JonnyH
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| posted on 15/10/14 at 11:56 PM |
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Wheel and tyre combo
Just wondering if anyone can offer any advice....
I'm planning to use the stock hubs and breaks of my donor car (RX8) the side effect being you cant fit anything smaller than a 17" wheel
over the calliper.
I want some pretty wide tyres as the thing will be pushing 350+bhp by time i'm done and going to need all the grip i can find.
Thinking something in the 255-285 range! 
Of course this means really low profile tyres too, 30-40
Ride quality of is no concern to me, going to be fully rose jointed wish bones. It's all about handling and performance. But is there anything i
should know about putting such big wheels on that might be an issue? Handling, control etc, wise?
Ta muchly
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Jon Ison
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| posted on 16/10/14 at 01:26 AM |
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you need lots of tyre wall on a 7 to give predictable handling, fit the combination your suggesting and a similar car on 13" wheels will run
rings round you.
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JonnyH
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| posted on 16/10/14 at 01:41 AM |
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Can you explain why?
Obviously rolling diameter plays a part in acceleration.
Does not having a lower profile tyre equil less tyre deflection when cornering and thus better handling?
I could be totally wrong there, but I'd like to know how and why it effects handling rather than just a statement 
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loggyboy
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| posted on 16/10/14 at 03:11 AM |
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Will be about getting heat in the tyres. RX8 will weigh 3x as much as most kits. Wider and lower profile will not give you any latteral movement in
both tread and sidewall.
brakes that size will be unnecessary, either change the brake choice, or reduce the disc size and customise the brackets so you can retain the
calipers. 15s is as b7g as you want to go on a light car, and even then 13s are considered optimal.
Mistral Motorsport
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carpmart
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| posted on 16/10/14 at 04:13 AM |
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Big alloys the size you suggest are heavy. The effect of that much unsprung mass will not be good.
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britishtrident
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| posted on 16/10/14 at 06:25 AM |
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Aquaplaining is another problem as is the loss of steering feel.
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adithorp
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| posted on 16/10/14 at 06:55 AM |
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quote:
Originally posted by JonnyH
Can you explain why?
Obviously rolling diameter plays a part in acceleration.
Does not having a lower profile tyre equil less tyre deflection when cornering and thus better handling?
I could be totally wrong there, but I'd like to know how and why it effects handling rather than just a statement
With a fraction of the weight there's less tyre deflection when cornering so low profile isn't neeed. Less side wall means less flex so
the tyres conform less to the varied surface so less grip and with the lower weight this is worse. Tyres in the sizes you're looking out are all
designed with 1500+kg cars in mind. Add smaller rims are lighter so less unsprung weight...
"A witty saying proves nothing" Voltaire
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Nickp
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| posted on 16/10/14 at 07:02 AM |
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I doubt you'll need a rear tyre in the 255-285 range. Remember you should have very little weight to shift and going wider creates more issues
with added weight, tramlining, aquaplaning etc You'll probably find a good sticky 225 (even 205) rear and 205 front will be more than enough.
Maybe some of the high powered turbo'd guys on here can back this up?
Mine should be good for 230+bhp and I'll be sticking to 205/50/15s all round.
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JonnyH
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| posted on 16/10/14 at 11:01 AM |
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Ok, there are some sensible reasons to go with smaller here.
From best i can tell if i upgrade the breaks to 4 pots the callipers are actually smaller (go figure) and can get a 16" wheel on.
If i modify things a bit i'm sure it won't be too hard to get away with a 15".
Maybe something like 235/50/15 will be sufficient.
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Doctor Derek Doctors
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| posted on 16/10/14 at 11:45 AM |
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quote:
Originally posted by JonnyH
Ok, there are some sensible reasons to go with smaller here.
From best i can tell if i upgrade the breaks to 4 pots the callipers are actually smaller (go figure) and can get a 16" wheel on.
If i modify things a bit i'm sure it won't be too hard to get away with a 15".
Maybe something like 235/50/15 will be sufficient.
Are you using the RX8 front uprights/hubs as well? Will be interesting to see how the geometry i when fitted to a Seven with Double Wishbone
suspension.
