mx5cartalk.com

Standard NB8a + Torsen diff.
I'm not intending to chase more power.
I'm not intending to increase brake rotor diameter.
Is there any point in going to a larger wheel diameter?
Is there any point going to a wider wheel?
I'm thinking of getting a second set of standard wheels and R spec tyres.
Or should i get some other wheels of different diameter and/or width?

Nuddy wrote:Standard NB8a + Torsen diff.
I'm not intending to chase more power. Ok
I'm not intending to increase brake rotor diameter. Don't need to at std power
Is there any point in going to a larger wheel diameter? No
Is there any point going to a wider wheel? Yes
I'm thinking of getting a second set of standard wheels and R spec tyres. Good idea
Or should i get some other wheels of different diameter and/or width?Yes

If planning on running r specs for track work, then 205/50 on a 15x8 is the sweet spot for a std engined car

Thanks rascal
So now I'm looking for some 15x8 wheels.

Nuddy wrote:Thanks rascal
So now I'm looking for some 15x8 wheels.

I would like to offer another opinion. I have a modified road registered NA6 that can do the times listed below on 195/50 15s on a 6.5 inch wide wheel, in supersprints. Last year I moved to 205/50 15s on a 7 inch wheel and have yet to match any of my best times, despite my new wheels being lighter than the old ones (Yoko A050 tyres in both cases). I have more power than a standard 1.8 litre engine.

Why is this so? Well, I hypothesise there are a number of reasons the 205s A050s are apparently slower than the 195s *ON MY CAR*.
1) I have increased all my overall gear ratios, noting I *never* hit redline in 5th;
2) The small NA6 brake rotors I run, due to Type 2 State class rules, are now even smaller relative to the tyre diameter thus reducing brake force for any given clamping pressure;
3) The tyres (not wheel rims) have a greater mass moment of inertia affecting braking and acceleration;
4) It takes a little longer to get the larger tyres up to temperature, noting that the only time I ever overheated the 195s was on a hot day at SMSP South circuit, which also gave me my best time, below;
5) They have increased air resistance and possibly rolling resistance.

So, please be careful about over tyreing your car. I will be going back to 195s on the 7 inch rims as my next combination to try.

I would love to hear the *informed* opinion of others on this topic, especially MangaBlue.

Edit: I would love to try a 185/55 tyre on a 14x6 or 6.5 rim.

Unless the car is grip limited then wider tyres are not needed.

My car is not stock... WTAC 2015 I ran 15x6.5 with 205/50/15 AD08R's, in 2016 I ran 15x7 with 205's again and was 2 secs faster. Time gain I put down to driver improvement in the majority. Of all the cars at WTAC I'm running the narrowest tyres, but not the slowest...

I can consistently get 1.3 of braking/cornering with the AD08R's on 15x7's and still not grip limited yet. I think that at 1.5g I will find the limits of grip.

However, this all depends on alignment, pressures, temps and tyre combination. Agree that a second set of rims with R specs is a good decision. Look at the R specs you want to run then pick a rim based on that, for example A050's/AD08R's in 225 only come in 16". NTO1's however come in 225 for 15"...

If I run in WTAC this year it will be on 15x8's again with 205's. this was after discussing rim size at the recent MX5 meet at Sandown in January.

You might consider getting some experience with what you currently have on the car, before moving to R-spec tyres. If you increase the straight-line/corner speed of the car too early in the learning process, it could just mean you will have a faster accident.
When I started driving at track days, I deliberately ran the car on road tyres (Khumo KU31s) and with std power and brakes for over a year, to gain experience on the track and learn from the inevitable mistakes.

StillIC wrote:

Nuddy wrote:Thanks rascal
So now I'm looking for some 15x8 wheels.

I would like to offer another opinion. I have a modified road registered NA6 that can do the times listed below on 195/50 15s on a 6.5 inch wide wheel, in supersprints. Last year I moved to 205/50 15s on a 7 inch wheel and have yet to match any of my best times, despite my new wheels being lighter than the old ones (Yoko A050 tyres in both cases). I have more power than a standard 1.8 litre engine.

Why is this so? Well, I hypothesise there are a number of reasons the 205s A050s are apparently slower than the 195s *ON MY CAR*.
1) I have increased all my overall gear ratios, noting I *never* hit redline in 5th;
2) The small NA6 brake rotors I run, due to Type 2 State class rules, are now even smaller relative to the tyre diameter thus reducing brake force for any given clamping pressure;
3) The tyres (not wheel rims) have a greater mass moment of inertia affecting braking and acceleration;
4) It takes a little longer to get the larger tyres up to temperature, noting that the only time I ever overheated the 195s was on a hot day at SMSP South circuit, which also gave me my best time, below;
5) They have increased air resistance and possibly rolling resistance.

