Hi All,

I am trying to get the combination right for my build and I wonder what the best diff would be to use in a BEC locost.

Cheers,

Omni

IT depends on

1) engine rev range
2) what wheels you're using
3) what the ratios in the gearbox are
4) whether you want top speed or acceleration

More info needed!

this depends on the engines power and torque, the gearing of the bike, the intended usage and the wheels and tyres used.

However, the answer is between 3 and 4.

You want your top geared speed to match how fast the engine can actually push the cars shape. With a zx10 your gearing mayas well run out at 130 or 140, though you could pick a different ideal ratio for road or any given track. 3.38 would probably be ball park, 3.62 a little shorter but no doubt adequate.

Obviously some bike engines are longer than others, i dont know exactly where to zx10 falls in the scheme of things.

There is also weight to take into account, the freelander diff is 7kgs lighter than an open sierra diff but 3.21 is too short for most bike engines. Maybe a busa or zx12 would manage with one on the road though.

[Edited on 25/6/07 by JoelP]

A 3.92 will give the best acceleration if you're happy with a 100 - 110 MPH top speed and with the engine revving at 9000 RPM on the motorway.

A 3.14 will give nicer crusing at speed and the highest top speed (probably) but you've have to try harder to pull away from rest and you won't have as much acceleration in each gear.

That leaves 3.38 and 6.62. I have the latter with my carbed R1 engine and it is fine.

Which one is best? It depends on the engine in question and has been said, what you want out of it.

Hi BenB,

Cheers for the reply. I am looking at either a 05 R1 or 05 ZX10R at the mo. Again I am torn between 13" mini lights or 15" wheels. I am really looking for all round performance as I will be using the car for road (fun) and track use.

Does this help? Prob not. lol

O

hi 3.62 lsd is a good one

quote:

---

Originally posted by smart51

That leaves 3.38 and 6.62. I have the latter with my carbed R1 engine and it is fine.

---

It's important to decide on your wheel choice as it's a major factor. The relative percentage difference between a 3.38 and 3.61 is 7%, the difference in circumferences between 15" and 13" is double that (and that's not taking tyre profiles into account).

If you narrow it down to engine, wheel, tyres etc you can then work out your diff requirement.

ps Bigger wheels and tyres = greater unsprung mass...... (go for the 13s unless you're going IRS at the back when 14s are often used).

Hi All,

I am not going for IRS so it sounds like 13" wheels are for me.

Looks like I will have to make some serious decisions soon then.

Many thanks all for the advice. Much appreciated.

Omni

http://www.locost7.info/gearcalc.php

will let you know the max speed in different gears given engine redline, tyre measurements, wheel size, diff type etc..... Decide what your realistic top speed will be and go from there....

One thing to remember is that the 7 shape is not so aerodynamically friendly so top speed is limited by that rather than power when you get above 120-130mph...

On the other hand, some people use a long diff ratio so that the theoretical top speed is faster than they'll ever go. The advantage then, though, is that the rpm at "legal limit" motorway cruising speed will be lower (ie more bearable)...

So once you've worked out your choice of diff, use the gearcalc to work out the rpm @ 70mph (honest officer) and see if that RPM for a few hours next to your ear sounds fun....

For the record my car revs to 8500 which gives a theoretical top (given my tyre, wheel, diff, gearbox etc choice) of 118mph but means @ 70mph I'm only doing around 6000rpm which isn't too bad noise wise...

[Edited on 25/6/07 by BenB]

The choice of diff in this case is going to depend on your choice of engine as the two engines you mention have quite different gearing. As an example, using 205/60 x 13 tyres and a max rev limit of 13000 and a 3.38 ratio diff

2006 R1 = 132mph
ZX10 = 121 mph

As you are wanting a trackday capable machine, you should ideally gear your car for at least 130mph (achievable in a 175bhp car) so that you don't hit the limiter on long straights - this suggests that for the R1 3.38 is ideal, although you should be able to get away with a 3.62 as the rev limit is around 13500. For the ZX10 you are ideally going to need a 3.14 - a rare ratio. you could get away with a 3.38 with careful choice of wheel/tyres.

