final drive ratio

[Beej]

Can anyone tell me what the correct (if there is one) final drive ratio is on an HPC racer?

 

I am a tad confused at the moment. I have bought a very well known Vx engined racer which purported to achieve an average 85mph over 24 hours. However, the car currently manages around 70 mph (speedo doesn't actually work, so this was measured by sign language with my wife in the car in the next lane - v. scientific!) at 4000rpm in 5th (which on the type 9 box is apparently a 13% overdrive). It currently has a 3.9 diff and runs 13" wheels with (from memory - cos the car is back at C) 175 Avon ACB10s

 

I have been advised that changing the tyre width will change the rolling diameter and hence restore a sensible ratio of around 19.5 mph per 1000 rpm.

 

Does anyone know how realistic a simple tyre change is at improving the final drive ratio or should the car really have a 3.6 diff?

 

I should add that the factory are trying to be helpful here but we are all struggling for an answer. I know that they are trying to get hold of Jez Coates to see if he can remember what wheels/tyres/final drive were orginally fitted.

 

Cheers

[Chris W]

Beej

 

Tyre width itself doesn't affect rolling diameter. The confusion is that the aspect ratio figure eg: the 70 figure in 185/70 means the tyre height is 70% of the tyre width. So for a tyre width of 185mm the side wall height will be 70% of 185 = 130mm.

 

The rolling diameter will therefore be twice the sidewall height (since there are 2 sidewalls across the full diameter) plus the wheel diameter. So if you have 13" wheels with 185/70 tyres (note the crazy change of units used in the tyre industry) the rolling diameter will be

 

2 x (185 x 0.7) + (13 x 25.4) mm = 589mm or 23.2"

 

Therefore in each revolution of the tyre, the car will move forward (23.2 x pi)" = 72.9" (over 6' per revolution)

 

If you tell me the tyre size fully eg: 175/70R13 or whatever, I can calculate your 5th gear ratio from the other info you supplied and suggest a tyre/wheel combination that will give you your desired mph/1000revs.

 

Hope this helps

 

Chris

 

1.8K SV 140hp see it here

 

 

 

Edited by - Chris W on 30 Apr 2003 19:06:21

[westy]

Have a look on George Polleys site for rolling circumferences here

They do vary on different widths with the same ratio

[Chris W]

Westy

 

They do vary on different widths with the same ratio

 

You need to understand the calculation I just posted and the way tyres are marked.

 

If the width varies but the ratio figure stays the same, eg: 185/70 compared to 175/70 then the sidewall height is DIFFERENT because 185/70 gives a sidewall height of 130mm and 175/70 gives a sidewall height of 123mm. viz: the sidewall height is the aspect ratio figure in percent (eg: 70) multiplied by the width.

 

So changing the width but keeping the same aspect ratio WILL change the rolling diameter, but only because the aspect ratio is a percentage of the width, not because of the change of width itself.

 

cheers

 

Chris

 

1.8K SV 140hp see it here

 

 

Edited by - Chris W on 30 Apr 2003 19:33:31

[Paul McKenzie]

VX challenge cars used 7/21/13 on 6" rims

Paul

[Beej]

Paul, sorry to be thick but what does 7/21/13 mean? - I guess 13 is 13" wheels.

 

Thanks everyone else and for the avoidance of confusion I was assuming a constant aspect ratio (hence rolling diameter would increase with tyre width). I am also aware that actual tyre width at a stated measure eg 175 seems to vary with manufacturer and tyre type (dunno why, it just does).

 

My main concern was how a modest change in rolling diameter could put 20 mph on the top speed? I suppose I just need to do the maths.

[Beej]

Doh! I've done the sums now and it makes naff all difference. The rolling diameter of a 175/70/13 tyre is 71.09" and that of a 185/70/13 tyre is 72.82" = a difference of just 2.4%. In other words, all other factors being equal an increase in speed per 1000rpm of 2.4% = not a lot!

 

May be thats why regular road HPC cars run 16" wheels?

