I have 16" rims and need to choose new tires. My thoughts are to go with the Azenis RT-615 and a 205/40-R16 front and a 225/50-R16 rear.

Is it bad to run such a big difference in diameter between front and rear?

I could run a 215/45-R16 at the front, but it will be really tight and I might have some clearance issues.

I don't see why it would be any sort of problem at all.

The ford GT for example P235/45ZR18 front and 315/40ZR19 rear which is more than 65mm difference in diameter.

The other thing is you basically have to have bigger tyres at the rear to take advantage / counteract the rear weight bias of the middy layout.

Without tyre data I'm guessing that the the ratio of front to rear width ratio should be equal to the ratio of the max load on each outside tyre during corning.

Doug.

just thinking that the stiffness of the low profile front wall compared to the much taller rear wall would cause the car to behave unpredictably at the limits of cornering.

just theorizing, but since I have to make a purchase I figgered I would ask here!

I might be stating the obvious here but:

Tyre and wheel size have a real impact on roll centres for the same ride height - i.e. the bigger the wheel you fit on an existing car the lower the roll centre on that axle will be for the same ride height and the less distance you will have to the bump stops.

So I think the decision on wheel and tyre combos is best made in the design and spring and shock selection and placement stage.

Otherwise if the design allows for it I don't think the difference in diameter will have much impact on vehicle dynamics.

I have 16" wheel and 50 series tyres all round.

I think 40s are a bit low for the the weight of the car as the walls are built to support far heavier vehicles and make the car very stiff and may cause problems.

most people think that low profile tyres are not realy suited to Locost type vehicles

Fronts are 205 50s
rear 235 50s


Had no problems so far and the car seems to handle Ok.

If you want to fit 40s its up to you but a difference in profile front to back makes little difference to the vehicle.

Personally I would adjust the profile height to be as close as possible.
I went for 205/40/17 on the front and 235/35/17 on the rear - about as level as you can get.

[Edited on 2-12-06 by RazMan]

I just need to see if the 215/45 will fit the wheelwell. I have about 1/2" to spare.

Moving from a 205/40 to a 215/45 the sidewall height increases from 81.78 to 96.52mm (3.22 to 3.8 inches) For reference the rear tire, a 225/50R16 has a 112.26mm (4.42 inch) sidewall.

Increasing the diameter does change my scrub radius. Making it increase from 0mm to just under 2mm negative to centerline. I think this effect will be almost imperceptible.

Hi Steve
I'm not sure what to think about tyre (tire) profiles. Low profile is assumed to be better but Formula 1 cars use 13 inch wheels and high profile tyre and they seem to go OK.

Cars usually look best with wider tyres at the back and this particularly makes sense when you have more weight at the rear. My experience is that straight line stability is best when the front tyres have same or higher profile than the rears and are wider relative to the rim width than the rear (less sidewall stiffness). This makes the front less affected by minor road imperfections. To achieve this you usually need rims an inch or two narrower on the front. Also worth considering is ride comfort. If all else is equal, a softer ride will make your trip (and your passenger's) more enjoyable. You are going to drive it arn't you. Long trips? why not. Tyre choice: Frankly, go for the softest rubber you are allowed to use on the road and your budget can cope with. They just work so well on light weight cars. Regular road tyres are built for cars weighing two tons and they hardly flex at all. Well, just my 5c worth. Cheers, Brian

I know its an extreme example but an enzo runs a 235/35/19 front and a 345/35/19 rear....... so the profile size is very different front to rear.

quote:

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Originally posted by Ratman
I'm not sure what to think about tyre (tire) profiles. Low profile is assumed to be better but Formula 1 cars use 13 inch wheels and high profile tyre and they seem to go OK.

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Andygt, just out of interest, do you know the actual rim sizes that those tyres are fitted to?

not 100% but I think the fronts are 8.5x19 and the rears are 13x19.

I have decided to run a staggered wheel setup on La Bala for two reasons. Run a taller/wider tire at the rear and keep the sidewall heights similar front to back.

It's very difficult to find a wheel design thats available in these sizes and bolt pattern combo.

Look at the figures below and let me know what you think please. Thanks!

