I'm just about to purchase some track wheels: 8x17 and 9x17. While it's tempting to slap some fat rubber on these bad boys, the Puhn book makes it clear that narrower is better. In fact, he suggests you go with a tread width that's a couple inches narrower than the wheel (not to be confused with section width). The reason is that you will add sidewall stiffnes by using treads which are narrower than the rims.

Now, if you look at pictures of Porsche GT1's and other race cars you'll see that the wheels are as wide if not wider than the rubber. However, the "hot setup" which is commonly referenced here and on other boards involves using the widest possible tires on your wheels.

I'll probably go with Kumho's and use 225(8.9") up front and 255(10.0") at the rear but is there a better choice? I plan to use these wheels for track and auto-x events only, so street driving is not a consideration--it's all about performance. Any thoughts?

 

My summary of a very complex thread elsewhere -
Ignoring 'mechanical grip', the grip is a function of the tyre compound, temp., pressure, etc., and may actually be reduced by wider tyres.
In the real world, mechanical grip plays a very large part, so wider tyres will produce more grip (except in the wet).
Increased sidewall thickness of lower profile tyres will improve turn-in and steering response, but (of course) steering effort will increase with the width of the tyre.

If I can find the thread (from a Lotus engineer), I'll post it.

 

I hope this is the thread - haven't had time to read it!


Nick Adams
Member posted 30 October 2001 19:32
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Whoo Hoo, real questions!! Thanks Steven....
The effect you describe is a function of the gyroscopic forces generated by the rotational motion of the wheel, known as precession. The faster the wheel turns and or the greater the mass of the wheel, particularly that at the rim of the wheel, the greater the force generated. Although fairly impressive when held in your hands, the actual magnitude of these forces generated by a road wheel are not that significant in comparison to those generated by the tyre against the road and hence they play little part in the forces on the suspension bushes or the consequent camber change through bush/suspension/chassis deflection. Having said that, we do effectively take them into consideration when setting up the cars suspension as they are present when we tune the springs, dampers, roll bars, steeing rack friction and bushes and therefore affect the ultimate specification of those parts. The biggest effect that precession has on the car is the amount of feedback you get through the wheel, as the forces tend to resist the wheel twisting on it's vertical axis as you pointed out. This makes the wheel less inclined to push or pull against the steering when approaching the limits of grip and masks the feedback so essential to understanding how close to the limit the car is. We therefore try very hard to minimise the rotational inertia and therefore precession forces generated by the wheel by using the lighest rim possible (while maintaining adequate strength for normal road use) and the lightest tyre (achieved through the use of fancy composite carcass construction and smaller section tyres). This also benefits the unsprung mass of the car, which governs the ride quality of the car and the ability of the dampers to keep the tyres in contact with the road surface over bumps.
As you so rightly say, friction should be purely a function of the load applied to the two surfaces and the coefficient of friction (mu) between them. The width of a tyre should not therefore affect the levels of grip. While correct in pure terms, this fails to take account of the surface irregularities in the road and tyre, which interlock when one of the surfaces is flexible like a tyre. This interlocking creates additional resistance to lateral motion, boosting the apparent friction. A wider tyre therefore generates more grip than a narrow one of the same compound even with the same load applied. This applies less well when there is water on the surface of the road; if the load on the tyre is on the low side anyway because the car is very light and the tyre relatively wide then the surface pressure between the tyre and the road is going to be low, and may not be great enough to force enough water out of the surface irregularities to generate sufficient interlocking to achieve good grip. In this instance a narrower tyre, with a higher contact patch pressure can force the water out better and get a better interlock with the road surface, therefore generating better grip. This is what we did with the S2, the narrower tyre generates more wet grip than the old Pirelli ever could, and in the dry the softer compound we specified allows better interlocking than the Pirelli and therefore it generates more dry grip too, despite the loss of width. Being narrower and therefore lighter it would have generated less precession forces as well, but unfortunately the stylists insisted on larger diameters which negated that benefit and increased the unsprung mass.... It is true that the wide tyres on a Formula 1 car are partially necessary to reject the heat from the tyres and brakes, but as Ads says, this is really just a means of running a softer compound with better interlocking properties. Ads has unfortunately got his argument for bigger wheels back to front, although the rotational speed of a bigger wheel is very slightly lower, the rim speed remains the same while the increase in effective radius of the masses make balancing far more critical on big wheels.... (Good try though Ads, thanks for contributing )
You're also basically correct with your statements about the chassis rigidity; it is essential to keep the suspension mounting points in the correct place and to therefore avoid the geometry moving away from the optimum. The mass of the car plays a part in this as it affects the load inputs to the system, but a good proportion of the effect is down to the perpendicular distance between the point at which the forces are input to the chassis (the suspension pick ups or hard points as we call them)and the axis about which the chassis tries to twist. The greater this distance the greater the torsional load on the chassis and the stiffer it has to be to resist twisting. The design of the chassis has to therefore take into account the manner in which the chassis wants to distort when loads are applied and make sure that the loads are applied in such a way as to minimise the torsional inputs. As this would require the hard points be moved, and assuming that we have already defined the geometry we want to achieve the desired camber change with bump and droop, it is better to redesign the chassis to minimise this torsion than redesign the suspension and compromise the geometry.
I hope this answers your questions, I certainly enjoyed trying.
Cheers, Nick