[Peter Carmichael]

7 inch tread width

21 inch diameter (varies quite a lot)

13 inch rim

 

This is standard sizing for crossply tyres like ACB10s. Assuming a 7 inch ACB10 ( diameter from Avon racing website is 20.7inches)

 

Numbers for a Quaife high first 5 speed are:

 

Gear   Mph per 1000 RPM   Mph @4000 RPM   Mph @8200 RPM

--------------------------------------------------------

1 7.70 31 63

2 10.20 41 84

3 12.98 52 106

4 15.71 63 129

5 18.06 72 148

 

Mph RPM (in Gears)

-------------------------------------------------

1 2 3 4 5

-------------------------------------------------

5 649 490 385 318 277

10 1299 980 770 637 554

15 1948 1470 1155 955 831

20 2597 1961 1540 1273 1108

25 3246 2451 1926 1591 1385

30 3896 2941 2311 1910 1661

35 4545 3431 2696 2228 1938

40 5194 3921 3081 2546 2215

45 5844 4411 3466 2864 2492

50 6493 4901 3851 3183 2769

55 7142 5392 4236 3501 3046

60 7791 5882 4621 3819 3323

65 6372 5007 4138 3600

70 6862 5392 4456 3877

75 7352 5777 4774 4154

80 7842 6162 5092 4430

85 6547 5411 4707

90 6932 5729 4984

95 7317 6047 5261

100 7702 6366 5538

105 8087 6684 5815

110 7002 6092

115 7320 6369

120 7639 6646

125 7957 6923

130 7199

135 7476

140 7753

145 8030

 

Gear Change RPM drop (change @4000) RPM drop (change @8200)

-------------------------------------------------------------

1 -> 2 -980 (to 3020) -2010 (to 6190)

2 -> 3 -857 (to 3143) -1757 (to 6443)

3 -> 4 -694 (to 3306) -1423 (to 6777)

4 -> 5 -520 (to 3480) -1066 (to 7134)

[Tight fart]

Peter is that with a 3.9 or 3.6 dif?

I always assumed my 1989 de-dion had a 3.92 diff, but the car is stripped at the moment

and the sticker on the top of the diff looks like it's a 3.62.

 

SLR for sale

T.F@O.F.

www.griptv.com

[Beej]

Blimey, 18.06 mph per 1000 is not a lot - no wonder they still accelerate so hard in 5th!

 

But 8200rpm? Never having owned any Vauxhall or Vauxhall engined car before (no mean feat when you have owned as many cars as I have) I asked caterham what the red line was and they said 6500rpm..

 

that probably explains why the shift light wasn't coming on.

 

But then it doesn't explain why the engine cut at 6500 - unless it was just a serious misfire?

 

Does the XE really rev that high.. safely?

[Paul Ranson]

When I first read this I thought that the gearing sounded about right for a race car. I'm sure that 6500 isn't the rev limit that carb'd VX engines were restricted to. 7500 sounds more likely, giving 130mph flat out. Crossplies usually grow slightly so raising the top end a bit.

 

4000rpm/70mph is the same gearing that a 3.62/6-speed/ACB10 car runs, if you have an overdrive top gear but shorter (3.92) rear end then that would seem plausible.

 

To make things more comfortable for road usage you have to raise the gearing, larger radius tyres or a longer rear end. Moving to radials, 185/60 for example, might be enough. You will need to consider what ratio first gear you have, 60mph/7500rpm is fine for competition but probably a tad impractical for road use, if you have a close ratio box you may need to consider changing it.

 

Paul

[Beej]

Hmm.. one small problem, its not on carbs. It runs the standard Vx injection set up. This was apparently retained for reliability. Caterham reckon its a 175bhp engine (although it was advertised, by them, at 190bhp).

 

The key here is, am I getting what I bargained for? They knew I wanted a track day car that could pull a decent top speed down the straights - by decent I mean 130 mph.

 

I accept that I cannot expect them to pay for modifications that were never present in the original car and that for posterity I need to stay true to the original spirit of the car. But I still can't figure how with that rev limit, tyre size and final drive ratio they managed an AVERAGE 85mph, including stops for driver change, fuel etc over 24 hours? They must have been pulling 120mph+ down the straight? How can they do that when the gearing says 117 at 6500?