My main question would be is it a road car? If it is then fully rose jointed wishbones/suspension are going to make the vibrations on the road pretty
horrific. If its a track/race car then its worth doing but on road driving it could be utterly unbearable, especially when combined with large wheels
and low profile tyres. It will also put loads more wear and fatigue into the suspension and chassis.
I dropped from 15 to 13" wheels and the cars compliance, handling and predictability improved massively, making it much faster round a circuit.
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JonnyH
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| posted on 16/10/14 at 12:00 PM |
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quote:
Originally posted by Doctor Derek Doctors
Are you using the RX8 front uprights/hubs as well? Will be interesting to see how the geometry i when fitted to a Seven with Double Wishbone
suspension.
My main question would be is it a road car? If it is then fully rose jointed wishbones/suspension are going to make the vibrations on the road pretty
horrific. If its a track/race car then its worth doing but on road driving it could be utterly unbearable, especially when combined with large wheels
and low profile tyres. It will also put loads more wear and fatigue into the suspension and chassis.
I dropped from 15 to 13" wheels and the cars compliance, handling and predictability improved massively, making it much faster round a circuit.
Yep planning to use the RX8 hubs, they are ally as standard and pretty light.
Will have to design myself the wishbones to fit and figure out how it's all going to work. But that's nothing a couple of hours with
Solidworks can take care of.
Going to be a road car, but want to use it for track days and hill climbs. Pretty sure i can live with the ride, minus a roof and windscreen
it's never going to see long distance or heavy use with UK wether 
I think it will be a struggle to get anything under a 15" on without some serious modifications....
[Edited on 16/10/14 by JonnyH]
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daniel mason
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| posted on 16/10/14 at 05:03 PM |
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It's ok living with the ride, but as a hillclimber it will not be very much use are all!
The car needs to be as light as humanly possible.the chassis as low as possible (but enough to clear speed bumps if road legal) but most of all it
needs compliant handling and ride. I'd go watch a hillclimb and see what everyone else In class runs. And when you do you'll see they
nearly all run 13"s and a well sorted geometry setup!
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JonnyH
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| posted on 16/10/14 at 05:10 PM |
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Light weight will be easy, I'm very lucky to work for a company that has a whole site dedicated to making composites that I will be very soon be
next door to, and I'm a CNC Machinist. Carbon fibre will be featuring heavily... I'm aiming for something around 500kg or lower.
Getting 13's on will only be possible if I completely scrap the standard breaks and heavily modify the hubs.
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ettore bugatti
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| posted on 17/10/14 at 11:10 PM |
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Donkervoort uses on their D8 GTO Toyo R888 in the sizes 215/45 R17 and 235/45 R17. 8x17 and 9x17.
The D8 GTO might be an interesting benchmark to your design.
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JonnyH
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| posted on 17/10/14 at 11:34 PM |
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quote:
Originally posted by ettore bugatti
Donkervoort uses on their D8 GTO Toyo R888 in the sizes 215/45 R17 and 235/45 R17. 8x17 and 9x17.
The D8 GTO might be an interesting benchmark to your design.
Thank you!
I hadn't even heard of it tbh, but looking at the set up and it's performance it's not something to be taken lightly!
Now all i need is to get a good look at the suspension set up
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ettore bugatti
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| posted on 18/10/14 at 04:56 PM |
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Its performance does come with a price.
But a 200hp rotary will do nicely in a Haynes Roadster.
As for the wheels Donkervoort are using Japanese Rays forged alloys weighing around 6.5kg, which is quite light for a 17" wheel.
Donkervoort also uses Tarox brakes. Alloy 4 or 6 pot brake calipers can save quite a bit of unsprung weight compared to cast iron ones.
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JonnyH
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| posted on 18/10/14 at 05:07 PM |
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My performance figures have changed a little, i was offered a 13b REW and just couldn't say no. So now i'm looking at about 450bhp
I was surprised at how heavy the GTO is!
If i can use 17's will make life easier as everything can be standard, maybe 16's if i upgrade to the slimmer 4 pots. Impressed at 6.5kg
for a 17" wheel. Well worth a look at!
Getting unsprung weight as low as possible will always help.
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