TR with 18" P-Zero tires, car tends to follow road grooves

try 225's on the front.

 

John
You have to realize that there is a reason these cars originally came with 15 in wheels. The higher aspect ratio, (higher sidewall height) allowed the suspention as designed to work properly and the flexing sidewalls were part of that design and made the TR much less prone to "tramlining". What you are describing. F1 cars use 13 inch wheels with massive sidewalls. This does limit brake size but they still seem to be able to go around turns pretty well. Modern cars (which your TR is not) have suspentions designed for low profile tires and larger wheels BUT fitting even larger wheel and lower profile tires than are specified by the mfg. while it may look bling,actually HURTS handling esp. on the street.
I would go back to your original wheels and tires. The car will handle much better and be less prone to rim and tire damage from potholes.

 

try 225s and a little more caster

 

This is an 86 TR. That's my point.

 

I had the same setup on my 88 TR with no problem. The ride improved and so did the handling. Make sure your alignment is right. Wider tires will always tend to follow road groves more and might be more sensitive to bad alingment. I had a Viper Coupe with 285/35/18 fronts and it did wonder alittle but not much.

 

Sorry to bring an OT question but I've always try to find out the reason for this setup on F1 cars. Unless is is dictated by regulations (which I wouldn't undestand either), I can't see why they still use such small rims and such tall sidewalls. Suspensions are re-designed every year or even more often and I take for granted that you get overall better handling and more precise steering with lower profile tires. Why is it then?

BTW, I agree that the difference of stock setup for 86 TR in comparison to 18" rims should not be so annoying. Maybe it is a combination of the rimsize difference and the thread of the tire itself? I've heard of tires prone to tramlining just because of their thread desing. I think the Pzero's thread pattern is based on parallel grooves, maybe if you try another design like Goodyear F1 or similar the problem decreases???

 

F1 wheels size is regulated. It keeps the size of the brakes down which slows down the cars which is what they are trying to do as if they allowed bigger wheels they feel the cars would get dangerously fast.

 

However bigger brakes would allow faster lap times but not much higher speeds, don't you think?

Also, it seems to me that it was the early 90's when they started wanting to cut off power and speed increase. And rim-tire sizes had been the same for years before... I've been searching FIA regulations for some time but never found about rim-tire sizes. Are you positively sure they're regulated?

 

F1 wheel size is most certinly regulated.

Also, F1 tyres have incredibly stiff sidewalls that actually perform the BULK of the suspension work in the car. There is little comparison to a road car, where you want the tyres to do no suspending, and only flex just enough to properly engauge the pavement.

The cars are now powerful enough that given better brakes, they could go much faster before having to switch to brakes. Higher peak speeds would thus be attained.

Best!
Ben.

 

Napolis,

Does that mean I should put the 14" OEMs back on my 308? LOL
Actually, I'm refinishing some now for shows. 14" VR is no longer supported by manufacturers.

Point well made on suspension design. The 512 was touted at the time as a TOTAL suspension upgrade from previous TR.

Goodyear is #1 in Racing

 

I prefer to think they don't want F1 tires to do most of the suspension work. What is the point then in designing such advanced suspension systems, and in working so hard to get the right susp. settings? I think it's just the same than in road cars, I mean they'd prefer the tires not to suspend and thus being able to control all the suspension functioning by adjusting springs, shocks, etc. Obviously the tire sidewall flexes, though I don't know how much or how little. Any way it flexes I guess they want to "control" everything by themselves (adjusting springs and shocks).

I'm willing to be corrected if I'm wrong.

However I insist that if they could brake, say 50% later, speed would not increase in a 50%. Just a few mph in the longest straights.

 

To address Jim Schad's question, I have not noticed any tramlining with my 512TR. It has P Zero's with the original wheels.

My car tracks very straight with little input necessary.

 

how's the tire wear on the set of wheels you took off? is there any unusual wear ? also, what tire pressure do you have your new tires at? if you car is tramlining (over expansion joints, etc) that's normal when going to a larger wheel/tire combo, but excessive pull to either side implies an alignment problem, or a problem with tire pressures (check your toe settings, and make sure you're running app. pressure in your new tires).

when you move to larger/bigger wheels with smaller sidewalls you're effectivly hardening up the suspension b/c the reduction in sidewall increases the rate of weight transfer b/w all 4 corners of the car ; with your previous set-up the sidewall dampenened/absorbed the rate of weight transfer, and inherantly gave your car a more supple ride and instilled it with a lot of body roll -- the taller sidewalls acted like additional bushings that flexed under load. (if you tracked your car w/ your new setup v. your old, you'd find it a lot crisper during turn - in, flater through a series of esses , but a little less comunicative at the limit of adhesion -- because you'd be cornering faster).

common parlance is accepted to be , that: shorter sidewalls / bigger wheels stiffen up your suspension for the reasons above; this stiffening make turn in more sharp, and overall grip is increased, but communication at the limit is reduced. as is "street ride quality." therefore , since you've effectivly sharpened your suspensions acumen, it's not unlikely that faults that were previously hidden, now come to the surface; i.e., poor alignment, sensitivity to correct tire pressures, etc.

about the f1 comparison... there is none. f1 cars run small wheels 1. b/c they have to (regulations), and 2. b/c they need to (drag considerations). if you think about it, the most parascitic drag that an openwheel racer sees is the front wheels; which is why you'd want the sleekest possible tire profile available (see the michelin v. ferrar debacle), but for a sleender oncoming profile , you give up some feel in the way of sidewall height; however, you can make up for that in a milleu of ways : 1. more caster, 2. stiffer carcass, 3. chassis rigidity, etc.

lastly, f1 cars don't need huge brakes becuase the carbon disks provide ample heat sinks to where the giant iron heat sinks required in endurance racing wouldn't be required (which is why those cars run huge wheels; to clear the giant brakes -- if the rules allowed carbon brakes in other forms of motorsport, you'd see a likely drop in overall wheel diamteters), and even in CART (where iron rotors are used) you still see small wheels. then again, you'd never want an open wheel racer with a 18 inch wheel to accomodate 16 inch front rotors; your aerodynamicist would likely kill himself faced with that design compromise.

in closing: check your alignment.

