ChrisC Posted May 25, 2003 Share Posted May 25, 2003 I have a 97, DeDion car, with a Caterham 6 sp, powered by a VX 2.0l on carbs. I know that 6th is 1:1. Can anyone tell me the rolling diameter of 7.0/21.0 x13 ACB10’s on 7x13 wheels and the final drive ration I might have fitted???? I hit the rev limiter on the straight in 6th yesterday at Bedford, I have a DL90 fitted which gave a top speed of 122.4 mph, how realistic does this sound?? Chris.... H15 SVN Get it ! Link to comment Share on other sites More sharing options...
Chelspeed Posted May 25, 2003 Share Posted May 25, 2003 The 21 in 7.0/21.0-13 on the ACB's means 21 inch diameter. So the nominal circumference is 1.68m. Standard diff ratios from Caterham are 3.6:1 or 3.9:1. With a 6 speed box and it's low top I would think it's very likely Caterham would have chosen 3.6:1. The only thing you forgot to mention is what the rev limiter is set to. Link to comment Share on other sites More sharing options...
Chris W Posted May 25, 2003 Share Posted May 25, 2003 Sorry to disagree but the Caterham 6 speed box is coupled to a 3.62 final drive ratio normally as standard. Also the calculation with 21" diameter wheels plus the standard Caterham 6 speed box and a 3.62 final drive means you should get 17.1mph/1000rpm in top. That would equate to 7158 revs at your speed of 122.4mph. If you happened to have a 3.92 final drive ratio with the same gearbox, that would give 15.9mph/1000revs in top which would in turn give 7698rpm at 122.4 mph. So you should be able to confirm your final drive ratio from the revs achieved at 122.4mph. Chris 1.8K SV 140hp see it here Edited by - Chris W on 25 May 2003 21:16:37 Link to comment Share on other sites More sharing options...
ChrisC Posted May 25, 2003 Author Share Posted May 25, 2003 I only know the shift light was set at 7200, the limiter cuts in a little way past that. I assume the limiter is a function of the Caterham supplied MBE 906 inginition unit (this must be a safe assumption) but I don't know the cut point. 7600 seems a little high, and 7160 is below the sift light setting Chris.... H15 SVN Get it ! Edited by - ChrisC on 25 May 2003 21:22:08 Link to comment Share on other sites More sharing options...
Chris W Posted May 25, 2003 Share Posted May 25, 2003 The accuracy of the calculation is of course dependent on the accuracy of your speedo reading. Any ideas how accurate it is? Chris 1.8K SV 140hp see it here Link to comment Share on other sites More sharing options...
ChrisC Posted May 25, 2003 Author Share Posted May 25, 2003 The speed comes from a DL90 Data Logger. The speed is from the GPS, not the Caterham POS they call a Speedo . How much effect will tyre wear have on the calculation, ie the ACB10's are near the end of there life, so would this acount for say an extra 100 rpm at that speed. This would mean 7258, 58 rpm after the shift light. Chris.... H15 SVN Get it ! Link to comment Share on other sites More sharing options...
Chris W Posted May 25, 2003 Share Posted May 25, 2003 Well if the limiter is set at say 7300rpm (you said it's just past the shift light setting of 7200) then a 1.9% error in the calculation due to tyre wear (ie: different diameter) or speed reading accuracy would account for the difference. I think you can fairly safely say you do not have a 3.92 final ratio as the figures would be way out. Also how accurate is the setting of the shift light... or the rpm reading itself? Chris 1.8K SV 140hp see it here Link to comment Share on other sites More sharing options...
ChrisC Posted May 25, 2003 Author Share Posted May 25, 2003 The shift light is a SPA digital unit, It has problems at idle, but who knows how accurate any of this is. I think the final drive must be 3.62:1, not 3.92:1. Many thanks *thumbup* Chris.... H15 SVN Get it ! Link to comment Share on other sites More sharing options...
Chelspeed Posted May 25, 2003 Share Posted May 25, 2003 > Sorry to disagree Chris W - Now I'm confused, could have sworn I said the 3.6 was more likely that the 3.9? Sounds like you're agreeing with me. The 21.0 is a nominal diameter, you can get a better idea from http://www.avonracing.com/ follow the race tyres, road legal link and you get 20.70 inch for the actual diameter. Link to comment Share on other sites More sharing options...
