RE: 17" wheels on a Spider

[Alex Jenner <alexj@domain.elided>]

Glenn Wasserman wrote:
> More precise handling at the expense of ride comfort. Depending on the
> suspension design and alignment, you can get more grip due to less flex
> of the sidewall. Could keep more rubber on the pavement.
Or not depending on camber changes etc.

The main reason for 17" or larger wheels on a road car is fashion. The
reason it's fashionable is because it's used in racing (at least Touring
cars and GT's - F1 still uses 13") and the reason they are used in
racing include....

1. as racing gets more competitive the only way to pass another car
might be under brakes and 17" wheels might allow you to run larger more
effective brakes and maybe brake a few metres later than your
competitor. Of course, many people who upgrade the wheels on their road
cars don't fit the big brakes, cause they only want to look fast.
2. Many Touring car classes introduced tire/wheel width limits (e.g.
Group A) that are measured with a caliper around the tire (fitted to the
rim). Therefore you want a tire with very little sidewall bulge (i.e.
low profile).
3. With restrictions in width, a taller tire (with associated suspension
modifications) might allow you a larger contact patch and therefore less
heat build up and the possibility of running a softer compound than your
competitor (Of course the down side is higher sprung weight and centre
of gravity).
4. Some classes state the minimum ground clearance in the following way
- the car must not touch the ground when both tires are deflated (on
either side of the vehical). So to pass this rule with minimum ground
clearance, 40 profile tires on 17" wheels are better to pass this rule
than 55 profile tires on 15" wheels.
5. In consideration of the above, the teams are using the largest wheels
consistant with low unsprung/sprung weight ratio. This means spending
$$$$$ on ultra light wheels (some classes are introducing minimum wheel
weight rules) and of course the sprung weight has been increasing over
the years. A mid 1980's 2.5 GTV6 in ETTC Group A would be around 960kg
and ran 15" or 16" wheels, but a current 2L BTTC car is about 1150kg and
runs 18" wheels.

It's quite possible that not all of the above reasons apply when
selecting wheels for a Spider.

By the way, does someone have an accurate figure for the weight of the
14x5.5 turbina wheels?

Best Regards
Lex Jenner
Auckland/New Zealand
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