Death of the 17" tyre?

[e36jon]

I should have clarified a bit that most of my tire talk was track / auto-x focused. I agree that hydroplaning is an issue, such that I bought all-seasons to replace my 'super-duper-extreme-summer' shoes.

Here's the article I was aping with my 'bigger is always better' quote: http://www.vorshlag.com/forums/showthread.php?p=58689 Again, track centric.

Lastly, be careful what your reference standards are. It was only after I bought my e36 BMW that I learned they understeer like pigs, so there's a whole aftermarket devoted to improving that aspect. 360 Modena? Don't go to their message board as you will be sad... You could do better than the F1 by going with one of his big influences and look at the original NSX (Maybe even get a ride in one!). Or a Miata, frankly.

Big props for wanting to design and build your own car! That's epic.

[Phil]

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?

My car has just shy of 300hp. Give or take around 300hp/tonne.

Traction is very good, though bare in mind my car is mid-engined (MR), so weight distribution is around 40/60. The engine sits right on the rear axel, so it takes a lot to break traction.

Also; my car isnt the torqiest. Revs to 8500rpm and the bulk of power comes at 4000rpm and over 6200rpm. The semi-slicks definitely help. Never ran normal tires.

Interestlingly, the newer Lotus Exige models come in at 1200kg with the larger V6 engine and depending on tuning state, offer around 350-430hp of performance. They come standard on 265/35/18 AFAIK, not 17”s anymore.

[strad]

All of this planning needs to take wheel width and back spacing into consideration.

[AngusF1]

Jon, good point about the influence of wheel size on the brakes. Some basic calculations suggest this hypothetical car would do a bit north of 300km/h so braking would be rather important. It's clear that 17" wheels should be sufficient due to existing supercar use, but smaller could pose problems. I'm not too concerned with ultimate cornering power as the use case would be sport-touring road use.

Greg, nice find with the Carlton, I hadn't heard of that car before. On further research it seems SS Commodores around 2010 were sold with 245. I'd like the hypothetical car to be reasonable in the rain, again based on the use case.

Phil, thanks for the info on your Lotus and the Exige. I've looked up the Evora and it seems like they came with 255 rears standard.

Strad, when designing your own car from scratch I believe you have the freedom to choose whatever tyres you want, then work backwards to wheel size, spacing and upright design etc. That said, I know nothing about back spacing so could be wrong.


It's looking like a "narrow" rear tyre may actually be feasible. By narrow, I mean 245 or 255. Thankfully it's still possible to find 45/17 tyres in these widths. For example, later versions of the original NSX came stock with 215/45/16 front and 245/40/17 rear. I could look at say, 215/45/17 front and 255/45/17 rear. (I think someone earlier mentioned 275/40/17, but I couldn't find any among the "big four" tyre manufacturers.)

A point this brings me to is if, say, 255 can be suitable for 450hp today, why on earth did the major supercars of the late 80s/90s use such gigantic tyres? Was 345 really necessary for 540hp in the XJ220, for example, or 335 for 470hp in the F40? Was the rubber really that much worse? Was there an aspect of pushing into the unknown with the questionable driving abilities of the cars' purchasers, so they wanted to provide as much grip as possible? I'm not ruling out an attitude of "excess", either... Perhaps they wanted the cars to be as over-tyred for road use as they were over-powered.

[e36jon]

Greetings Angus F1

I can only speculate as to the reasoning behind the seemingly oversized rear tires on 80's/90's supercars.

1) Launch from standstill: A very narrow part of the use cycle but critical to selling cars. Witness the Porsche Turbo.

2) Fairly un-sophisticated rear suspension geometries: I appreciated your insight that maybe the rubber just wasn't that good, as it hadn't occurred to me. I would counter that when fresh and new, on dry roads, the tires then were a match for tires now. Outside of that narrow definition I think you are probably right that modern tires pull away. Having said all that, rear suspension design is vastly more nuanced now then it was then, both in terms of gross geometry layout, but also in terms of dynamic behavior.

3) I don't think it was because "It looks cool": Sure there were market pressures, but it was a different time and I think that performance elements were driven by engineering and not sales or marketing. Feel free to laugh and call me simple...


Regarding brakes: Are you designing around carbon-ceramic rotors? I harbor a deep lust for said rotors on my car but am really limited in options. Basically only the Porsche 996 Turbo rear rotors will fit, at 350mm X 30mm. Everything else is 390mm or more, which won't fit in a 17" wheel (At least not one that I know of.).


OK, off to watch the F1 race at Austin...

