Death of the 17" tyre?

[AngusF1]

Some background: I'm considering designing a basic mid-engined car from scratch at some point in the future. The goal of the car would be to have high, although not ludicrous, performance, as well as being comfortable and civilised enough that I would actually want to drive it. Somewhere around 400 to 450 horsepower, with a weight around 1,100 to 1,200kg.

Towards that end I've been researching tyres. I recently spent some time driving a Chevy Malibu with 225/55/R17s and really liked them. They were high in grip, felt meaty and solid, yet were also still comfortable enough for long stretches of highway driving and even a dirt road or two.

With this in mind I searched the websites of the major suppliers, Pirelli, Michelin, Bridgestone, and found to my horror that sporting tyres suitable for the rear of my postulated car, say 285/45/17, do not exist! The widest 17-inchers were 255 - not really suitable. Further research showed that most sporting tyres exist in the 18" to 22" (!) range.

Who is buying these tyres? (Masochists?) Which perfect roads, free from the slightest imperfection, are they driving their cars on? How often do they bend the rims? What is their chiropractic expenditure? Do they get out to take a break after twenty minutes, or half an hour? Do they wear ear plugs to cut down the road noise?

In more seriousness, can anyone recall when 17" tyres went the way of the dodo and why? If they're good enough for the Ferrari F40, Jaguar XJ220 and McLaren F1, surely they ought to be good enough for the likes of you and me.

[Ted68]

A first gen Boxster comes close to what you are describing, at 1250 kg. Porsche chose 225 section rear tires for the standard suspension and 255 for the sport option. That car has a ton of grip.

Something I learned years ago in Solo I competition is that when I went to a bigger tire, my times slowed. The older racers there told me that first, the tires can be too big to get heat in them for a given weight and two, the bigger wheels and tires add unsprung weight which slows suspension reaction. Gyroscopically, the bigger tire/wheel combo requires more brake to get equal performance to the smaller combo I switched off. Making the switch a double whammy. So bigger is not always better.

All this was on a race track. If your car is going to be driven on that canyon road behind your house, you'll never heat them up all the way, regardless of size and horsepower.

Building from scratch, I would suggest you concentrate more on suspension geometry than tire size. Anti-dive, anti-squat, anti-roll. Learn what makes mechanical grip. Look at the skinny tires on a Formula Ford racer and then think about how many G's those pull.

[AJI]

https://www.tirerack.com/tires/TireSear ... Code=51400

F40 rear 335/35/17, Pirelli P ZERO, only $579 each and in stock!

[Just_a_fan]

Sadly people seem to want to have tyres with short sidewalls on large rims for aesthetic reasons. This is a classic example of form before function. A deeper sidewall gives better ride and, on the public road anyway, gives perfectly good handling response.

[krisfx]

FWIW, I ran 225/45/17 Bridgestone RE070s on my DC5 Integra and they're a brilliant road tyre. They last ages, too.

[Edax]

Sadly people seem to want to have tyres with short sidewalls on large rims for aesthetic reasons. This is a classic example of form before function. A deeper sidewall gives better ride and, on the public road anyway, gives perfectly good handling response.

Is that true? I always understood that the sidewalls in f1 were preferred due to restrictions on the suspension, but the preferred option would be skinny tires and active suspension. Intuitively I would say that the function of a tire is to make contact and you should try to seperate it from the function of the suspension.

[Just_a_fan]

Tyres on road cars have to be able to deal with varied road surfaces. Race cars usually run on smooth tracks.

Very shallow sidewalls give little compliance - good on a race car but not so important on road cars. Yes, you can gain response by having shallow sidewalls on road cars but you usually also get more immediate loss of grip at the limit too.

If you want to drive a race car on the road, shallow sidewalls are great. For the rest of the driving public, some sidewall depth is no bad thing.

[e36jon]

I have a 1994 BMW 325i that I updated to 17" rims, which were the M3 size of that same era. I was running Dunlop "Direzza Z1 Star Spec" in 235-40 at all four corners. Great track day tire, and good grip on the street, but not comfortable or fun in the rain.

