;D