 

I guess I will have to wait and see but on the face of it they will have to change either the rolling radius or the diff ratio? Which would you do?

[Ken elle]

I reckon a 13" 195/50 aspect tyre with a 3.92 diff will be doing about 4000 reves at 70,

at 6500 revs that will be about 115mph max.

a 3.62 diff would give you 125mph at the same revs but at 70 mph it wil rev at 3600.

 

Can you borrow A GPS (Blackspot would do) to check you speed.

 

[Tight fart]

Hmm Just checked my gearing for my new engines rev limit.

 

186mph 😬

 

Standard ford 5 speed, 3.62 diff and 14" wheels.

 

Somethings gotta change

 

 

SLR for sale

T.F@O.F.

www.griptv.com

[Peter Carmichael]

Sorry. The 8200rpm was a leftover figure in the gearing program that I didn't change.

 

I might have made a few mistaken assumptions if the car is an injection Vauxhall. It would seem unlikely that it would be specced with the Quaife box that I picked. This means that if you have a stndard type 9 gearbox you probably get a few more mph/1000 in top

[Peter Carmichael]

Whatever, it doesn't sound like an HPC racer...

[Paul Ranson]

VX Injection would be 6500rpm, the standard Bosch ECU would control that.

 

There are photos of the Nelson Ledges car in Chris Rees book. It is confirmed as a Vauxhall with the standard plenum injection and a Motronic ECU but a long primary 4:1 exhaust. The tyres are Yokohama A008R. Dymag have a logo on the side of the car so that would suggest what the wheels are, are these original JPE spec? That would be 205/50-15 on 6.5x15 front and 225/50-15 on 7.5x15 rear.

 

Anyway the rolling radius of the rear wheels would have been substantially more than a 21inch ACB10 on a 13 inch wheel, and that's where the gearing difference comes from.

 

So your choice is to return to 'original spec' wheels and equivalent tyres, you can get Yokohama A032R in the right sizes, the rears have a diameter of 24inches, I'll let you work out the resultant gearing, or to change the diff to 3.62 and live with the small wheels/tyres.

 

FWIW I'd put original spec wheels/tyres on the car. Perhaps Caterham can sort you out.

 

Paul

[Beej]

Thank you Paul and I am impressed with the details you could work out from the book. The car was fitted with ACB10s when it was rebuilt at Caterham 3 years ago. I don't know what tyres were fitted previously but it stands to reason that they were Yokos - clearly I would prefer ACB10s given the choice but I am a fan of AO21s for wet weather.

 

The minilite style wheels look identical to the ones in the picture on the front cover. I haven't looked closely at the pictures inside the book to see if they show the wheel style. None of this became an issue until I drove the car 2 weeks ago. I am the first person to have driven it on the road since it was brought back from the US. It lasted 24 hours before developing a major misfire. Since then it has been back at caterham having a whole bunch of stuff put right.

 

For the doubting Thomases it is claimed that it was a stock Vx racer with a blue printed XE retaining the original factory injection system. It has a Quaife straight cut box with an o/d 5th (I thought at first it must have been a direct 5th because of the revs/subjective road speed- don't forget I didn't have a working speedo). All the other usual stuff applies - non adjustable dampers but with adjustable spring platforms, 250lb front springs, thick anti roll bar (don't ask me how thick cos they all look weedy compared to most other cars but thicker than the usual spaghetti), AP 4 pots at the front. It did have APs on the back but they have changed them to the standard set up so that there is a mechanical handbrake (the hydraulic one wouldn't get thru the MOT). Feels like a quicker rack but it could be my imagination - and obviously it has no screen, doors, trim, passenger seat etc. I am told it has a slippy diff in the Dedion but I am now starting to doubt anything I have been told.

 

I'm not sure about the exhaust primaries - they look exactly like a normal competition set - ie 4 pipes exiting the engine bay and then merging to a single pipe shortly before the silencer. But then, I thought all caterhams did that? My previous 2 did!

 

Am I having my plonker pulled?