Mr2 Roadster Wheels

March 3, 2003

I have a 2000 mr2 roadster and want to buy some momo tuners

Should I get 225/40/16 rear and 205/45/16 front

Or just go for 205/45/16 all round ?

March 3, 2003

Not knowing the Mk3 I thought I'd cross post this to another forum to see if I could get a quick response, thanks to Martin H from IMOC who replied with....

"The 2000 mk3 has as stock 6x15J+45 185/55 & 6.5x15J+45 205/50.

TRD recommend using 7x16J+45 all round with 205/45 & 215/45 for use with their Sportivo kit, though I don't know anyone who's actually using that. Most people seem to have gone for 17x7 with 205/40 & 215/40.

Whatever you get, make sure it's light - this isn't a tubby and needs to stay light in order to get the best out of it. After that, its just the usual stuff, keep your rolling radius the same, etc. Oh, and don't skimp on your tyre choice. "

Info from Martin Holden, IMOC-UK.

Thought it only polite to quote name and location of person providing the answer :D

March 3, 2003

Another response for consideration from Mark Jones at IMOC-ROC....

"The sizes should fit ok depending on what width rim he is looking at

(more than 7" is not a good idea especially if he wants to lower it.),

however you need to consider offset (45mm is ideal), and weight, Momo

wheels are very heavy, on a MK3 this will have a very profound effect

on acceleration and handling! I found that out the expensive way.

Finally 205 all round is not a good idea as this will make it very tail

happy. I'd suggest 195 front and 215 rear, or at the most, 205 front,

215 rear.

Mark.

Stock wheels way 14lb front and 16lb rear. "

By the sound of it MOMO's may not be the best choice for the MK3

March 3, 2003

Thanks for the speedy replies !

I had not really put the weight into consideration but will do now . Having had momo's on my other 2 models i liked the look but will possibly consider the tte alloys .

March 3, 2003

As stated, not very knowledgable on the Mk3, luckily have a number of people on other forums who drive them and take them on track and are a good source of information :) :) :)

HTH

March 3, 2003

Now for a reply in full, again from Martin Holden From IMOC-UK and IMOC-ROC on tyres for the Mk3 MR2, this is a lengthy topic but once read I think you'll agree worht the read....

"[umm, this turned out to be a bit longer than I anticipated. E&OE ;-)]

I'm still trying to get my head round these tyre specs, alloy centre told me today that these size tyres would add 10mm and 20mm to the rolling radius front and back respectively.

What we're doing here is changing the *ratio* between wheel and tyre. So when we increase the wheels from 15" to 17", we're decreasing the size of sidewall of the tyre by 2" so that the wheel+tyre ends up being as close to the same as possible.

Let's give you what I've done as an example.

On the front, the stock tyres are 185/55R15 (the wheels are 15x6.0J+45). That equates to a circumference of 1836mm. I've replaced those with 205/40R17 (using 17x7.0J+45). These equate to 1871mm. That's an increase of 1.9% - pretty good.

On the rear, the stocks are 205/50R15 (15x6.5J+45) = 1841mm. I've replaced with 215/40R17 (17x7.0J+45) = 1897mm. This is 3.0% bigger, not quite so good, but certainly not that bad.

I don't know, but presuming the speedo is feed from the rear wheels, then the speedo says 100mph, then you're actually doing 103mph; says 50mph, actually 51.5mph - you get the picture. Considering Toyota have an officially state their speedo has a tolerance of +/-10%, this isn't something to be concerned over.

While I'm at it, your wheels' PCD (pitch circle diameter) is 4x100. All the maths from above can be checked here:

http://www.miata.net/garage/tirecalc.html

How does it affect things?

Are you sure you wanted to ask?

This is where I'll leave the maths behind as I don't have the qualifications for the level of maths required. :-)

There are two parts to this since you're changing two components: the wheels and the tyres.

The performance of the car can be increased in many ways, reducing the weight of the car is an obvious example. Now wheels+tyres count in two important measures: rotational inertia and as part of the unsprung mass of the car. Wheels act as a big fly wheels, so the lighter they are, the

easier and therefore quicker they will accelerate.

Now alloys are heavy, and increasing the size of is only going to adversely increase the weight. The stocks are (front/rear) 14.6lb/15.2lb. Expect 17x7s to weigh 21lb* which is ~40% increase - and

don't forget that more weight is further from the centre, increasing its interial value. Of course, you can get 17x7s as light as 13lb - but less is more, and we're talking $$ now.. In theory some of this weight is offset by the reduction of tyre weight, but I don't have the information available to give you figures on this.

* source: http://www.fiercecars.com/specs/rimweight.shtml

To try and make all this tangible, let's try and give something real. As a driver, I hope that you notice that your '2 isn't quite as responsive on the ol'throttle if you've got a passsenger in there with you (I

suggest not imaging your skimpy partner, but someone a little larger perhaps). Now take the passenger out and put your 17s on. Same effect.

Unsprung mass is everything that is the ground side of the suspension. The increased weight causes a mixed blessing here. With the wheels being heavier, it requires more energy to more them. Going over a bump, its more likely that the tyre will flex than the wheel move up again the suspension spring. But when it does go up, you load the suspension harder (more energy entering the system) and that will work your dampers more. All this translates to a slightly bumpier ride.

Ok, so we said that straight line acceleration isn't going to be as good as with 15" wheels. But that's not the whole story when we're talking performance.

Going round corners is what a Roadster is all about, and lower profile tyres will help. Less side wall (let's quickly compare: 102mm vs 82mm though around about 30mm is trapped around the rim edge so effectively you have 72mm vs 52mm or 30% reduction) means less flexing under lateral loading of the wheel. The translates that the more of the contact surface of the wheel stays in on the road, giving more grip. Of course, there is a downside to all this grip. There's a point under lateral

loading the tyre will no longer grip the road and start slipping. With normal tyres, there is that feeling, termed 'progressive loss', that the driver will get as more and more of the tyres losses traction. With

lower profile tyres, the extra grip is at the price of a sharper loss of traction.

Of course, I've been talking about dry weather performance. I'm guessing you're putting on wider tyres when you upsize your wheels. Well in that case, pressure equals force over area - translated bigger tyres means less down pressure means less water clearing power meaning more chance

of aquaplaning. Working out the figures is a bit to much for me so I can't quantify it, but its something to bear in mind.

Going back a bit, perhaps I've confused you earlier: when talking about unsprung mass, I mentioned the tyre wanting to flex more, yet one of the characteristics of low profile tyres is that they don't flex so much.

This results in far more stress being absorbed by the sidewall, which requires a far more complex design and composition than that of normal tyres, and yes, that means $$.

So, in summary, the possibility is there to buy 17" wheels lighter than

your stock 15" (increased acceleration/brake performance) and have

better grip too boot. The reality is you don't have the $$ to afford

superlight 17" and they're be heavier than stock and you're not (insert

favourite racing driver star here) to be able to correct a TLT (total

loss of traction) at 100mph round that hairpin corner you drive

everyday.

And that's really the point of it - anyone who's serious about performance will list out all of the above and shake their head sadly at someone upping their wheel size. However, I don't drive round hairpins

at 100 everyday (or even every month sadly) and a great deal of this is to do with appearance and I doubt you do too. If you remember this is predominately an appearance modification, with the buzz that you'll get a stiffer ride and improved cornering in the dry, then enjoy it for what it is. I know I do, and I have.