[Just_a_fan]

A point this brings me to is if, say, 255 can be suitable for 450hp today, why on earth did the major supercars of the late 80s/90s use such gigantic tyres? Was 345 really necessary for 540hp in the XJ220, for example, or 335 for 470hp in the F40? Was the rubber really that much worse? Was there an aspect of pushing into the unknown with the questionable driving abilities of the cars' purchasers, so they wanted to provide as much grip as possible? I'm not ruling out an attitude of "excess", either... Perhaps they wanted the cars to be as over-tyred for road use as they were over-powered.

"Only" 315 for the rears on the McLaren F1 and that had 627bhp.

I think part of the issue back then was that, a. wider tyres look cooler on a supercar, and, b. the cars didn't have the sophisticated engine controls and traction control systems that just about everything has these days. Odd that the F1 should have relatively narrow tyres, but then it was intended to be the "ultimate driver's car" so there may have been an expectation of driver involvement / skill. Or maybe it was because the F1 was designed to ride properly and so had better traction than the XJ220 / F40 types. Certainly the F40 was more of a "race car for the road" than the F1. The XJ220 was originally intended to have a big V12 engine and four wheel drive. Perhaps when they changed to rear wheel drive, they decided extra rear rubber was needed. Perhaps the engine is "spikey" and might overwhelm lesser rubber.

[strad]

I don't know where you are located but you must have access to Tire Rack or someplace like them.
https://www.tirerack.com/tires/TireSear ... adRating=S
I am really bummed that Pirelli quit making my size in the P Zeros but they suggest going plus 1.
Plus 1 in 18s are almost identical circumference but I don't feel like buying new 18 inch rims.
.
You are correct that when designing your own you can set your needs as far as back spacing but it once again brings rotor size and caliper size and suspension placement into consideration. Tire width will affect wet weather performance and aquaplaning.

[Just_a_fan]

Wow, Goodyear F1 GS-D3. I had that on my Mk4 Golf back in about 2003. Didn't realise Goodyear still made it - it was discontinued in the UK some time ago. Good tyre back in the day.

[AngusF1]

Does anyone have any thoughts / experience of traction in cars with 400-450hp? What tyres did the car(s) have, and how was the traction? eg could you break the rears loose on a flat, straight, dry road at X speed just by mashing the throttle?

Tire Rack or someplace like them

Thanks Strad, that site's great. Clearly the manufacturers' websites are just marketing fronts for the most common tyres, as there are several listed on there which were not on their websites.

I am really bummed that Pirelli quit making my size in the P Zeros but they suggest going plus 1.

In this case would you need to drop down a couple of aspect ratios, eg 55 to 45? Depending on how optimised your car is, you might also then want to adjust the dampers (if possible!) and dodge all the pot-holes.

it once again brings rotor size and caliper size and suspension placement into consideration. Tire width will affect wet weather performance and aquaplaning.

Yes, I would need to make sure the wheels are large enough for whatever calipers are required. 17" wheels seem to be the smallest used since the 80s for road cars of this speed performance (max ~320km/h). (Racing single-seaters, on the other hand...) Re water performance, yes, hence I'm trying to discover the narrowest tyres feasible for the power. Looks like 255 or even 245 rears might be feasible.


Just_a_fan,

"Only" 315 for the rears on the McLaren F1 and that had 627bhp.

Or if some sources are to be believed, maybe even up to 670hp! Out of those three cars, the only one I'd count on for purely engineering-related sizing is the F1. The F40 doesn't even have a space frame (ha), and the Jaguar is just bloated everywhere except in height. Perhaps if they'd sized the tyres properly they would have been able to fit the V12 in! Good point re the traction. If 315 was ok on the F1, that really seems to imply that 255 might be ok for 450hp. Perhaps time to pop over to Pistonheads and query Flemke about the traction...


Jon,

1) Launch from standstill: A very narrow part of the use cycle but critical to selling cars.

Good point. Pretty useless in reality, but great for selling cars. 60-200km/h times would be more relevant.

Fairly un-sophisticated rear suspension geometries

I would have thought some numeric optimisation would have been available for suspension geometry by the 80s, but you're right definitely not as sophisticated as today (at least for the pro's).

Regarding brakes: Are you designing around carbon-ceramic rotors?

No idea on the brake material - if I ever do end up really designing the car that would be a decision to make at the time. Carbon brakes are definitely much lighter (great), but at least for now apparently have worse feel (bad), seem to be extremely expensive (very bad), and restricted to particular single sources of supply (very very bad for long term supportability). So at the present I would tend to lean towards metal, but carbon could become a better option in the future. Who knows, maybe in five years there'll be a revolution and all sports cars will have them?

[e36jon]

A few quick thoughts on carbon-ceramic rotors:

When I went from my 15" X 6" wheels to 17" x 8.5" I managed to save a good deal of weight. Stock was 22 lbs per corner and the new wheels were approx. 15 lbs each. Winner! I lost 7 lbs per corner!

The tires were going from 205-60-15 @ 20 lbs ea. to 235-40-17 @ 24 lbs ea., so I gained ~4 lbs per corner. So sad.