Graber

Front wheel - 16x7 rim - 4x100 40mm offset -
Tire - Falken Azenis RT-615 - 215/45-R16 ~$95/per tire
Specifications:
Section Width: 8.46 in 215 mm
Rim Diameter: 16 in 406.4 mm
Rim Width Range: 7 - 8 in
Overall Diameter: 23.61 in 599.69 mm
Sidewall Height: 3.80 in 96.52 mm
Radius: 11.80 in 299.72 mm
Circumference: 74.17 in 1883.9 mm

Rear wheel 17x7 rim - 4x100 40mm offset -
Tire - Falken Azenis RT-615 - 225/45-R17 ~$135/per tire
Specifications:
Section Width: 8.85 in 225 mm
Rim Diameter: 17 in 431.8 mm
Rim Width Range: 7 - 8.5 in
Overall Diameter: 24.97 in 634.23 mm
Sidewall Height: 3.98 in 101.09 mm
Radius: 12.48 in 316.99 mm
Circumference: 78.44 in 1992.3 mm


Wheel Choices 16x7 et40mm 4x100 17x7 et40mm 4x100
Konig Brite-lite Yes Yes
Konig Helium Silver - Yes Yes

What does this do to your suspension geometry calcs?
I would expect there to be a change in the roll centre heights if the tyre sections have changed.
But the CofG height will be moved proportionally too, so is there any real effect on the geometry in terms of the lever effects and weight transfer?
I guess the only way to tell would be to do the math, or scream around the neighborhood and see if you can feel the difference

Hi there AndyGTT

Thanks for the Ferarri wheel rim widths. This makes the tyre width about 20mm wider than the rim width for both front and back wheels. I was just wondering if they might have a greater difference for the front wheels, but it seems not. I suspect there is not a lot of choice here, these very low profile tyres are made in sizes to match standard rim widths. Cheers, Brian

I'm interested in your findings, I have gone for high sidewalls both front and rear, but most specifically I went a lot taller on the front than the enzo (I used the enzo tyre on the rear of my GTT).

I also went as wide as I could on the front... my tyres are 255/40/19's which is a lot bigger than the 235/35/19s the enzo runs on the same rims.
However I think I will be having a larger sidewall overhang on the rear than the front.

However I never considered stability in my choice and it was purelly to improve ride comfort and traction

[Edited on 8/12/06 by gttman]

Hi Steve
That choice seems fine. As I have opined before.. I like the look of one size smaller wheels at the front. One thought... This is my own thoughts and there might not be sound automotive practice behind it... but your front tyres are tighter on the rims, tyre width is closer to rim width, then the rears. I have the same situation on my car (175 tyres on 6 inch rims front, 205 tyres on 7 inch rims rear) and I feel it is wrong. When I changed the front tyre size to one a bit wider (previously was 165) the straight line stability of the car improved with no loss of turn in. So my reasoning would suggest that you might be better off with 8 inch rims at the rear. 7.5 if they exist might be better. But.. as I say.. This is just my own theory. What I am thinking about is things that will make your car nice to drive on the road, and still handle well on the track. Hard front suspension is great to get good handling.. but again makes the car very twitchy on regular road surfaces. You can get away with quite soft rear suspension, and this helps avoid pitching by being very different to the front. This also means you dont bury the front of the car in the road when you brake hard, and you can take speed bumps at any speed and the car just launches into the air dead flat.. this is great fun. The middy layout is different to other cars in this regard. Cheers, Brian

I finally found a decent rim with a fantastic variety of widths, offsets and bolt patterns. - Team Dynamics Pro Race 1.2 - http://www.teamdynamicsracing.com/pro-race/Pro-race12.htm

Now the question is: Is a 9" rim too wide for the rear? Should I go with the 8". I assume wider is not always better?

It's a great looking rim btw:

Steve, you might want to check out Compomotive too - loads of sizes available and the CXR is fairly similar (if you squint a bit ) which is why I chose them




[Edited on 8-12-06 by RazMan]

Thanks for that link. Looking closely only one Compomotive rim comes in both 16 and 17 inch size and I don't fancy it too much.

But your wheels look nice!

quote:

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Originally posted by RazMan
Steve, you might want to check out Compomotive too - loads of sizes available and the CXR is very similar (which is why I chose them)



[Edited on 8-12-06 by RazMan]

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How about-

15 inch 185/55 - 205/50 (Original Elise and Toyota MR2)

16 inch 195/45 - 225/40

15 or 16 inch 195/50 - 225/45 (earlier Toyota MR2 IIRC ?)

So your suggestion is to go way smaller on the rims and tires? don't do the stagger? I know it's less unsprung weight, but I don't like that idea!

quote:

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Originally posted by cymtriks
How about-

15 inch 185/55 - 205/50 (Original Elise and Toyota MR2)

16 inch 195/45 - 225/40

15 or 16 inch 195/50 - 225/45 (earlier Toyota MR2 IIRC ?)

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The rear tyre size I suggested is exactly the same width, at 225, as your plan. I suggest going narrower at the front as this is in line with most other mid engined cars.

I can't see any problem with bigger rim at the rear or with biger overall diameter at the rear.

I would still suggest-
195/45-16 front narrower and less than half an inch bigger than your 205/40-16
225/40-17 rear (if such a size exists) which gives you a staggered look.

Gotcha! I just didn't quite get your previous post. Makes perfect sense to go similar to what has obviously been working for others. No need to try to re-invent the wheel, so to speak.