I'm trying to work out what wheel / tyre combos I can use on the trike, but I'm running into problems as none of the online calculators allow for a 'primary transmission ratio' as found on a BMW shaftie. Does anyone know of a programme that has this functionality?

These are the figures I'm working with:

Primary Transmission :
1.734

Gear Ratios:
1st 2.176
2nd 1.625
3rd 1.296
4th 1.065
5th 0.939
6th 0.848

Final Drive:
2.75

Cheers

Can you not just multiply the gear ratio's by the primary and use those new numbers or does it not work that way? (or something similar)

Try this, http://www.catherineandken.co.uk/sti/tyres.html

I'm pretty sure the GearingCommander website will do what you want, it may well have all the correct gearing already on their database depending on the BMW model you are using.

Just use a normal one but put in 4.7685 (i.e. 1.734*2.75) as the final drive ratio.

Scotty - I have sent you a spreadsheet gear calculator with your gearing added.

Use the link below to calculate your tyre sizes for input into the spreadsheet:
Tyre size calculator

Doug.

Cheers Guys!

Meant to add that the reason I asked is because I'm struggling to get a BMW HP2 Sport rear wheel at sensible money, so am looking at having a split-rim made instead. The guys at Windle only make 13" rims (the BMW is 17", so I wanted to see what the gearing would do with a smaller wheel.

I've been playing with Dougs calculator and it appears that a smaller wheel actually makes the gearing a bit better for a road going trike. Might not look so cool, but who cares!

I have a Excel spreadsheet that does calcs for any variations of gear ratios and diff ratios and tyre circumference u2u your email and Iwill send you the file.
Steve.

[Edited on 14/7/11 by stevec]