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BEC Diff ratio issues...
tegwin - 27/4/08 at 09:50 PM

Following on from this thread:

http://locostbuilders.co.uk/viewthread.php?tid=87946

Im trying to work out if my engine/axle combination will be vaguly usable....Its an all out track day car so practicality or fuel consumption are not an issue (within reason)!

Really struggling to get my head around it, can someone with more brain cells than me please have a look at these figures and see whats what?

Engine- ZX9R (C series I think)

Rear axle- BMC A-series. (input shaft approximately 2.2 turns to 1 full wheel revolution...only one wheel was turning though, if that makes any difference?)

Wheels- 160/515/13


Any help on this would be really appreciated.

[Edited on 28/4/08 by tegwin]

Andy W - 27/4/08 at 10:21 PM

I'm using a 3.62 diff with 15" wheels, gives me about 10mph/1000rpm in top.

Andy

adithorp - 27/4/08 at 10:22 PM

If only one wheel is turning then it turne twice as fast so that'd make you diff about 4.4:1 which is pretty low geared. Thats if I've got it right!

adrian

mark chandler - 27/4/08 at 10:42 PM

Really low diff that 4.4:1

So had a quick google and a zx9 C model engine 1990 - 1993, this also applies to D model 1993 - 2001

Primary 1.637
1st 2.800
2nd 2.000
3rd 1.590
4th 1.333
5th 1.153
6th 1.035


Which then gives on 205/55/15 tyres

Speed in 1st (mph) 49.78092836
Speed in 2nd (mph) 69.6932997
Speed in 3rd (mph) 87.66452793
Speed in 4th (mph) 104.5660911
Speed in 5th (mph) 120.8903724
Speed in 6th (mph) 134.6730429


Which looks pretty good. But do not shot me down if its wrong!

bigrich - 27/4/08 at 11:25 PM

i think the calcs are incorrect, i would say with a 4,4 and a 1,6 primary you would be lucky to get much over a ton, flat in 6th

my zx10r has a 1,6 primary and does just over 130 on a 3,21 diff so is roughly 25% taller geared .
so loose a quater of the top speed to be nearer the mark, IMHO

Paul TigerB6 - 28/4/08 at 02:53 AM

quote:
Originally posted by bigrich
i think the calcs are incorrect, i would say with a 4,4 and a 1,6 primary you would be lucky to get much over a ton, flat in 6th



My gear calculator agrees with this - 101mph at 12000rpm in top assuming a 4.4:1 ratio and 205/55/15 tyres.

So what size tyres are you actually using??

tegwin - 28/4/08 at 08:28 AM

Not sure on the actual tyre, will have to check. But they have a total outside diameter of 15"...(Im guessing they are 13" rims)

Which will make the diff ratio even more useless! Bother!!!

I have found a couple of places to get different diff ratios....not cheap mind you, but I have little choice.

How do I work out what ratio would be ideal for the engine/wheel combo?

[Edited on 28/4/08 by tegwin]

Paul TigerB6 - 28/4/08 at 11:26 AM

15" (381mm) total diameter!! Thats tiny and if they are 13" rims that means the tyres are rubber bands with 1" sidewalls. Definately only 15" total diameter???

Top speed with a 4.4:1 diff would be 63mph anyway!! I'm guessing you will be wanting a top speed of around the 120mph mark with the power of the ZX9R to make best use of it. That would suggest a diff ratio of around 2.3:1 if those tyres really are 15" total diameter which would give you 121mph at 12,000rpm in top.

tegwin - 28/4/08 at 11:48 AM

You have a very good point, 15" total diameter does not sound right atall!.....

I will check when I get home so we are not p133ing in the dark!

Paul TigerB6 - 28/4/08 at 12:05 PM

quote:
Originally posted by tegwin
You have a very good point, 15" total diameter does not sound right atall!.....

I will check when I get home so we are not p133ing in the dark!


If you get an accurate tyre size then the figures can be popped into the speed calculator i have here (its in the site somewhere too). If you know roughly what top speed you want then we can calculate the required ratio easily

tegwin - 28/4/08 at 06:07 PM

The wheels are Yokohama Advan 160/515/13 (515 seems a weird number!)

What is a decent top speed to aim for on a track car? 120~130mph?....Although acceleration out of corners is more important!

Paul TigerB6 - 28/4/08 at 07:38 PM

Yeah - racing radials are coded differently (and i'm not sure how exactly - maybe width in mm / diameter in mm???) but a search of t'internet says an Advan of that size is 516mm diameter.

So, with a 4.4:1 diff it calculates out to give you a top speed of 86mph. For the track i'd say 120mph is probably a good guess - not sure if the ZX9R will get it up to 130mph due to the aerodynamics - anyone????

If you want to gear for 120mph then you will need a diff ratio of 3.14:1. So what ratio's can you get between 3 - 3.5:1???

tegwin - 28/4/08 at 08:11 PM

Thanks for that ...86MPH perhaps is not ideal!....

According to the highly knowledgeable chaps on 750 formula.co.uk...

The standard diff ratios for this axle are 5.3, 4.9, 4.5, 4.2, 3.9, 3.7. The 3.7

It is also possible to get a 3.5, but that is non standard and therefore might be prohibitively expensive!

I know I can deffinately get a 3.7.....If its not too difficult, could you tell me what the relative top speeds would be with a 3.7 and a 3.5?

At a guess, the 3.5 will be favourable?

Paul TigerB6 - 28/4/08 at 08:16 PM

3.5:1 gives you 108mph max, 3.7 gives 102mph.