Debating 8" vs 6" rears on R400

[mph]

Hello folks,

I know this has been discussed a lot — I did a search and have found lots of threads over the years, but none seem to answer the questions in my head. So here goes, another thread on 8" vs 6" rears.

I have an R400 with 15" wheels at the moment. I am changing to 13". I'm debating whether to go 8" wide on the rear, and have been told that for greater than 200hp it's more sensible than 6" because they get overloaded with the power too easily. But that's a racer's opinion, and I'm not a racer.

The car gets used for road blats and track days. I am fine with driving it hard on track, and I have no problem with the tail being happy. In fact, I like a slightly waggy tail, it's fun! I'm not chasing lap times, I am not after perfect power transfer. But if 6" rears (with planned Avon ZZS tyres) are way too small for an R400, then maybe 8" is the right way to go.

Currently I am gravitating towards 6" all around — I like the symmetry and as mentioned, I am fine with a wayward backend. I will absolutely be getting the geometry sorted and corner weighted once the 13s are fitted.

Your thoughts?

--Mark

[356DNA]

My 420 with 8" rears effortlessly spins up it's rear tyres on the road. In most situations though, I wouldn't be pushing the car anywhere near the physical limits of grip on the road and so 8" works just fine - by which I mean the balance of the car is harder to determine and less relevant during road use IMO.

I think the issue is that they don't get any meaningful temperature on the road and so especially over crests, bumps and general undulations it can be quite interesting... I wouldn't want less rear grip...

On track with hot tyres it's completely planted and I have been adjusting the car to take of a bit of rear grip to reduce understeer.

What I couldn't say is whether the 6" rears (with equivalent narrower tyres) would warm up more easily during road use. I doubt it TBH

David.

[ScottR400D]

I have never felt that my car (220bhp) had too much rubber with the 8" wheels and ZZRs. It used to spin up its wheels, or at least one of them, very easily, not so much now with the SPC diff but it can be provoked easily enough into a slide or drift. 
I also think they fill the arches better!

 

[mph]

Nice input, thanks both! Scott - going off on a tangent a bit, did you fit the SPC diff yourself? I've messaged them about a price (but haven't heard back yet).

So far then it's +2 on the 8"s. Maybe I'll have to listen to that racer's advice then 

[ScottR400D]

I didnt fit the LSD to the BMW housing, Steve Perks did that (mine was the first fitted to the BMW housing, I believe) and I stripped and rebuilt the car. 

Do you have a number for Steve? +44 7538 382729, he's on WhatsApp too. 

[mph]

Most helpful Scott, thank you! I'll give him a call…

[Blackmamba]

I had 7.5 x 15s all round on my R400 and recently switched to 6+8" x15". Feels great and still very playful. I wouldn't want less grip! I think you'll be told go 6+8" 95% of the time for good reason.

[aerobod - near CYYC]

No problem in inducing oversteer with 6" fronts (185/60-13 tyres) and 8" rears (235/45-13 tyres) when my R400D is driven hard, otherwise car is quite neutral on the track when not using large amounts of throttle in turns. My other set of "street" wheels that are 6.5" wide with 195/50-15 tyres all around are less grippy overall and always tending to oversteer vs a more neutral stance when driven hard.

[mph]

Nice, sounds like the perfect setup James, thank you

[Geoff Brown]

I selected 13' x 8' rears partly for esthetic reasons - they fill out the rear wings laterally but do look a little lost diameter wise.

Cannot really comment on the grip advantage. As with others here the wheels can be provoked to spin up & be tail happy though I would suggest with more rubber on the ground to go side ways requires a little more effort.

[mph]

Thanks Geoff! I used to have a 911, which of course had much larger rears than front. It could be

with some gratuitous flinging around of its weight

[adz]

I prefer the 'feel' of 6+6 - the car seems a bit more communicative and I have the confidence to press on. thats on track with 220bhp,  on the road I cant tell much difference.

. I went to 6+8 as the 13" *6 looked lost in the rear arch. I wouldnt say theres any more grip from 8" rears - if you keep the tyre pressures the same, then the contact patch will be the same, just a slightly different shape.

 

[ScottR400D]

There is unquestionably more grip with wider tyres both in terms of traction and cornering and, though it's less important on the rear, in braking. 

It's too simplistic to say the contact patch is the same, with the same pressure, just as it is to use the empirical theory about friction depending on just the COF and load so the grip's always the same regardless of tyre size. 

[mph]

Decision made. Ordered 6x13 + 8x13 wheels today 

[Grant_7]

If it's any help at all, I had a Sigma Supersport (140bhp) with 6" rears. It moved around loads, which was great fun.

I now have a 420R (220bhp) with 8" rears, and it can be made to move around just as much as the Sigma. However I will confess it's not quite as tactile as the Sigma was on 6" rears.

I actually think 6" rears with >200bhp would probably just be frustrating.

[Garth]

Personally I just think the wider tyres on the back look more purposeful so that did it for me! Good choice me thinks!!

[Nigel B]

My 420R was fitted with 15 inch wheels when I bought it a couple of years ago, shod with 195/50 ZZS tyres all round. 

I actually rather liked the front/rear grip balance with those tyres. It could be persuaded to slide relatively easily, but still had decent traction, in the dry at least. It felt pleasantly lively, without being scary.

I’ve never liked the look of 15 inch wheels on a Seven, so I’d planned to swap them for 13s. I actually had a chat with the salesman at Caterham Gatwick about it & he advised me that the 8 inch rears were ‘necessary’ on a 420. To be completely honest, at that point I hadn’t twigged that Caterham’s 15 inch wheels were of equal width front & rear. It’s something that never seems to be mentioned on threads about higher powered cars & wider rears.

Several months later I bought a set of 13 inch wheels, on ZZSs with the wider rears. The car immediately felt more ‘planted’, but was definitely quite a bit more reluctant to oversteer on the throttle. More sure footed, but not quite as enjoyable for me.

I’ve since had the car set up & flat floored by DPR & it now feels closer to how it felt on the old wheels, in terms of the balance of grip front to rear. I’d say the traction is better in comparison to the previous set up with the 15s, but it’s still possible to point the nose using the throttle at road going speeds. The best of both worlds really.

I think I’m essentially agreeing with your comment that reviewing the geometry & corner weighting is essential after changing the size & width of the wheels! 

I hope you're pleased with the results.

[mph]

Great feedback Nigel, thank you!

I've booked the car in for full geometry + flat floor when the new wheels arrive. I intend to give them the "I'm fine with it being a little tail happy" story

Quite excited. Next 'upgrade' will be the SPC LSD.

[Nigel B]

No problem Mark! I thought I'd share my recent experiences. I think it's a good idea to let whoever's setting up your car know what sort of balance you prefer, though no doubt they'll ask the relevant questions! 
 

I fitted a Tracsport diff last year too, which I'm very happy with. It's much quieter than the previous (knackered carbon plate!) Titan. I've covered 5500 miles with the Tracsport so far & it isn't any noisier than when it was first fitted. I changed the oil last month & it came out looking the same as when it went in.
 

I find the Tracsport provides a bit more traction than the Titan, so it might make sense to fit the diff before you make changes to the set up, so you're looking at the whole picture when you make the final tweaks.