Upgrading to 205 rear tyres

[Shaun_E]

As I am planning a power upgrade to c.200bhp I feel that I ought to fit wider rear tyres. I am currently running 185/60 R13 A048Rs on standard fit 6" KN Minatour (minilight style) wheels.

I have read through the archives and it would appear that there are some issues with moving to wider rears.

My first choice would be the new Caterham 8 spoke 13" rims in 6" and 8" but there appear to be issues with fitting the rear wheels to earlier De-Dion cars (mines a 1997). Is this easy to get around (I don't have an angle grinder) and are there any other problems beyond moving the position of the ARB drop link and grinding the De-Dion? My ARB is the over the diff one. Has anyone got a picture of where to remove metal from the De-Dion or is it obvious?

If this is a real pain, has anyone used other suitable wheels. I have seen the Compomotive CXR and the ML used on sevens - can these be ordered in the correct backspace and offset?

 

Yellow SL #32

[Gambo]

It's obvous where to grind the dedion, takes about 5 mins a side. CXR's were designed for the Caterham and alike so offsets are not a problem. The ARB drop link is easy, just unbolt and bolt on the other side of the mount, do same to ARB. If you do not have a grinder, hire one. Your spedo will read slow as well, so watch out for speed camera's.

 

RED 2.0 HPC now on throttle bodies

[I reply to every thread]

I needed an angle grinder & it still took about 15 minutes. It's easy enough to do but I am going to keep an eye on the welds attaching the endplate to the de dion tube & maybe invest in a new tube.

 

Other than ARB the only other issue is ride height. The lower profile tyre takes mine (from the old 60 profile 32's) just a little too low. Obviously that's not a problem if you have adjustable platforms.

[SLR No.77]

I may have misread something, but 60 profile 205s should sit slightly higher than 60 profile 185s, so shouldn't be a problem. It may actually improve the handling.

 

Stu.

 

[EFA]

60 profile 205's are quite a bit bigger than 185/60's so you will need to change your diff!

[Shaun_E]

Speedo not a problem as Stack sensor reads off front wheel (and is adjustable anyway).

Ride height will be corrected with adjustable platforms (and new dampers 😬).

Diff is 3.62 as fitted to Superlight - I presume this is also the ratio fitted to SLR/R400. I hope that should not therefore cause a problem with a 200bhp engine. Anyone know for sure what ratio the SLR/R400 uses?

 

 

Yellow SL #32

[Shaun_E]

Quick check on George Polley's site and the difference in diameter between 185 and 205 is 23mm. If stretched over an 8" rim will this difference be reduced?

 

Yellow SL #32

 

Edited by - Shaun_E on 5 Sep 2005 18:33:07

[Gambo]

I don't think a signifcant enough amount to matter.I agree about your diff,if you have a 6speed box 200bhp on a 3.6 diff will be fine. Ask no limit's for his opinion, that is what he has got in his R400.

 

RED 2.0 HPC now on throttle bodies

[F355GTS]

Shaun

 

I've got an angle grinder you can borrow

 

re the tyre size, the reality is that a 12mm difference in wall height will make little difference to the overall performance, on paper it will have a higher top speed but slightly slower acceleration, if you strethc them over an 8" rim (as I do) I suspect it will help a little and alowing for 6mm odd of tread you are getting into the range of a worn/ new tyre diameter difference, fit the 205's and see how you get on

 

Mark

 

Edited by - F355GTS on 5 Sep 2005 19:18:48

[EFA]

"the reality is that a 12mm difference in wall height will make little difference to the overall performance"

 

I disagree if I may Mark.

 

It will noticeably blunt the acceleration. It will also mess up the ride height. If you are going to do this I would almost bet you'll be fittting a 3,62 diff at the same time. Aslo the raise in ride height (if you do not have adjustable platform dampers )will totally negate the increase in mechanical grip - maybe more.

 

You will not get any better hjandling on the road fitting these tyres. If you dr5ag race at traffic lights you may get more traction, but the advantage is lost as the compound gearing increases.

 

The only upgrade is to go for the ultra low profile 32R's or CR500's (or ACB10's if you are into a twitchy ride) on the SLR/R500/"new grey alloy" type wheels

 

 

[Shaun_E]

Arnie - diff is currently a 3.62.

Current power is 135bhp (1.6K SS) and new power will be 200bhp (1.9K with ported VVC head, TBs, etc.)

Ride height is irrelevant - this will be sorted with a flat floor set-up as new dampers will be fitted at the same time.

 

So the real question is: with 200bhp should I fit 205 section tyres? I want the power upgrade to be very noticable - a 50% hike should be! Is traction going to be a problem with 185 section tyres? Since SLR/R400 use 205s with a 200bhp upgrade they seem the logical size to use.

 

I am prepared to go to CR500s if necessary but would prefer to stick with Yoko A048R as they are a lot cheaper and offer similar performance.

 

 

 

Yellow SL #32

[EFA]

Shaun

 

If you want to discuss I'm on 07710 ****** as you have raised a few points which would result in too much typing!

 

 

 

Edited by - EFA on 7 Sep 2005 15:56:56

[Mick Day]

What's wrong with using 6" wheels for 205/60/13 tyres? GP's website says they'll fit.

[Shalders]

I've an R400 racecar which had a 3.62diff as standard, I changed to a 3.9diff ratio as I went from the old slicks to 205/60 Yokos (on 8" rims incidentally, I think I'm correct in saying this gives a stiffer sidewall to fitting the tyres on 6" rims). This ratio diff for the tyre size seems to be a common choice and is the best compromise for most tracks. I've never come near the rev limiter in 6th....3.6 perhaps only useful if you plan on going Autobahn blasting!!

 

 

[Shaun_E]

Having chatted to Arnie and played around with gearcalc I understand that 205/60 Yokos are significantly taller than 185/60 to the extent that acceleration would be appreciably blunted (difference in diameter is 23mm). As I wish to see a significant performance difference with the engine upgrade, it would be pointless to waste some of that with taller effective gearing. Avon CR500 in 205/55R13 have an almost identical diameter to the 185/60R13 Yokoham A048R so this would be the most effective tyre with the 3.62 diff. If I really wanted to stay with the Yokos then a 3.92 diff would be more appropriate. Avon recommend 8" rim for the 205 CR500, so looks like new Caterham 8 spoke wheels in 6" front, 8" rear with 175/55 and 205/55 CR500 tyres is the best choice for me.

Mark - I'll take you up on the angle grinder offer when I order them.

 

Yellow SL #32