And then the brakes. I had cast iron calipers and rotors all around, with solid rotors in the rear. I went to big-boy floating rotors (@ 330mm front and 328mm rear) on aluminum hats with aluminum calipers. I was SURE this was going to be a win. Nope. They were ~12-13 lbs a corner heavier, even with all of that aluminum. That's all due to the bigger cast iron rotors. So, I gained un-sprung and rotating weight.

Braking performance is greatly improved and stays happy all day at track days, but, comfort around town took a big hit. I have the same Bilstein B6 (Sport) shocks and springs before and after, so the differences are down to un-sprung weight and sidewall aspect ratio.

All that to say I would very much like some carbon ceramic brakes, especially those made by Surface Transform. They're stock on the Porsche GT3 in Europe and guys here are ordering them in as replacements for the Brembo carbon-ceramics as they are vastly more durable and provide much better feel, and they shave ~10 lbs per corner(!). They're getting 14-15 track days out of a set, or 'lifetime' on the street.

And, some 5-way Ohlins TTX46 through-rod shocks would also go a long way towards making everything all right.

[strad]

Yes Angus I would have to drop to a lower aspect ratio if I went Plus1.
My Mustang Cobra has a few power additions that bring it up to 450hp or so from 400hp.
It is lowered 1½ inches on Eibach springs with Eibach anti roll bars.
In first I can break the tires loose at any speed below say 30mph. If I'm on it I can break them loose in the first three gears in the dry. In the wet you best be decent at car control you can get completely sideways in the wet. With the supercharger a certain amount of finesse is required with the right foot. Especially on corner exit and especially in the wet. I run Koni adjustable dampers all round but feel that even in their softest setting they are a little too stiff.
I live with it because I have become cheap but if I had it to do over I would have lowered it on adjustable coil overs.

[AngusF1]

Jon, thanks for the reference to Surface Transforms. I had a look at their site - very interesting. Their technology of using cloth rather than compressed, shredded pieces of carbon makes a lot of sense. I had a look at the Porsche GT3 rotors; they don't list the weight but charge 8,500 GBP for 4 rotors. $$$ ! This sort of thing will be well worth considering though, depending on budget, and the cost may decrease over time.

Agreed that carbon brakes save massively on weight. I've had an opportunity to hold an iron, then carbon rotor (same car and application) in succession, and the carbon rotor weighed probably 1/3rd that of the iron.

Strad, to clarify, you mean you can break the rears loose at up to 30mph in a straight line, and up to third gear powering out of corners etc? That sounds something like the behaviour I would actually want in a car, although I'd be happy to fall off the road at slightly higher speeds (45mph?) if I was dumb enough to willfully mash the throttle.

[strad]

All depends on road conditions.
In warm weather with my Pirelli's I had better traction than with the B.F. Goodrich G-Sports I'm running right now.
I avoided the Goodyear F1s because I had poor experience with them and don't think they are worth the price.
I live where it rains and I used to have to drive it more often when it was wet hence the B F Goodrichs.
If I only drove it in the warm and dry (like if I lived in L.A.) I would run the NItto NT05 as it is I'm considering one of their other tires that they say has good wet weather traction.
Like I say with my car one has to have control with their right foot. Especially if it's wet. I have a friend with a 2003 and he has done a complete 360 exiting a wet corner when not paying attention.
In the wet I have gotten completely sideways when hitting 3rd gear whilst drag racing from a rolling start.
Almost scared myself.
Between cops and traffic I can't have as much fun as I used to.

[AngusF1]

All depends on road conditions.
...

Thanks Strad, sounds like a fun car!

So, I've thoroughly confirmed that recent Holden SS Commodore V8s (LS3 engine, ~430hp) came with 245 rears. This is enough for me! That puts the widths into Honda NSX-esque regions and something like 245/45/17 and 205/45/17 should be reasonable. The only challange is finding Y-rated tyres in this size. Off to Tyre Rack, and 3 tyres are suitable:

• Michelin Pilot Super Sport

• Bridgestone Potenza S007A

• Vredestein Quatrac 5

Not exactly a long list, which leads me to think there is future risk of Y rated tyres in these sizes being abandoned by manufacturers. By contrast there are 20 tyres in these sizes in the W range (270km/h). While it would run completely against my principles, it looks like a speed limiter would significantly reduce the risk of future tyre supply problems...

Perhaps such a limiter could be "switchable" by the driver when parked for "track only" applications!

[strad]

A ton of rears (multiple pages)not so many fronts
245/45/17 : https://www.tirerack.com/tires/TireSea ... iameter=17
.
205 45 17:[urlhttps://www.tirerack.com/tires/TireSearchResult ... iameter=17[/url]
Why the need for drastically different size fronts?