I'm currently working on putting an R1 in my seven. I'm trying to gather as much information about diffs as I can. I reckon the optimal diff ratio for the engine's torque curve would be the one that gives the same max MPH in each gear as that of the bike that the engine originally came from. I assume that Yamaha have already done the hard work to optimize the use of engine's torque profile.

People seem to umm and err about diff ratios a lot and I'm very interested to find out more and make the right choice, I'm sure there's more to it than what I understand so far, so someone please tell me why I'm wrong...

I believe most people fit the 3.62 to BEC or have diffs made specificaly, suppose it depends on what you have too spend.

my 3.38 seems to work pretty well with first gear hitting about 60mph and when motorway cruising it doesnt feel too strained sitting in top at 70mph

I haven't yet looked in to what is available/buildable, purely interested in what would be optimal so I can make an informed decision.

depends a lot on the engine, but from the sierra camp of diffs, for big bokes (busa etc) 3.14:1 is liked as it gives more legs on the long straights and the engines have enough torque to deal with the taller gearing, for smaller bike engines 3.38:1 is better. 3.62 or above on a BEC would be very low geared.

[Edited on 2/11/09 by Dan.]

Yoy're wrong because the bike has far less aero drag than a seven and a lot less weight. With the same ratio diff as the bikesfinal drive (chain sprockets and wheel) you'd never get the car to set off without stalling in 1st. In 6th the R1 is geared to almost 200mph and in your 7 will top out at 130ishdue to the drag.

3.36:1 diff seems the best option in a 7. I'm running a Freelander 3.21:1. It'd be a bit high in a 7 but with the better aero of the Fury it's OK. It all depends upon the year of R1 as the ratios alter from year to year.

adrian

OK, so because of the extra weight and the aero effect, it's best to gear for a top speed based on drag predictions. It's an 06 R1 and I have all the ratios for the calculations.

My '06 R1 with 3.21 diff gets to 72 in first (a bit long), 150 in top (in theory) and cruises at just over 6000rpm at 70mph.

There's a gear calculator thread/sticky at the top of the BEC section but you'll have to play with the ratios for a later R1.

adrian

72 does seem a bit long in 1st, but then the bike will do 140 in 1st according to this. What is the engine comfortable cruising at?

ive got a 3.92:1 on my r1 and to be honest it feels like its wasting a lot of the power. it gets to 40 in 1st if i push it, and tops out easily at 108. im looking for the likes of a 3.38, but want an lsd.

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Originally posted by charlie_pank
72 does seem a bit long in 1st, but then the bike will do 140 in 1st according to this. What is the engine comfortable cruising at?

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You can LSD up a 3:38 either with the vLSD from a 3.62 or buy a Quaife ATB.

Road and Race transmissions can help with fitting it if you need it.

not my normal calculator but could be of some use to you, http://www.angelfire.com/fl/procrastination/rear.html

my normal one doesn't seem to be letting me on now.

i'm running a 2.73, this give me 61 mph at 6000 rpm on 18" 295 tyres, but i got a bit more power than you guys and its in viento

3.38 in mine

You need to work out which wheels and tyres you'll be running before deciding on a diff. Even a small difference in circumference wil make a big difference.

Then you can pick a suitable diff from here.

Thanks, I know I'm going to be running 185/70/13s and have been using the diff calculators based on that.

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Originally posted by charlie_pank
Thanks, I know I'm going to be running 185/70/13s and have been using the diff calculators based on that.

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That's an interesting question. I currently use the car most of the time on the road rather than the track, but I am more interested in setting it up to give the best handling and performance possible, than making compromises for the road. I have had a string of sports bikes and have done some touring on a GSX-R. If I can tolerate that, and given that I wear earplugs in the 7 as it is anyway, I can't really imagine having the usual objections to 'sporty' gearing in a bec.

quote:

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Originally posted by charlie_pank
That's an interesting question. I currently use the car most of the time on the road rather than the track, but I am more interested in setting it up to give the best handling and performance possible, than making compromises for the road. I have had a string of sports bikes and have done some touring on a GSX-R. If I can tolerate that, and given that I wear earplugs in the 7 as it is anyway, I can't really imagine having the usual objections to 'sporty' gearing in a bec.

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an r1 bike will do about 110 in first.

yeah loads of bikes will do silly speeds in first if you ring the neck out of them although in truth 1st to 2nd change is done early and probably 2nd to 3rd also its just too much on the road for no real gain, and its gonna loose you your licence sooner or later

on my busa geard for top speed events its about 100mph in first and change just before the red line (only 10,500 on the busa) this is a balancing act to get the front wheel almost or just off the road in first and second for max acceleration.

my bike will do xxxxxxx in first its just bragging rights really and not at all relvent to BEC...

if you want to go fast as errrrrrrrrrrrm, someone said build in lightness, but also build a full boddied car with reasonable aero's

otherwise the ideal (to go as fast as you can in the locost) would be a gearing that will see you at the peak BHP of the engine (often 1000rpm or so below red line) at the theoretical top speed of that car with that BHP, trying to gear any higher than this will not only reduce the top speed but also loose you acceleration. probably though shorter gearing and a slower top speed is preferable as it will make it more ""sporty"" and quicker accelerating, even on mst tracks your not going to be doing huge top speeds anyway

aitch