Typically it does not make a huge difference between 13 and 15" wheels as the tyres on 15" wheels (for most common/economical sizes) have lower aspect ratio so the overall circumference is about the same . The difference between 205/60 R13 and a 205/50 R15 is about 2mph. Don't choose 17" rims unless you want to make your car handle and ride poorly.

Food for thought

Paul

Cheers for the GearCalc. I am getting some strange results. lol

earCalc v2.0.0.3 Copyright ©2001, Peter Ogden
Gear Ratio, Speed and RPM Calculator

Gear set: ZX10R
Gear 1 2 3 4 5 6 Final
Ratios: 2.553 2.053 1.737 1.524 1.381 1.304 3.620

RPM Limit: 13000
Tyre Size: 185/60 x 13
Tyre Diameter: 21.74 in
Tyre Circumference: 68.30 in


Gear Mph per 1000 RPM Mph @13000 RPM
----------------------------------------
1 7.00 91
2 8.70 113
3 10.29 134
4 11.72 152
5 12.94 168
6 13.70 178

Mph RPM (in Gears)
----------------------------------------------------------
1 2 3 4 5 6
----------------------------------------------------------
5 714 575 486 426 386 365
10 1429 1149 972 853 773 730
15 2143 1724 1458 1279 1159 1095
20 2858 2298 1944 1706 1546 1460
25 3572 2873 2431 2132 1932 1825
30 4287 3447 2917 2559 2319 2190
35 5001 4022 3403 2985 2705 2555
40 5716 4596 3889 3412 3092 2919
45 6430 5171 4375 3838 3478 3284
50 7145 5745 4861 4265 3865 3649
55 7859 6320 5347 4691 4251 4014
60 8574 6895 5833 5118 4638 4379
65 9288 7469 6319 5544 5024 4744
70 10003 8044 6806 5971 5411 5109
75 10717 8618 7292 6397 5797 5474
80 11432 9193 7778 6824 6184 5839
85 12146 9767 8264 7250 6570 6204
90 12860 10342 8750 7677 6957 6569
95 10916 9236 8103 7343 6934
100 11491 9722 8530 7730 7299
105 12065 10208 8956 8116 7664
110 12640 10694 9383 8503 8028
115 11180 9809 8889 8393
120 11667 10236 9276 8758
125 12153 10662 9662 9123
130 12639 11089 10048 9488
135 11515 10435 9853
140 11942 10821 10218
145 12368 11208 10583
150 12795 11594 10948
155 11981 11313
160 12367 11678
165 12754 12043
170 12408
175 12773

Gear Change RPM drop (change @13000)
---------------------------------------
1 -> 2 -2546 (to 10454)
2 -> 3 -2001 (to 10999)
3 -> 4 -1594 (to 11406)
4 -> 5 -1220 (to 11780)
5 -> 6 -725 (to 12275)

Any ideas?

O

You mean the speeds in each gear seem to high? Is there a 'final drive' within the bike gearbox that you haven't included? Don't know much about BECs so I wouldn't know if the nunmbers look right or not.

you need to add in the primary drive ratio to your calcs aprox 1.6 to 1 for the zx10r

I found this very useful spreadsheet on Bob Carters website (I hope he doesnt mind me linking to it). It can Calculate everything for you, and even has graphs that illustrate achievable speeds in different gears, and accelaration(theoretical). It also calculates wheel circumference when entering type, size and width.

http://freespace.virgin.net/bob.carter/files/gearcalc.xls

I am using a 3.62 for my 06-07 CBR 1000RR. If it doesnt work for me, a diff change is not the hardest job in the world.

Good Luck

Don't forget the primary reduction ratio - this effecively acts as a torque multiplier (for those CEC naysayers....).

For the ZX10 the info is :

Primary reduction 1.611
1st 2.533
2nd 2.053
3rd 1.737
4th 1.524
5th 1.381
6th 1.304

6th is not as high as the R1 and the primary reduction ratio is greater, hence the difference in gearing.

- Paul

You can download the spreadsheet in my avatar below, just edit the ratios page for gear ratios and peak RPM for your particular engine then you can select diffs / tyres from the dropdown list and see what combinations give you in terms of top speed.

Chris

Thanks very much all

Omni