I decided to go with high end all season tires when it was time to replace them and found that my size was not supported. In the end I went with a size wider, 245-40, and got the same tread width and rolling diameter, but had to get spacers, roll fenders, etc, to make room for the wider side walls. The punch-line being wow, I can't believe no one makes an all-season tire in this size... If I had gone to 18" there are a ton of choices but I think they look funny on my car, and as mentioned earlier the sidewall gets even shorter.

[NL_Fer]

Aren’t cars getting bigger, fatter? Rim sizes are growing with them. Also suspensions are getting better, most biggers sedans in Europe drive comfortably with 18 or 19”

[Phil]

My Lotus is on 17” too. Standard is 16” for fronts, rear 17”.

Currently running Toyo R888R semi slicks on them (225/45/17). The width is ok, given the car only weighs 900kg..

[AngusF1]

Thanks all for replies.

AJI, wow! Looks like you've found the only "cheap" 17" tyre larger than 245. Of course the McLaren F1 and Jaguar XJ220 still have wide 17" tyres, but those are too expensive. On further research some of the Ultima GTR owners run those F40 tyres. The problem is that 335 was considered to be sufficient for 600+hp back in 1990, so for somebody running only 400-450hp today 335 seems excessive and unnecessary. On the other side, 245 seems too narrow.

Ted, yes, agreed that the details of the suspension design are extremely important. However, the suspension cannot be designed without choosing a tyre first! In fact, the tyre heavily influences several objectives of the suspension design, for example the camber and camber changes, bump steer etc. As far as I can tell the process goes something like this:

• Choose what type / format of car you want, how much power it will have and approximate weight.

• Select rear tyres wide enough to handle the power.

• Select front tyres sized in proportion to the front, depending on the estimated weight distribution.

• ... A few steps later, suspension design can begin.

Phil, very interesting. How much power does your Lotus have? How's the traction? Do you think it could run 400-450hp with sufficient traction?

On further consideration I'm severely tempted to drop down to 16". The reasoning being: Well, 17" was big enough for 500-600+hp back in the late 80s / early 90s, so surely 16" would be sufficient for 400-450hp with the rubber available today. The problem still remains: Surely 245 is too narrow.

Does anyone have any thoughts on this? What's the narrowest tyres used on a 450hp car? The narrowest I can find is a 275/30/R19 on the rear of a 2017 Holden Commodore SS (6L Chevy V8).

[strad]

Not sure why you need that wide a tire. My 2004 Mustang Cobra has north of 400 HP (450 actually) and works fine with it's stock size 275x40x17 tires. However my favorites, the Pirelli P Zero is no longer available in that size but many other brands do supply that size tire.

[e36jon]

Greetings Angus F1

I would add cornering and braking to your list of design factors:

Your brakes are usually a lot more powerful than your engine. Rotor size, while admittedly a cosmetic factor on some cars, can and does drive wheel diameter in some cases. Front tire width, balanced with other factors, has a lot to do with maximum braking force. A big part of my 17" upgrade from my stock 15" was to get larger rotors for better track day performance (less fade). No solution that would fit in a 15" wheel would give the same results.

Cornering is another area that's hard to cheat. There's a reason why every class of street-car based track competition has stiff rules about wheel / tire width. It's been well documented that wider is always better for cornering provided the suspension can handle the loads and be adjusted to take advantage of the width (Camber, usually).

All of the above ignores the level of sophistication of high-end factory cars (Often released with bespoke tire designs!) or F1. On the 'tuner' level you can do a surprising amount with coil-overs and some aftermarket bits to allow adjustment and / or stiffen things up.

Cheers,

Jon "Maybe 200 RWHP, so I should just sit down..."

[Greg Locock]

The Lotus Carlton wasn't quite 450 hp (by 70hp), but 235/45R17 V 91

One obvious issue is that for a road car aquaplaning is a big deal, so you can't go too silly if you want to use it all year in the UK.

[Greg Locock]

I'm not sure that a wider tire always gives more grip, but i suppose in the small range near factory fit it is probably true, if it is the same construction and rubber, and rim widths are modified to suit.