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Most of this year comments have been made that the best tyre combination for a MX5 iwith 205/50/15's is on a 8" rim. Interestingly Ken at Accurate Suspension mentioned that a 7.5" would be the best choice.

Whilst on the subject of Accurate Suspension comments Ken looked over my tyre data and agreed that the AR-1's temps/pressures were about right. Why so high, well it is because they are on 6.5" rims, wider rims would require less pressure...

Back on topic, These pictures make it plainly obvious that a 205/50/15 on a 8" rim will provide some extra grip in corners. The AD08R's do look a little stretched on 8" rims, however there is almost an extra 25mm of lateral contact patch for each wheel over the 6.5's. Will be interesting to test the 8" against the 6.5's at Norwell.

Stacked 4 high
205/50/15 x 8.0" - 945mm (AD08R's)
205/50/15 x 7.0" - 875mm (AD08R's)
205/50/15 x 6.5" - 842mm (AR-1's)





AR-1's on 6.5" rims


AD08R's on 7.0" rims

Simply playing devils advocate, and realising the difference would be hard to measure .... but doesn't sidewall strength/flexibility/construction play a part in how much contact patch is available ? Not just rim width, tyre size and compound but in combination of all elements ?

A stiff side wall on a wide rim would give less flex placing more emphasis on the suspension keeping the contact patch maximised on the road surface.
A less stiff sidewall or on a a smaller rim, may allow the tyre to remain in contact (as it squerms) before lifting a edge.

I'd respect your thoughts and experience of this Magpie.

Yes, the construction of the tyre also influences the way it will behave on different sized rims.

It would be nice to have another set of AD08R's to put on the 6.5's and then do a back to back test. In the past I have run the AD08R's on 6.5's, WTAC 2015' in 2016 on 7.0" and this year on 8.0". However I do not think it is fair to use the times from 2015 as I have improved a bit since then.

I'm doing a bit of research to see what studies have been done on tyres and width. There is a lot on bicycle tyres/rims but not much on 'race cars'.

FWIW - I have sat different brands of 205 'track' tyres side by side (on the same rim) and found alarming differences in width

So far this thread raises so many questions, provides so few answers yet makes me want to say so many things.

As Ned Loh says, comparing different brands/types of the same nominal size is worthless if the actual size is different (and they usually are). E.g. the Yokohama A050 205/50 is a 217 mm wide tyre when measured on the 'standard' measuring rim (which from memory is 6 or 6.5 inch wide). Other tyres are generally closer to 205. So let's stick to the AD08 comparison from rim to rim.

Magpie wrote:These pictures make it plainly obvious that a 205/50/15 on a 8" rim will provide some extra grip in corners.

Looking at the photos of the AD08s, the extra rim width makes the tyres wider in the sidewalls not the tread...not surprising. Is it this that makes it "plainly obvious" they will provide extra grip? I am not sure what is obvious.

Magpie wrote:however there is almost an extra 25mm of lateral contact patch for each wheel over the 6.5's.

When it comes to contact width...so what? The contact patch area is predominantly a function of tyre pressure. Pounds of car (on one tyre) over square inches of contact patch = tyre pressure. Well, actually, the contact patch is a little smaller than this calculation suggests as the tyre has some stiffness, unlike, say, a party balloon. A wider tyre patch = a shorter tyre patch for the same pressure. And anyway, the extra width is in the sidewall not the tread.

Magpie wrote:There is a lot on bicycle tyres/rims but not much on 'race cars'.

Lots of work has been done on tyre traction of racing tyres, but the results are not publicly available. When I raced FSAE, teams would pay tyre companies for data about their own tyres, doing specific tests on different load/pressure/rim width/slip angles. We were given some basic free information about some tyres, but could not afford more comprehensive data.

From this I am pretty sure that for any tyre/car combination there will be a rim width that proves optimal for lap times. But what is it? Is wider always better? I doubt it. I am about to change from a '205' tyre on a 7 inch rim to a 185 tyre on a 6 or 6.5 rim, chasing lap times. The 205s proved slower than the 195s I previously had (on 6.5s). So the obvious experiment is to go in the other direction or try a different (softer) tyre compound.

in any case this is a really complicated area of research. There are academic books on the subject. Talk about a can of worms!

The tyre size is the nominal size, not actual size. So there can be up to 20mm difference on the tread face width. As to going smaller rims and tyres to go faster, I can only imagine this would work on a very low HP light weight car. Track lay out and surface can make a difference as well. I ran a stock 1.8 motor (so not very powerful), on 9" rims and 225 R specs. Pretty sure it has set the lap record at Phillip Island for the fastest untuned MX5 with a 1:50.1, could have gone quicker with a few easy fixes.
And the last MX5 I had with SE turbo also ran 9" rims with 225 R specs. It set the fastest lap time outright for any MX5.
Kind of proves that with modest horsepower, or double the stock horsepower, the wider wheel and tyre is faster. No I didn't test with skinnier rims or tyres, but going faster than anyone else must mean the combination works.

StillIC wrote:So far this thread raises so many questions, provides so few answers yet makes me want to say so many things.

in any case this is a really complicated area of research. There are academic books on the subject. Talk about a can of worms!

