Hitchcocks Forum

Well, either my math is screwed up or my bullet is getting 53mph per 1,000rpm!

I have a bullet with all the standard drive train fixings; 25T primary, 18T gearbox and 38T rear wheel, std gearbox ratios (1:1 in top). I'm trying to calculate rpm at given speeds and speeds at given rpm, my tyre is about 27" in dia, +-1/2".

So, can anyone help me?

This gives me a 1.38:1 P:G ratio and a 2.1:1 G:W ratio (inverse is 1:0.476).

Here's my calc.

1 x 1.389 x 0.476 x 27" x pi x 1,000 / 1,760 / 36 = 0.885 miles per 1,000 revs = about 53mph!
Prev Grev Wrev Dia

You are right there your maths are wrong

Strewth, this takes me back a few years. I used to do this sort of calulation on my old D10 bantam. The first thing you want to know is: How many revolutions of the engine (in top gear) will it take to rotate the rear wheel once. On my Bantam is was 6.57 to 1. Then I measured the circumfrance of the rear tyre (about 80" I think). Then I calculated how many rotations of the rear wheel it would take to cover a mile. I'd then take this figure and multiply it by 6.57 to give me the number of engine revolutions per mile. Once I had this figure, I could work out the RPM for any given speed. I don't bother with this any more as my Lightning has a tacho.

Hi guys ------- You don't mention the clutch drum teeth No...you'll need this to get the top gear overall ratio....( As per BARRY Q's post----you can then do those quoted calcs. to get that engine rpm per mile.... For the lower gears just increase by the reciprocal of each gears reduction ratio eg}-- A reduction ratio of 1/3 will increase the speed by a factor of 3, a reduction ratio of 1/4 increases the speed by a factor of 4 etc.. ie}--the engine rotates faster in the lower gears)..................................... or just buy an electronic tachometer like this}-----http://www.ebay.co.uk/itm/New-White-2-5 ... ge-Car-Mot

Sorry, that didn't work out ------try this instead}----- http://www.ebay.co.uk/itm/New-White-2-5 ... 5d3f07cfdc ----------

--------------- just a bit more }---- If you are doing an indicated 60 mph then that's 1 mile per min...and if your tacho says your engine is turning at 5,000 rpm then,--- BINGO !! ---- you are doing 5,000 revs per mile = simples really !!------------- I use the motorway distance signs this way to approximate time as well......( In town 30 mph = 1 mile in 2 mins, etc, etc)

When your vision becomes blurred, your fillings are rattling in your head and you get pins and needles in your fingers and wrists you are doing 50 mph. As to RPM, don't worry!

Hi
If you have a 3.50 rear wheel I have a couple of Excel spreadsheets that will help.
Edward

It is quite easy to see where the original calcs are wrong, he hasn't taken into it the fact the engine sprocket is driving the clutch sprocket. I forget how many teeth on the clutch sprocket but that is where it is wrong

A couple of spreadsheets; here
http://www.whfolk.co.uk/REOCHUMBER/Articles/revs.xls
And here
http://www.whfolk.co.uk/REOCHUMBER/Arti ... edcalc.xls