Apologies now if I'm treading old ground.
I was advised (incorrectly as it transpires) by S-T to use a 3.89:1 diff for my CBR1000FP (Damn me if he didn't have one to hand: lucky me). Having now got it on the road it is way undergeared (I sort of guessed some reduction in gearing)Today I did the calcs, with a top bhp declared at 9,500rpm the bike is geared, long legged, to do 147.5mph.
This was assuming the bike wheel at 1921mm circumference, with final drive at 2.47:1 (17t:42t). My 185/60x13" tyres are 1694mm around. With the 3.89 diff I'm looking at 83mph in top @ 9,500rpm!, and with the newly acquired 3.54 diff, 91mph. Feck. To get to 115mph @9,500rpm I'll need a 2.80:1 diff. Unless someone knows of a Mk1 or 2 diesel escort I have a few serious decisions to make. I cannot be alone in this dilemma (bloody feels like it). I've had a few thoughts (some a little less sensible than others).
1. Source a crown-wheel/pinion to give me the right ratio (could this be done by mix-n-match? - I think not). Therefore a custom jobby.
2. Any way sierra bits can be bastardised to fit (m/c gears?)
3. Go IRS/de-dion
4. Make an axle using a sierra, or other, diff (yes I'm serious!)
All comments welcome, those that are helpful and constructive even more so.

p.s. I'm sticking with 13" wheels
pps. 3.54 and 3.89 diffs anybody?

This is almost blindingly simple.

I think.


GTS De dion axle - fits your chassis and allows Sierra diff.

Gawd - I hope I'm right.

I got this excel spread sheet from Yahoo groups I think, It calculates speeds in gear for given bikes and diff ratios.

If the rev limit is 1100rpm then the top speed would be 95mph with 3.89 or 105mph with the 3.54.
It does seem to be the lowest gearing of the few bikes in there.

Chris, trust me, I've done the sums. The CBR has a low speed output at the sprocket, god knows why (somebody sowed this seed of doubt with me a while back). Just did another check: taking the tyre circumference and diff/final drives cumulatively I am ~44% down on the bike ratios, therefore 147 becomes a theorectical 83.
Cheers Zil, Gawd, I hope you're right!
Just wonder how much welding I'll have to do to get the diff mounts in. Car weighed in at 430kg wet. How much will a de-dion add to it?

did you already looked / thinked @ your clutch??

how do you know if it loses for 100%??

can you make/show some pics of the M/C used for it??

have same prob..

TKS

tks: You have u2u

Kissy,

Am i right in thinking that you are using a modded F27 Chassis?

If you are i have plans for the rear end IRS for a F27, a mate of mine is already looking at taking on the conversion next winter.

Might be worth a go, at least it will free up a lot more diff ratios for you. I estamlished early on that the CBR was one of the more poorly geared engines available thats why im looking for a 3.38 to chop about the internals and make an LSD out of it. would still top out about 110 i think.

It's more locost than F27. Mine is a 'ground up rebuild with updates'! Hence has trailing links to rear axle. Could never understand why Steve Porter used forward facing links. His answer was "...Bugatti did it...", my reply was that even Etorre got it wrong sometimes. What rpm did you calculate 110 being achieved at with a 3.38?

cant remember the rpm but i seem to just remember the 110mph bit, maybe i was pissed at the time and it was nothing like 110?

HI there,

Yep - this is a problem for me too!

Heres the cure - as far as I can tell anyway...

According to the bike gear ratio calculator posted earlier on (which I have used to work out my ratios) I'll be getting around about 120-125mph with a 3.38 rear diff and 195/50/15 wheels and tyres running at 10,000 rpm in 6th.

I can live with that as a top speed - its just the sligt buzziness on long journeys which might get to me after a while but it'll be worth it for the low down acceleration - just get ear plugs!!

Jeef

BMW diffs come down as low as 2.56:1, E36 325 TD, you can also get 2.9 something then 3.07, 3.15 etc. They're plentyful, cheapish and the case can be modified by welding on a couple of bracks and grinding off a couple of others to make it compatible with Sierra chassis mounts.

Bit heavier than a Sierra diff though. Can get the LSD gubbins too for £200 extra.

Nice one, I've also seen an X-Type Jag diff on ebay with suitably high ratio (diesel), but he was being a bit greedy (can't think where he got a brand new diff from??). I can't believe someone has not, at some time, made a high ratio c/w & p for the english axle - what are the chances of modifying those out of IRS diffs??

Jambo, is that chart referring to the CBR1000F or RR? Just a thought.

I recon its the 1000F , isnt the RR a fireblade?

Yeah it's the latest 'blade - just waiting for one to pop up in a car! Somebody must have dropped their CBR1000RR by now.

This diff ratio thing is a problem for Trike builders too. The best Diff I could come up with was a SIERRA 2.3 V6 Mk1 Ghia on a "Y" plate it had a 3.14:1 diff. the same diff may be fitted to the Diesels. Look for a donor with a "M" on the VIN plate.

I've just taken a flyer on a Mustang diff on ebay (internals only) 7.5", 2.46:1. It really is a flyer as I am hoping to get the bits into either a Sierra or English diff, bought for 10 quid. I'm keeping my options open as I may do GTS de-dion. Whatever there has to be a way to get high ratios into these diffs without making custom gears.

Yep - is the CBR1000F - I've got the Haynes if you want all the gear ratios and primary reduction onto the clutch basket if you want it?

Have you got it sorted now?

Not sorted, but adamant my sums are good! Have just bought s/h GTS de-dion beam, so on lookout for the other bits now!

Disc-braked English axle 3.89 & 3.54 ratios, all brackets, and discs (no calipers I'm keeping them) - any takers??

Are bigger wheels and tyres not an option?

I want to stick with 13", partly aesthetics, partly weight/drag, partly clearances. Now have GTS de-dion beam. Am also looking at the possibilities of getting other CWs & Ps into the english axle - just to be different, and for the challenge.

Bah.... You advertise your 3.54 now that I've got one!

Fair enough. I hope mine's OK i got 16" and a 3.54 to help resolve the gearing problem but the offsets are very large so i've just ordered some spacers to stop them rubbing on the trailing arms, hopefully this'll solve the problem - if not they'll be going on Ebay!

That sounds a bit more serious if the offset is that profound. Are they intended for RWD application? Sure you can correct for the rear wheels, but what about the fronts, and your geometry (KPI etc)?

I'm currently stuck with the same CBR gearing problem and was considering going the bigger wheel route to get round this.

Matt - Is your wheel offet positive or negative. I thought most common wheels have a negative offet, meaning if your wheel are catching on the inside a greater offset is required.

Is this correct or I have got things the wrong way round (Again!)?

Thanks.....

I hate posting links to eBay but this looks promising too . . .

eBay Mercedes LSD diff

Is the book KPI not zero? The spacers should set the scrub radius to be slightly positive as required.

Matt

That Merc LSD Diff is a relist. There was a brand new X-type diesel diff in last week, never sold (bit too pricey)
I doubt Ron Champion knew what KPI was when the book was first written.