Been using the very helpful gear calc spreadsheet on the Westgarage (chain diffs) website.

Thing is, I'm struggling to interpret the data in a way that my pea-brain can understand!

The rear tyres I intend to use are almost the same diameter as the donor bikes rear wheel. I could therefore also use the same sprocket sizes as the bike too. As you can imagine, this input produces fantastical figures on the spreadsheet (90mph in first gear anyone!). Perfectly achievable on the bike I guess, but how would that translate when the 'bike' now has 4 wheels and weighs 250% more?

Do I go straight to adjusting the sprocket sizes on the spreadsheet so the figures become more sensible, or do I carry out a rough adjustment of the output data first to reflect the weight / drag of the car over the bike?

Whats the car and motor? Whats the "fantastical" top speed?

520(ish)KG chain driven midi.

2009 Fireblade engine.

90mph in first... 180mph in top.

Works for the crotch-rocket, but how would those type of numbers translate to the BEC?

Do I tweak the figures to reflect the additional weight / drag, or go straight to different sprocket sizes to get more sensible numbers.

Apologies if none of this makes sense - my brain is scrambled with gear calcs!

[Edited on 15/7/17 by scootz]

I'd pick a ratio that reduces the gearing by about 25-30%. So top would be in the 125-135 range which is about the max you'd expect without full body.

For comparison, my Fury weighs 500ish, and is geared to just shy of 150mph... though won't actually quite pull to that. That compares to just shy of 200mph in the bike.

This is the gear calcs using the bikes sprockets, and a rear tyre the same diameter as the bikes...



[Edited on 15/7/17 by scootz]

Hi Scootz

As Adi says drop the gearing to circa 150, there's no tweaking required to the figures due to the size/weight of the BEC as it's a mechanical calculation - but you do need to change the gearing as it'd simply be too long for the size/weight.

Build into the diff mounting a range of adjustment to allow for chain movement and different sprockets as Douglas @ West Garage will, and for not much money, sell you alternative sprockets so if you want to have a play it'll not cost you much - as long as you've the adjustment range built in.

Though of course you can also change the front sprocket too, and that way, if space is tight for diff adjustment you can change front and rear sprockets in pairs.

Put a really strong chain on it..

Think you'd want a sprocket ratio of about 3.5:1 That'd be something like 14 x 49 but I don't know if thats possible... or if my maths is close.

Makes perfect sense Jim. Ta.

Your message just jogged my memory Adi - clean forgot that Douglas suggested a 14/51 combo the other week. That would give me a 3.6:1 ratio.

That'll be my ballpark right enough then!

Cheers.

If it helps.... I have 15, 53 on my Riot, gives me about 120 top speed on 195 50 15 tyres. Renthal sprockets and DID 530 chain.

[Edited on 15/7/17 by Phil_1471]