 

Hmm... it doesn't answer your question about relative tyre/wheel width, but it's interesting nevertheless!

 

Dear Joey and John,
One reason you play with tyre widths is to get the set up correct. You have to understand that talking C4 is very different to talking C2 as well. However the general rule in playing with understeer and oversteer is as follows.
To reduce understeer you increase front tyre width and reduce rear tyre width.
To reduce oversteer you decrease front tyre width and increase rear tyre width.
From here on it it gets complicated and a lot of playing with other issues come into play.
I am sure you can work out many combinations and variations based on changing tyre width.
However the best known and actually track combination for a C4 is actually 225 tyres up front and 245s on the rear with a thicker rear sway bar. On a C2 you would actually install a thinner rear sway bar, ala 964 RS 18mm adjustable version or something like this. I know this does not really help with your answer totally because there are simply no black and white answers. However anyone just contemplating wider tyres front and rear just for looks should consider the affect on the handling of the 964. The combination affect may lead to an increase in understeer or oversteer. Some basic knowledge of the affects is advised to be learned before spending hard earned cash. Then complaining to that it no longer feels like it used to. The issue of a particular tyre dimension on a particular wheel width is dictated by the tyre. I highly recommend you follow the specifications laid down by the respective tyre manufacturer. they will advise which is the best wheel widths for their tyres. Too wide tyre on a particular wheel width and it will under stress roll off the rim.
Ciao,
Adrian
911C4

 

A related conversation <a href="http://forums.pelicanparts.com/showthread.php?s=e454a9922d0ef24bbc972f81ce777880&threadid=79047" target="_blank">Tire Width discussion</a>

 

JB- My Puhn book is 1976 edition so may have changed in a later edition. In reading my edition I came away feeling Puhn recommends widest tire, widest wheel that is approved for tire. Selection of ultimate size to be determined by diameter required and width clearance issues.

Generally a given tire developes greater cornering force with increasing rim width. This is because the stiffness of the tire/wheel assembly is enhanced and the tread is less distorted during highly loaded service.

Generally a wider tire puts more "rubber on the road" which at first glance doesn't make sense that a bigger surface area patch would be better, since each square inch of patch would have less contact force. The key is that the coefficient of friction of the contact patch is load sensitive. This means that at high surface loading the tire becomes "less sticky". This means a highly loaded small patch will stick less efficiently than a large patch that is less highly loaded (See Carroll Smiths books: Tune to Win andDrive to Win)

Another point about the wide contact patch was discussed in a BF Goodrich newsletter I unfortunately threw away some years ago. The long narrow contact patch of a "high profile" (High aspect ratio) tire is less efficient than a "low profile tires" (low aspect ratio) contact patch. This is because the leading edge of the contact patch in the high profile tire is actually turning around at a smaller radius corner than the trailing edge of the contact patch. The leading edge of the patch tends to be over worked while the trailing edge is under worked.

