hi,questions of 17' rim on NA6 thanks

Boags · Postby **Boags** » Tue Nov 13, 2007 6:34 am

lanoshow wrote:anyone using 17' on their na6 to TRACK DRIVE? any good?

For the track? No good at all.

Boags

adamjp · Postby **adamjp** » Tue Nov 13, 2007 8:06 am

When I bought my car it had 17x7 Speedy wheels on it.

I think that the most polite way I can express my regard for this size on an early MX5 is to tell you that I am now running 15x6.5.

The problems with the 17's....

Tyres are very expensive, and hard to get. Good tyres even more so.
They weigh far too much - this affects the ride, and the acceleration of the car.
The thin sidewall can setup some difficult handling issues.
It does not take much to get a flat.
The rims rub against gutters so easily it is not funny.
You lose any ability to deal with rough roads.

The advantages of going to 15's....

Good tyres are cheap (~$100 each per rim less than 17's).
The selection of tyres is excellent.
Good rims are available cheaply in 15.
The police cannot hassle you over wheel size.
Rough roads OK.
Acceleration better.
Handling better.
Ride better.
Gutter rash gone.
Normal sized tyre = no problems with a repair.

In short - don't do it.

Steampunk · Postby **Steampunk** » Tue Nov 13, 2007 10:10 am

Well made points. You would only put 17\"'s or above if you like the look of big wheels and low-profile tyes.
If you want a more \"racy\" look then stay below 16\". Just take a look at ANY car racing and you never see huge wheels with thin tyres, extreme case in point, look at what F1 cars run.

Rob E · Postby **Rob E** » Tue Nov 13, 2007 10:13 am

Another \"dont do it\" aside from the looks (hate big wheels on small cars) from a performance standpoint its all bad.......taller gearing (= slower acceleration), more weight = worse handling and braking.

adamjp · Postby **adamjp** » Tue Nov 13, 2007 10:28 am

Acceleration does not just drop due to the increased diameter.

It also drops because the wheels weigh more AND that weight is further out on the circumference of the wheel. Both of these bad things increases the rotational inertia of the wheel, which reduces your ability to change the rotational speed (accelerate or decelerate).

In simple terms, it takes more energy to accelerate a heavy object if it is further out from the central point. Think of a fishing sinker on a piece of string, simple to twirl fast on a 5cm string, harder on a 10cm string, harder still if you double it's weight.

AB7 · Postby **AB7** » Tue Nov 13, 2007 10:51 am

I read somewhere that 1 KG inclease in unsprung weight equal to around 10 KG in actual rotational inertia of the wheel? Is that correct?

BTW just say that the 17" + tyres size and weight combo is the same as 15", would it be the same?

adamjp wrote:Acceleration does not just drop due to the increased diameter.

It also drops because the wheels weigh more AND that weight is further out on the circumference of the wheel. Both of these bad things increases the rotational inertia of the wheel, which reduces your ability to change the rotational speed (accelerate or decelerate).

In simple terms, it takes more energy to accelerate a heavy object if it is further out from the central point. Think of a fishing sinker on a piece of string, simple to twirl fast on a 5cm string, harder on a 10cm string, harder still if you double it's weight.

Adam_NAclubman · Postby **Adam_NAclubman** » Tue Nov 13, 2007 12:48 pm

As everyone else has said, from a performance angle, don't do it!

But even if you decide to get them anyway because you don't care about the performance or the feel and just want to look cool when you pulll up outside the cafe, I'd still say don't get them. +40 offset on a 7 inch rim won't look at all tough, they'll still be an inch and a half/two inches inboard of the edge of the guards. If you get big wheels just to make the car look cool, do it properly

meanmx · Postby **meanmx** » Tue Nov 13, 2007 4:40 pm

Do a search. This topic has been done to death. If you like it do it!!! Plenty of NA drivers have 17's and love them, myself included. You may want to definately lower it afterwards as it will sit quite high.

Ben

corners · Postby **corners** » Tue Nov 13, 2007 6:04 pm

Sweet
I had this same question. So now I will shop around for 15's.

Can anyone recommend anywhere good?

adamjp · Postby **adamjp** » Tue Nov 13, 2007 6:28 pm

I read somewhere that 1 KG inclease in unsprung weight equal to around 10 KG in actual rotational inertia of the wheel? Is that correct?

Mate I can't remember the formula, just the principle

But if I remember it correctly a 1kg increase (on say a 8kg rim) at the outside circumference, is far more than a 1/8th increase in rotational mass. Probably a radius related function.

A quick google sends me to the moment of inertia page on Wikipedia...
http://en.wikipedia.org/wiki/Moment_of_inertia

BTW just say that the 17\" + tyres size and weight combo is the same as 15\", would it be the same?

No, not for the vehicle acceleration part. The weight is distributed differently. It is always going to be worse unless the rim is so much lighter it is not funny.

For other handling related issues (acceleration of suspension) it should be no different as the weight is now acting in a different plane.

MXkal · Postby **MXkal** » Tue Nov 13, 2007 9:56 pm

I took my 17x7.5 BSAs off after i wrote the NA8 off, Fixed the damage and put a set of 15s on it. Will never go back. Mabye some 16x8 RP-F1 from goodwin racing would be my max but the 17s did look nice.

EGG80X · Postby **EGG80X** » Tue Nov 13, 2007 10:07 pm

might as well bin my 17\"s the way this is going

MXkal · Postby **MXkal** » Tue Nov 13, 2007 10:14 pm

I did the AAMI driver safety course a few months back. I just used the day like a slalem/drift day. I had the 17s on when weaving in and out of cones. The steering gets REAL heavy when your trying to do quick turns left to right, No such thing happens with the 15s and i get no understeer with them either. Also the the car feels like it has 50hp more from a standing start.