Do modern low profile tires perform better on our Opels?

[azopelnut]

Every year sport cars seem to get taller wheels and lower profile tires.

Our Opels came with 13" rims and even though 15-17" wheels give them a great look I am wondering which actually works better handling wise.

I understand that sidewall flex is reduced with a tall wheel/short sidewall combinations but I assume that MAJOR changes in shocks and springs would be necessary to make the taller wheels work correctly with our cars?? COMMENTS PLEASE???

[Paul]

Paul
Don't you realize the can of worms you just opened up? You'll get at least 100 different opinions...

Mine is... wider tires & taller wheels is/are better... up to a point. To find the optimual combination, you need to do some testing... kinds hard to do on an Opeler's budget, however...

To illustriate this, Grass Roots Motorsports did a side by side comparison and the results are in this months issue. I just got my copy in the mail in the last week or so... Wait... its the February 2008 issue, so its probably on the news stands by now...

From MY direct experience running 14's is a definate improvment. My guess is that you could see some additional improvemt from running 15's with the proper sized tires... Above that, I think the increased rotational mass would be a negative from an acceleration and braking point of view...

Now for the flood

[Ooooner]

I have 17" rims on my GT and running (I think) 40 low profiles. They are 7 inch wide tires as well. The front and rear suspensions have been completely rebuilt. To answer your question about handling, I think it is a big improvement over the stock setup. I'm not saying the stock wheels and tires are bad, it's just what I have seems much better.

[GTJIM]

As long as tyres are selected for better performance ... rather than just 'better' looks then any modern tyre probably gives an improvement in handling.

The key is tyre/wheel weight and diameter - the combination needs to be as close to the weight of the standard 165R-80 X 13 tyre & rim and also have a similar diameter.

After a LOT of searching I found some P2000 Pirellis in 195R-60 X 15 on light-weight BBS alloy rims that have almost the same diameter and weigh within a pound and a half of the originals.

As the rim diameter gets larger the tyre aspect ratio has to become smaller to keep the diameter close to original. With the 'old' 165R-80 X 13 tyres the tall, flexible sidewalls are an integral part of the suspension system so as the sidewalls become lower as rim diameter climbs the suspension rating changes greatly ... until the ride becomes very harsh and keeping the tyres on the road becomes a problem. Certainly, there has to be a considerable change in spring rates and shock absorber rating with low aspect ratio tyres to ensure better handling.

The Dutch Opel Club once did an extensive test of tyres on a GT and found the original 165R-80 X 13 tyres hard to beat for all round handling, smooth ride and low price.

Maybe we are all second guessing Opel - again!

[Ooooner]

The 17" rims and 40 low profile tires I mentioned before are the same on outside diameter as the original 13" wheels. So I see improvement. My speedometer evens reads correct as I have checked it to be sure!

[StefanLee]

When it comes to go-fast racing, bigger is not better. There are several dowsides to having large rims. First, they are heavier, since the rim is bigger and the mass is further from the wheel center. Heavy wheels means slower acceleration, higher unsprung weight and so on and so forth.

The tire wall is lower, that impacts on the ride quality and the suspension characteristics. First, the higher tire wall acts as a part of the suspension. Also, the suspension effect of the tire wall has practically no unsprung weight! (ie, the rest of the suspension can be made with lighter and shorter shocks etc since the tires help out).

So why do some race cars have 18" rims? Well, to fit large brake calipers and discs. A racing porsche 911 or other large coupe weighs a bit, and needs large brakes, hence the need for larger rims.

Compare our GT´s to say a Formula 1 car (YEAH!). Formula 1's use 13" rims, pretty high tire walls (and low suspension) and the cars are LIGHT (no need for humongous calipers, except that the brakes on a F1 are pretty high tech...) Our GT´s are pretty light, so we don´t need very large brakes, hence it is better from a performance standpoint to use rims that aren´t to large.

I have 16" rims on mine, and I can honestly say I went for 16" because of the looks. I don´t mean to say that it has a large adverse effect on performance, BUT if I get a set of cup tires for proper racing, the will definetly be mounted on smaller rims, say 14 or 15".

Stefan

[hrcollinsjr]

Originally Posted by Paul Paul To illustrate this, Grass Roots Motorsports did a side by side comparison... From MY direct experience running 14's is a definate improvment. My guess is that you could see some additional improvemt from running 15's with the proper sized tires... Above that, I think the increased rotational mass would be a negative from an acceleration and braking point of view...

Paul, it's funny that you would mention this.... I kept an article from IIRC Popular Mechanics for years where they did the same tests on a car probably in the '80's. The 14's were a big improvement in almost every area over the 13's. The 15's improvements over the 14's was a much smaller improvement with some disadvantages over even the 13's. IIRC it was ride quality (who would have guessed) and it increased the tendency to hydroplane. As someone has stated in a later post than yours, the weight and/or rotational mass are also concerns. I personally don't like the idea of having a more vunerable wheel. My excuse for wanting to go to 15" wheels is for some increase in handling due to better tire selection.

Harold

[Paul]

Originally Posted by hrcollinsjr My excuse for wanting to go to 15" wheels is for some increase in handling due to better tire selection.Harold

The selection of 13" tires is extremely limited and in a few more years, they'll probably be non-existant. Even the selection of 14" tires has begun to dry up in the last 3 - 4 years. So, I've begun looking into some 15" rims just to improve the selection of good tires over the next 10 years or so...

[Phoenix]

I had Borbet T Type 14x7s with modern tires on the GT when I got and have recently changed to the stock 13x5 Ralley wheels with regular old school tires. I've noticed advantages to both.

The 14s gripped the road better/had a better feel.

The 13s are actually more smooth and have less road noise.

I was suprised by the orginal set-up but perfer the control and handling of the larger wheels with the performance tires.