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Discussion Starter · #1 ·
Recently purchased a mk2 1.8 NA for track use and have 15 by 7 wheels.

Pretty much set on the Federal 595 RSR given the good reviews and reasonable money.

However, looking for some advice on whether 195 or 205 width....lots of mixed views when googling!!!! 195 clearly cheaper so kind of where my head is at, but wondering whether any 'gain' from 205 for the extra pennies and if so what. Appreciate will be some what subjective as many things impact grip etc.

Cheers
 

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Been looking at these instead of R888s due to better in wet from what I can gather.

What price are you doing a set of 4 at 205 NickD? PM me as not wanting to hijack thread.
 

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Your talking less than £10 difference per tyre.
I know, but is the 205 better for the extra tenner and if so why.....looking for some education
[/quote]

I have no back to back data.

For the exact same conditions, car, wheels etc, 205 will give you an increased contact patch. Using some very basic (and I mean basic by assuming the contact patch is round, which it isn't) you get a 10% increase in contact patch. If you were to use a square shape you would get less of an percentage increase.

Anyway, what you can then translate this in to is then the question.

First up although the contact patch is bigger, the static load per square mm of tyre is less. In conditions where you can generate sufficient weight transfer the larger contact patch should equate to more grip. However as grip levels drop, so wet etc, to the point where you can't generate grip to increase that load per mm2 to the same as the narrower tyre, your grip levels will reduce. (I could have put that simply by saying you are likely to have less grip in the wet.)

So it is a nice sunny, warm day and grip is no problem, what does this mean. Firstly you will have more drag. Think of a piece of selotape stuck to the road, the bigger it is the more effort it takes to pull it off (innuendo alert!) more grip = more drag. So your speed down straights is also compromised. You will have a slightly higher gearing due to the bigger diameter, 1.75% in round figures so acceleration is blunted on top of the extra drag.
Extra grip should allow more corner entry speed and probably later braking. However it does not necessarily equate to more exit speed where extra grip will bog the car down. Think of a hydrofoil boat, it goes fast when it lifts out of the water, reducing drag. No doubt it can turn tighter when the hull is in the water, but so much slower and then has to accelerate again. MX-5's don't accelerate, the speed needs to be preserved, that extra grip being taken through a corner slows it down, you want the car to be up on its skis going fast, but instead it is stuck dragging its hull through the water.
There are all sorts of scenarios from this, wider tyres can certainly help with traction out of slow corners, higher gearing my take away the need for a gear change between corners and hundreds of other possibilities. However the main point is, for standard power, something MX-5's are not over endowed with, and a driver that can put the car at its limits, I would expect 205 section tyres to be slower than the equivalent 195 over a lap of a circuit.
 

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Would 195s also have a bit more stiffness in their side walls on 7" wide wheels?

My thinking is that the 205s would be a little "balloony" compared to 195s, and aside from all the good reviews Ive read about the RSRs Ive also read that they scrub the shoulders a fair bit and thought there may be a link between their conservative "195" and "205" dimensions and perhaps being fitted to too narrow a rim?

I do hope that makes sense there, as on top of what Rodders has said above, I think part of what Ive said could be true and Ill be going with 195s on a 7" wheel.
 

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That's a great explanation (which I just about understand!). The conclusion is backed up by some Ma5da info I've seen somewhere that 195 tyres will generally give you a quicker lap time than 205 tyres (assuming a standard power MX5).
I'll try and explain better. Ignoring modern F1 cars which are all about aerodynamics, then look at 70's F1, the Goodwood revival and anything that does not rely on down force and you will likely see the car sliding around. The best drivers got the fastest speeds by being able to drive the car beyond the normal limits of the tyres.

Modern tyres have far more grip than in the 60's plus being radial so you need more speed to take them to the edge. Things like RS-R, which have plenty of grip can very easily cope with the power output of a MX-5 and so the ability to drive beyond its limit is very much reduced. Of course you can slide them with entry speed but you don't have the power to slide them on exit. Driven at nine tenths, where the tyre is never taken beyond what would be considered a normal slip angle then it is quite possible that the wider tyre would be faster as it will generate more lateral grip before any drama

And I think I have just been renamed Rodders!
 

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Oops, sorry Rodders.


No comment on the sidewalls and wheel widths? Am I talking rubbish there or is it just negligible campared to the contact patch stuff?

Any thoughts on those CR500s posted by bashracing above for use on an MX5 and/or a CRX? Theyre only down the road from me, just spoken to the seller and all seems good. Just concerned they wont like the extra weight compared to a Caterham that they were originally designed for.
 

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Yea, but come on, T1Rs would have seen the R500 round there faster than a lardy Buggy.

Hmm. Struggling to find many reviews of the CRs, especially in 195/50, but what Ive read so far they could be a goer and a stickier, lighter, shorter lived option to RS-Rs which my sprint competition are using...
 

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Heat is a major factor too - a tyre has to be in the operating window to grip well.

A 195 tyre on an MX-5 won't overheat unless you are sliding about everywhere, so a 205 just adds weight and cost IMO.
 

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The standard with for a 195 is 6" and 205 is 6.5" so you are not exactly putting a ballon tyre on, in fact the recommended mazimum rim width for the 195 RS-R is 6.5".

The theory is that if you widen the base of the tyre than you reduce its distortion under load, however you are not exactly talking about floppy tyres here so I wouldn't worry about any of that.

The CR500 will be fine on your car, you have even smaller sidewalls so more stretch, but again, they will be fine.

Be aware that none of the tyres mentioned here is going to give you their best in the cold weather, so if you are after these to pop to the shops in the middle of Jan, I would go for the RS-R, if you want then for the track go for the cheap ones.
 

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Thanks, Nick. That all makes sense.

Id not use RS-Rs on the road, nor any other track orientated tyre. Ive seen a CRX get very dead this month on a cold, damp road with RS-Rs on it, wasnt pretty. Road tyres for road, track tyres for track in my little world.

One last question if you dont mind - what would you honestly go for for the 3 x 2013 Blyton sprints - those part worn through road use 195/45/15 CR500s or new 195/50/15 595 RS-Rs?
 

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Discussion Starter · #18 ·
Thanks for all the replies...most useful. Have to admit I was with skinny re: tyre pressure/weight of car defining the contact patch or shape of it.

Have done a few track days, but not a in a car that's specifically for it so doubt I will be driving beyond the grip of the tyre for a while yet
Car is also normally aspirated so think I will stick with the 195 for now.
 

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Doesn't the size of the contact patch depend on the tyre pressure? Going to a wider tyre changes the shape rather than the size of the contact patch.
That is not the case though, if it were there would be no reason to have tyres other than a standard width. It is the sidewalls that take the weight, not the tread. Think of a bridge, the road way has to be strong enough to bear the weight on it, but it is the parapets that keep it off the floor irrespective of its length.

Assuming, and it's a big assumption, that carcase construction is exactly the same, then for a given air pressure the weight on the sidewalls would still be the same, if the tyy were 2 inches wide or 2 feet. Assume the car weighs 1,000kg then that is 250kg per wheel and 125 per sidewall and they will deflect the same amount for that weight. So the longest chord length will still be the same but the axial width will still be wider.
 

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Very interested in this thread. I need some new tyres for track days and would also like recomendations. I'm also on 15" rims, and the tyres would be mainly for track use only, but i would need to drive there on them so road legal is a must.
 
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