Yesterday I did some minor suspension mods on a '57 Subaru Impreza WRX. The car had just had its 1000 'running in' service at the dealer and the owner wanted a few mods doing.
The car was fitted with OE 17" wheels and Bridgestone RE050 215/45 tyres.
When I'd done the work I test-drove the car and noted again how 'knobbly' these cars can feel on an imperfect road surface.
In my workshop I have a set of Subaru OE 16" wheels with Michelin Premacy 205/50 tyres which had come off a 2006MY Impreza due to a wheel 'upgrade' - they've only done 2000 miles.
So I thought, why not swap the tyres over and see how it feels? (the rolling radius, offset etc is the same). I did this with the agreement of the owner who was with me at the time.
It was an interesting experiment. We took the car over about a 6 mile run consisting of a very twisty B-road with 'dirty' road surface. A twist A-road with 60mph, straight A-road with 40mph limit, and 'bypass' type road with 60mph limit. Did this with 17", then 16" then back to 17".
I expected the 17" set to feel 'sharper', have slightly heavier steering and offer a little more grip. This was not the case.
The steering felt exactly the same, despite the slight difference in tyre widths. The 16" ran more quietly on the A-roads with a lot less roar and the ride was notably less 'agitated' than with the 17" tyres. Grip and steering response felt exactly the same, in fact the car actually felt rather more secure on the 16" set on bumpy roads. I was surprised by this because the Bridgestones are noted for having a stiff sidewall and I expected the softer sidewalled 16" Michelins to feel a bit 'mushy' on the sharper corners and make the steering a bit rubbery - which was not the case.
How much this was down to the difference in performance between the two tyre brands is open to conjecture, but in this particular case at least the 16" combo seemed better suited to the car. Obviously you don't quite get the 'look' of the 17", but in other respects, particularly ride quality, the bigger wheel combo is not as good, or at least no better.
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There has to be a limit to handling and grip improvement when it comes to larger wheels and tyres-after all, the car weighs the same so a wider tyre means the weight of the car is spread over a larger area, more area, less pressure per area unit. (I know you know this, just speculating out loud!)
I know that larger diameter wheels affect the geometry and response of a vehicles suspension, although in ways I do not fully understand.
Interesting experiment.
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>>a wider tyre means the weight of the car is spread over a larger area
Really?, How?
Number_Cruncher
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I too changed my Legacy tyres [not wheels] to a bigger profile and find them much more comfortable. They had been incorrectly fitted per the handbook by the previous owner.
Whenever I drive a large wheel, low profile-tyred car, I cannot understand why anyone wants them - apart from the looks. Even in town, every bump jars through the car like a ramrod, and as for sleeping policemen............mund you, that's lowered suspension issue mainly.
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@number cruncher
I'm thinking larger contact patch equals lower pressure exerted on the road per square unit of contact area. That is unless of course the wider tyre has the same surface area contact patch. In which case the pressure exerted per unit of contact area would be the same, obviously.
Edited by component part on 31/10/2007 at 11:37
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As the tyre is a flexible structure (less so near the sidewalls), the pressure between the tyre and the road is largely equal to the air pressure within the tyre. To a first approximation, the contact area is given by the weight of the car (not the mass!), divided by the number of tyres and the air pressure in the tyres.
As an example - if a car has mass of 1000kg, its weight is 9810N
if the tyre pressure is 30psi - that's approximately 210000 Pa
Area=9810/210000
~=0.05 m^2
per tyre, that's 0.012 m^2, or about 18 square inches - which gives the usual comparison that a car tyres contact patch isn't much different from the size of the sole of a man's shoe.
The size of the contact patch doesn't follow the tyre size - by fitting a wider tyre, the dominant effect is a change in the *shape* of the contact patch rather than the size.
Number_Cruncher
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call me old fashioned but i wouldnt want push bike 45 sidewalls on anything 50"s are better but 60"s are better still in fact 70"s are my prefered sidewall or was that 80"s,anyway personally i like squishy sidewalls and lots of gap between rim and road
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I think it's an age thing BB. I prefer more padding under my bum too!
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Fair enough-like the man in the orthopedic shoes, I stand corrected!
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What would I need to do if I wanted to swap my 17" wheel and tyre combo on my Outback for more comfortable 16". What details do I need so that the speedo still reads accurately and what should I do to ensure that the insurers don't complain that the car is not roadworthy?
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Espada
Browse through this!!
www.carbibles.com/tyre_bible_pg2.html
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as an aside:
my mate who used to run a souped up wrx found that with the bigger 4 pot brembo brakes the wheel rim would contact the caliper during track days and so had to go for 18" wheels.
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What would I need to do if I wanted to swap my 17" wheel and tyre combo on my Outback for more comfortable 16". What details do I need so that the speedo still reads accurately and what should I do to ensure that the insurers don't complain that the car is not roadworthy?
Bascially very easy, you just need to ensure that the rolling radius is the same. I assume your 17's are fitted with 215/40-17 tyres, in which case you need to go to a 205/50-16 There will only be about 1% difference in speedo. If you have 215/45-17 then 205/50-16 will give about 2.5% speedo error - if you think this is too much then get 215/50-16 which will be more or less spot on (but this size is more difficult to buy).
You need 16"-6.5" alloys with 5x100mm stud PCD and 56.1mm bore. Offset (ET) is 42-50mm.
See if your local dealers have a secondhand set they've taken off a car to do an 'upgrade' - this is very common. You will probably sell your old ones for more than the new ones cost!
Can't see the insurance co being too worried if the alloys are not of high value.
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Thanks
as per usual, concise and complete information.
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Little bit of a follow up to this. Driver of the Impreza with 17" wheels been mulling over our experiment and decided he wanted to swap to the 16" to get the better ride. Swapped then over this morning.
Must say, after 'experimenting' again that its amazing how much that 1" improves the ride (only 1/2" on the radius). It really really does take the 'business' off the ride on all but the smoothest roads. I suspect its all down the to the sidewall on the 17" tyres being more heavily reinforced and having little compliance - there isn't much deformation of the tyre sidewall where it contacts the road, the tyre stays 'round', if you know what I mean. The only downside is that the Suby 17" wheels (7-spoke) do look a lot nicer than the 5-spoke 16"-ers.
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