Many thanks for the comments, got me rethinking my logic...

1. Stacking tyres and measuring the height difference IS NOT the best method. The tyres should be mounted on the car and measurements done that way. However a method of 'measuring' the contact patch at home would be needed, plus I would need to know what the optimum pressures are for each set. Not to mention how to measure the contact patch when in a heavy corner.
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2. There will be no significant extra 'grip' with a 205/50/15 on 8" rim as the tyre will be slightly stretched, as shown in the photo. Yokohama states that for the AD08R's, the recommended rim width is between 5.5 and 7.5". But, Yokohama also say that the overall tread width is 7.7", this supports a 7.5 or 8" rim as the tyre will have 'square' sidewalls.
   .
   However, Feedback on a track based MX5 suggests that 205/50/15 on 8" is a proven 'faster' combination (google). Emilio on the 'big forum' has provided numerous examples of the 205/50/15's being faster on a 8" rim.
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3. For the AR-1's, the recommended width is 6.5" (stamped on the tyre) however advice from Nankang is that they can be used on a 7" rim. Again this 'recommendation' could be because the sidewall construction is different to the AD08R's. Have not been able to locate an actual tread width for the AR-1's, again it would be best to have them on a rim so that the sidewalls are 'square'.
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4. What has changed is the stresses on the sidewall and how much 'flex'/roll the tyre will have on the 8" rims.
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5. Excluding air pressure, the contact patch may have altered in shape but has not changed in area as weight remains unchanged.
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6. Are the followings statements true or false (can of worms)
   tire pressure = weight / contact_patch area
   contact patch area = weight / tire pressure

HOWEVER, what I have learnt is that the best combination of tyre and rim depends on the actual tyre being used and the construction of tyre. It will be back to doing testing of the 3 setups in June on either the National or the Sprint (where I have the most consistency). Then after the aero is changed repeat in July.

To add even more 'variables' consider the differences in versions of Toyos' R888's

lightyear wrote::
Kind of proves that with modest horsepower, or double the stock horsepower, the wider wheel and tyre is faster. No I didn't test with skinnier rims or tyres, but going faster than anyone else must mean the combination works.

I'd like to see what YOU could accomplish with skinnier tyres on those cars before that call could be made.
This would eliminate driver ability, cajones and car set-up as variables (though I suspect you'd have bigger cajones in the wider tracked configuration so would probably go faster).

There is more to lap times than lateral grip. Particularly with low powered cars like MX-5s.
For example it may be beneficial to sacrifice a little lateral grip if it brings a reduced rolling resistance.
Another example, it may be beneficial to sacrifice a little lateral grip if it brings a reduction in unsprung weight.
Yes it truly is complicated. And very difficult for those on a very limited budget (most of us) to do any meaningful testing.
The NC comes with 17" wheels. Is this for performance or just for looks? Would a NB get better lap times from going to 17" wheels? Would a NC do better with 15" wheels. It is easy to spend more money to go slower.

lightyear wrote:And the last MX5 I had with SE turbo also ran 9" rims with 225 R specs. It set the fastest lap time outright for any MX5.
Kind of proves that with modest horsepower, or double the stock horsepower, the wider wheel and tyre is faster. No I didn't test with skinnier rims or tyres, but going faster than anyone else [must] might mean the combination works.

Fixed the statement for you. Everyone needs to bear in mind that lightyear would be quick on 4 spacesavers.

As Nuddy pointed out you're gaining resistance and unsprung weight. You can't just assume that because you now have more grip that you're going to go faster, the time has to be found through your driving. Unless you maximise the grip you have through the corners then there's a chance your lap times will suffer. I'm with Lightyear the wider wheel and tyre is ultimately faster, the driver on the other hand

With low power cars there will be a point where the quest for contact patch results in such loss in acceleration and speed along the straight that the corner speed increase is outweighed by that loss, resulting in slower lap times.

Nuddy wrote:With low power cars there will be a point where the quest for contact patch results in such loss in acceleration and speed along the straight that the corner speed increase is outweighed by that loss, resulting in slower lap times.

Yep. Aero is exactly the same, If you increase drag/weight beyond a point where you can use the downforce in the corners you'll go slower.

Federal 595 RS-R 195/50ZR15 82W contact patch width = 185 mm
Yokohama A050 195/50R15 82V contact patch width = 195 mm
Hankook Ventus TD 215/45ZR17 87W contact patch width = 200 mm
Being a larger diameter the 17" will have a longer contact patch. How significant is this?

To settle the weight argument... The time I set on the AR-1 recently was on a set of wheels that added 11.2kg MORE unsprung weight. Now there could be a difference in tyre weight, but for the purposes of testing no changes will be made to ensure consistency.

205/50/15 x 8.0" (AD08R's) 13.4kg (6UL's)
205/50/15 x 7.0" (AD08R's) 13.3kg (Advanti Racing Storm S1's)
205/50/15 x 6.5" (AR-1's) 16.1 kg (Advanti Racing)

DSCN4301 by Eipeip, on Flickr

DSCN4299 by Eipeip, on Flickr

DSCN4298 by Eipeip, on Flickr