Tony C Posted May 25, 2003 Share Posted May 25, 2003 Chel, I think it was the 0.02 missing off the final drive ratio that was being disagreed with BRG SV 😬 aka Deep Throated Link to comment Share on other sites More sharing options...
Chris W Posted May 26, 2003 Share Posted May 26, 2003 Chel My apologies.... I misread your post somehow as "unlikely" rather than "likely". I must book that trip to the opticians! So I do agree with you. I checked the Avon site you suggested and the actual diameter they give for the "21's" is 20.7". Re-running the maths and assuming the final drive is 3.62 equates to 17.0mph/1000revs for the 6 speed box in top. This would give exactly 7200rpm at 122.4mph. So it sounds like that tiny difference (24mm less distance per tyre revolution) more or less clears up the gap. cheers Chris 1.8K SV 140hp see it here Edited by - Chris W on 26 May 2003 12:19:09 Link to comment Share on other sites More sharing options...
ChrisC Posted May 26, 2003 Author Share Posted May 26, 2003 *thumbup* Many thanks *thumbup* Chris.... H15 SVN Get it ! Link to comment Share on other sites More sharing options...
Chelspeed Posted May 26, 2003 Share Posted May 26, 2003 So are you happy with this? Was it just an intellectual exploration of what diff ratio you have? Or in the back of your mind am I correct in infering that you think 122mph is a little low for flat out, top speed, on the rev limiter in top type of thing? If so you have two choices, well three I guess but the third is probably not what you want to hear. First choice (and cheapest) is different tyres, a bigger diameter will give you higher speeds across the board. The most obvious would be 7.0/22.0-13 ACB10's with a diameter of 21.9 inch, this would give you 6% more speed or 129mph top speed. Second choice is a new diff ratio, Sierra diesels came with some ridiculously low ratios, luckily for Caterham. Both 3.38 and 3.14 are available. These would give you 7% more diff and 130mph or 15% and 140 mph top speed respectively. This is the route the SLR racecars went when they were using the 6 speed box and daft size 20 inch diameter slicks a couple of years ago. Thirdly (and likely to be least popular) option is that you could get a gearbox with an appropriate set of ratios. The 6 speed box is designed to tame a 1400 K series with a wild set of cams that makes power from 8000rpm to 9000rpm. With a torquey Vauxhall motor you can get away with a much higher 1st gear, say 2.3, and can pull a decent overdrive top gear, say 0.8. So a Quaife straight cut clubmans gear conversion on a type 9 Ford box would be a well proven, cheap and off the shelf solution, You could flog the 6 speed and easily cover the cost with some left over. An upgrade that makes money, could be a first. All you'd lose is the pub boast of having a 6 speed box. Fourthly (spanish inquisition anyone?) you could stick in a set of ARP rod bolts which would let you rev to 8000rpm. This would give you 11% more top speed (or 135mph) without changing anything else. On second thoughts this seems like the best option of all, about £50 of engine mods, keep everything else the same, and get another 13mph top speed. Bargain. Link to comment Share on other sites More sharing options...
Peter Carmichael Posted May 26, 2003 Share Posted May 26, 2003 Factual correction: 1.4 Ks didn't rev above 8000rpm. Link to comment Share on other sites More sharing options...
ChrisC Posted May 26, 2003 Author Share Posted May 26, 2003 Hi Chelspeed The truth is I am not disappointed with 122mph. If I making the following assumption, having the accelerator welded to the floor, all the way up the straight would mean I was getting the max out of my VX. Couple this with the fact that I was only hitting the limiter in 6th at the end of the day when I was carrying more speed into the straight, and braking later at the end. It would be pointless having a higher to top speed, at the inevitable cost of slower acceleration. However if the limiter was cutting in mid way down the straight then the gearing would have to be addressed. When I am a better driver (I think that should be if and when) and carry more speed into the straight, the gearing will become a problem. Until that time 122.4mph (.4 is all important) is fine with me. Chris. PS Like the plan with the bolts. 😬 Link to comment Share on other sites More sharing options...
Chelspeed Posted May 26, 2003 Share Posted May 26, 2003 Peter - Fair enuff, I exaggerated. Chris - Fair enuff too. It's not til you sit on the limiter for half the straight that it's worth changing anything. Link to comment Share on other sites More sharing options...
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