So, please be careful about over tyreing your car. I will be going back to 195s on the 7 inch rims as my next combination to try.

I would love to hear the *informed* opinion of others on this topic, especially MangaBlue.

Edit: I would love to try a 185/55 tyre on a 14x6 or 6.5 rim.

Excellent points thanks StillIC
We do have low power cars and I have found that I get zero wheelspin with the torsen even at full throttle in second and sliding with my half worn Kumho Solus HS51 195/50x15
I certainly don't want a larger rolling radius. I'm sure the change from 4.1:1 to 4.3:1 final drive is advantageous. I'm needing to change to 5th gear (5 speed box) 3/4 along the main straight at SMSP North.
Reading the above inclines me to stick with the standard wheels. In addition it will be handy to have 8 identical wheels for tarmac rallies.
I have 2 targets. MX-5 cup and Targa Tasmania 2018.
I want to get some wheels and tyres before I wear out the road tyres as I do drive a fair bit on the road. Then by this time next year have 8 R-Specs ready for TT.
So now I'm looking for a set of 4 standard NB8a wheels, idealy with R-Spec tyres on them.

My first ever supersprint was at SMSP North on Sunday 2nd. Running on the Kumho Solus HS51 195/50x15 I recorded a best lap of 1:29.282
I can see a few seconds to be gained with a few more laps of the circuit but was certainly limited by the tyres.
Having my first run at Wakefield on 23rd, probably still on the Kumho Solus HS51 195/50x15.

OK I bought 4 standard (15x6) wheels and some second hand R-Spec 195/50R15 tyres.
I will use them for the supersprint at SMSP North on 13th, State supersprint at Wakefield on 14th and club track day at Wakefield on 15th.
Meanwhile I have the hillclimb at Ringwood on Sunday where I will use the Kumho Solus HS51 195/50x15.
At Wakefield last Sunday I started with 38 psi cold and the tyres went up to 44 hot. I dropped to 38 hot and ran again and went to 40 hot.
Then I dropped to 36 hot and after the next session they were still 36. Now they are 32 cold.
My question is what cold pressure should I start with for the Hillclimb?
We only do 1 'lap' at a time so the tyres will not get hot. That's one reason why I'm not using the R-Specs.

I used to hillclimb an ST162 Celica, fairly successfully, on various tyres. Ultimately I was running soft compound 205/50 15 (on a 7 inch rim) Dunlop Direzza D03G at about 28 or 29psi cold, which warmed to maybe 31psi during a run. This seemed fastest.

But before that I used various other tyres which all had different optimal temperatures. One set of tyres I had were best at about 40psi, but they were a 60 profile 14 inch tyre, with less grip than the Dunlops. The rest fell somewhere between 28psi and 40psi at their best, when measured cold (which is how the tyres are during much of a hillclimb).

Of note, my MX5 has 20psi cold in the A050 Mediums, which warms, after a couple of runs, to 28psi in a supersprint.

I think experimentation is in order to find the optimal pressure for the tyres you have on the car you have.

Nuddy wrote:OK I bought 4 standard (15x6) wheels and some second hand R-Spec 195/50R15 tyres.
I will use them for the supersprint at SMSP North on 13th, State supersprint at Wakefield on 14th and club track day at Wakefield on 15th.
Meanwhile I have the hillclimb at Ringwood on Sunday where I will use the Kumho Solus HS51 195/50x15.
At Wakefield last Sunday I started with 38 psi cold and the tyres went up to 44 hot. I dropped to 38 hot and ran again and went to 40 hot.
Then I dropped to 36 hot and after the next session they were still 36. Now they are 32 cold.
My question is what cold pressure should I start with for the Hillclimb?
We only do 1 'lap' at a time so the tyres will not get hot. That's one reason why I'm not using the R-Specs.

What are the R-comp tyres? I'm almost certain you'd be quicker on those than the Solus HS51, heat up or not.

Short of doing it the 'proper' way with a pyrometer, most people will just have a target hot pressure based on someone with more experience/better times with the same tyres, and go from there. It's all just trial and error. Fact is that most people are not quick/consistent enough to rely on laptimes and 'feel' for data but absent anything else to go on...

With those tyres (Solus HS51)... who knows? They're just not up to the job in any event. I'd guess the biggest issue you'll have is a super-soft sidewall, so I'd probably target a high pressure to offset that, (eg 40+psi). But it's going to be a total guess and I'm pretty sure no one is going to have run them on the track, so you'll be on your own.