 

Hugh
What I'd like to see is 18" Carbon Brakes on F1 cars. I disagree that F1 cars have enough brakes. Several esp. BAR, seem to run out of brakes as the race goes on. I don't quite understand why wheel size introduces drag, as opposed to overall wheel and tire size. Does a 13" wheel with a higher profile tire produce less drag than a 19" wheel with a lower profile tire assuming the OA dia. and width remains the same?

 

james:
it's all conjecture, but i'd say it comes down to the pad compound development, caliper placement and ample cooling -- if you recall ferrari has done a lot of work on their brake duct setup , and the positioning of their calipers to accomodate the ducting design w/out sacrificing aerodynamic efficency up front. also, remember how much the BAR cars dusted? think back about why brakes dust? i bet if you could peek at the caliper temps of BAR v. ferrari and the temp celings of their respective pad compounds, you'd see a huge difference.

also, think about the implication of a wheel at the front of an open wheel racer, in dynamic motion, that covers an 18inch rotor; that would be a 20 inch wheel. think of the changes in ride height, aero etc that that would cause... it's a compromise. fenders do a lot for that .

lastly, it's not so much a question of overall diameter (since gearing changes could make up for that), but a question of the size of the front face of the car; the frontal area, the distance from the hub at which the mass is suspended and the overall BULK of the front of the car.

 

Hugh
That's interesting. I bet your right about pads.
Best

 

Hubert, I thought my english was pretty good but have found difficulties in understanding your paragraphs about "sleender oncoming profiles" and "heat sinks". Could you explain all that with easier words?

Basically I see your point, very interesting, but:

I'm with Napolis here, I know it's not a question of overall size. Of course not. Leave overall size the same and reduce sidewall and increase rim size... after reading all your posts I still cannot see why it is aerodinamically worse when frontal area is exactly the same.

 

It's here where I don't catch it... where is the compromise?

 

Hello,

I am the new owner of a 1988 Testarossa which has 18-inch rims and would like to get the advice of the experts here. I have a few quick questions regarding sizes:

1. Should the size of the front and rear tires be the same? Are there any advantages to having different sizes in the front and rear? Or keeping both the same size?

2. I am looking at putting BFGoodrich G-Force T/A KDWs 245/40ZR 18s. My decision was based off of so many online reviews I can't even count ;-) but they appear to be a very good tire. Any opinions?

3. Anything I am missing??? There is always something

Let me also say thank you in advance for your help!!!

Cheers,
Travis in Maryland

 

Increased wheel size increases drag not because of the front face (it should be the same overall size) but because of the ROTATING drag. This is a common problem with bicycle wheels (and part of why I was paid so much $$ to test aero wheels on the road and track).

Take a wheel with a tire and spin it. Put your hand near the rotating spokes. You will be able to feel the aero disturbance from the rotating spokes of the wheel, whereas if you put your hand by the tire you will feel noticeably less. The tire, because it is smooth, creates bvery little rotational drag but the wheel, because of its spokes, creates plenty. Increasing the size of the wheels increases the rotational drag.

Of course, the easy solution would be to create "disc" wheels like bicycle racers use. However, then you would have no air to cool the brakes. No one wants that, right? As a result, you have to settle for un-aerodynamic wheels. Increase the wheel diameter and the lack of aerodynamics from the wheels increases, even though the frontal area remains the same.

 

I have the same size wheels and tires on my TR. I have no problem, except I noticed that when the tire pressure gets low (after sitting in garage all winter, for instance), the tire will track and require more effort to keep straight. Check your tire pressure.

As far as someone's question about same size tire, I don't think that would work very well with TR. I also don't think the size tires that fit rear would fit the front, so you are talking about smaller sizing the rears, which are drive wheels. Not good.

 

Travis, congrads on your car!

1. Generally, try to get wider tires on your rears and smaller in the front. Also when able to choose, try to get the larger OA diameter to be in the rear and the smaller in the front. Both of these, along with proper air pressure, promote UNDERSTEER. You want understeer in a TR, else you might over turn and have some fun action mimicing a top. I have been advised by many people against having the same all-around in a mid-engined car.

2. Personally, I would be hesitant of the fronts being a 245. On my BMW I just jumped up to a 255/40-19 Potenza (up from a 235/60-16) for my fronts and the car tramlines like you will never believe, sticks like glue, but tramlines like crazy. What is the normal TR tire on the OEM 16? I will find out what sizes will be best for you then. Personally. I would try for a 225 front and a 285 rear, or may be a 295 max. Again keep in mind you want the OA diameter to be the same approx. as the OEM 16in diamer would have been, and if you can, to get the OA diameter to be slightly less in the front when compared to the rears.

3. Consider what you will be driving through, rain? or just dry? etc etc when choosing your tire.

PS I am in Baltimore! Where are you located?