In the low profile tire the length between the leading edge and trailing edge of the patch is shortened and the stress on the contact patch is more evenly distributed, so the tire has a higher limit of adhesion. The relatively shorter, stiffer sidewalls of the low profile tire also stabilize the contact patch similar to the way a wider rim stabilizes it, thereby improving grip.

The problem is what works best for a given situation depends on the driver, car and course. For example large tires may not generate enough heat in an autocross to outstick a narrower tire because it is less stressed and running too cool.

As the tire wheel unit is optimized it becomes less forgiving to drive. For example as rim width is increased the stiffer wheel/tire unit reachs its limit in a less progressive way, breaking loose more all at once and some drivers may have trouble with this. The same is true for aspect ratio. Low profile tires are less forgiving and break away more abruptly. An advantage for the sensitive driver, the limit is higher but M. Schumacher is necessary to exploit it reliably corner to corner.

So in general pick diameter first, then the widest tire at the lowest aspect ratio at that diameter that you can fit. Then use the widest rim that the manufacturer recommends.

Then drive Like Michael Schumacher to take advantage of your new donuts..... <img src="graemlins/yltype.gif" border="0" alt="[typing]" /> <img src="graemlins/burnout.gif" border="0" alt="[burnout]" />

 

Cupcar,
As usual a nice analysis! I remember that BFG literature also, packrat that I am I will start digging <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

 

[quote]Originally posted by Cupcar:
<strong>Then use the widest rim that the manufacturer recommends.</strong><hr></blockquote>

Is this tire or car manufacturer?

Cupcar, I believe your impressions are accurate. Puhn does recommend a move away from the narrow wheels provided by the car's maker and encourages use of the widest wheel possible for all the reasons cited above. But, once you have your wheel width determined, doesn't he recommend you use tires whose treads are narrower than the wheel?

BTW, how'd the fat tires work out for you? I thought you were moving towards 275's, right?

Appreciate the thoughtful analysis and comments from everyone. Adrian, your thoughts on 225's on a C4 echo my experience exactly. I think the Victoracers in 225 and 255 sizes will be a good starting point for me. I'll let you know how it turns out.

 

That would be the tire manufacturer. <a href="http://WWW.Tirerack.com" target="_blank">WWW.Tirerack.com</a> has a good database of the manufacturers specifications.

The rim width is usually listed with a design rim width which is the rim width dimension at which the tires specified cross sectional width dimensions are valid. There is then a minimum and maximum width below and above the design rim that is listed. You can figure that the width changes by 40% of the actual rim change from the design rim. For example increasing the rim width by 1 inch increases tire cross section by 0.4 inches.

As for my own R&D program I have not gone wider as yet. I will on the next set of tires. The plan is to go from 225/45 fronts to 235/40's and on the rear to go from 255/40 to 275/40. I am also switching from the Victoracer to the Ecsta Kuhmo tires. I use 8.5 and 10 inch wide Fikse wheels. My plan was to go to 265 or 285 rears on 18" wheels but Fikse dropped from production the "deep drop" rim halves I needed to convert my 17" wheels to 18's in the rear. I wanted the 18" tires on the rear for their shorter sidewall and smaller diameter than the 275/40 17 tires have.

<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

 

I'm putting a turbo-look kit on my car and I've heard that I can get 9" wheels in the front and 12" wheels at the rear. I was thinking of 255/40 in front and 335/30 in the rear. They are both 26.1" tall, according to tire rack.
I am fascinated with wider tires because they look cool. (atleast I'm honest!)
I figure that I'll get more grip, but I have heard that they can be a little scary in the wet, due to the "surfboard effect".
I would be just driving on the street for now. Also, I would like to be conservative about the inside clearances, so I may drop down to narrower tires, if necessary. Once the body is complete, I plan to drive the car (on the old wheels) to a local expert who will help me choose the right sizes...
I'd be interested in any comments from the field, and I'll keep you posted on the progress.
Here are some pics of the progress to date:
<a href="http://home.attbi.com/~jbalthasar/widebody.htm" target="_blank">http://home.attbi.com/~jbalthasar/widebody.htm</a>