Purpose of 13in wheels at HPDEs

[Bster13]

I can understand the purpose of 13in wheels at an AUTOX, in that they will help you accelerate on a tight course with lots of stop and go, but what is the benefit on an HPDE? Is it the same thing?

 

Would the 15in wheel with good rubber serve me better on an HPDE since there is less accelerating and 15in wheels would lend to a higher top speed (where we tend to get passed anyway)?

 

But that would only be true if a) you don't need the extra cornering ability the 13s would give you (lower COG?) and b) the 7 does not hit an aero damn before it runs out of revs on the back straights.

 

I play on running the first few HPDEs w/ summer tires as I learn the car. It'd be great if I could stay there and have my fun w/ 200 tread wear tires for HPDEs all the time on 15 inchers and never have to deal with swapping race rubber or grinding down calipers to fit the 13s.

 

Thoughts?

[supersportsp]

13" wheels tend to be lighter. That reduces your unsprung weight and thus improves the ability of your suspension to react to inputs, improving handling. It also improves acceleration and deceleration as there is less mass at the extremities of your wheel/tire combo.

[Bster13]

So it improves acceleration and braking and helps your suspension reacts... I guess the question is do those benefits entering and exiting corners outweigh a higher top speed down the straights. I suppose it depends on how long the straight is and if u think run out of revs b4 you hit the aero wall.

[supersportsp]

I am assuming that you are referring to run an overall smaller diameter tire with 13"?

 

I doubt you will be able to redline your car in 5th if it has standard gearing even with the 13" wheels. Aerodynamics will kill you before you run out of gearing. You would likely accelerate better at the track with 13" wheels.

[pi7ot]

In my view, The higher aspect ratio of the side walls has one ironic advantage to the right driver....The suspension and steering provide more feel as the weight transfers. This can greatly enhance driver confidence. So if the driver is happy, the car is most generally faster. Heavier larger diameter wheels and the lower profile tyres are quite numb, on a sub 1500Lb car. That makes it harder to determine how much grip you can confidently exploit, and harder to gauge the degree of the slip angle as you are cornering.

The taller, softer side walls also allow a little more vertical load on the outside front tyre which equates to more grip at turn in.

You could probably get a sense of what I'm trying to say by dropping your tyre pressures. You get the same read of the tyres and side walls but it not as glaringly obvious and not detrimental due to the poor support of the tyre carcass and tread contact patch.

 

m

[Slimjim7]

lets not forget the lowering of mass

[Hank]

13 inch tires are cheaper.

[Kitcat]

My car was transformed when I switched from the 10 year old street tires that were on the car when I purchased it, to Toyo R1's.

 

I did 3 track days on the street tires and learned nothing as it had no grip and what grip it had was not progressive-when it let go, it let go with a bang.

 

Only downside to grippy tires (wh/I run on the street) is they pick up stones like crazy and sand blast the rear fenders.

 

I have put abt 10K miles on the Toyos, including many track days, and they are still going strong.

 

Mike

[Bster13]

The car will be the Brunton Super Stalker with 275hp/300tq. Mid 1400s for weight I'd guess.

 

If aero will get to me before gearing, I understand why someone would take all the other listed advantages of the 13in tires for an HPDE.

 

I'll most likely run Dunlop Direzza Sport Z1 Star Spec tires at first, and move to 13 inchers as I learn the car and my driving progresses.

 

@Hank - good point. haha.

 

(Sorry to put this in the wrong category, never knew there was a Tire & Wheel section until today, doh.)

[Ian7]

There's another aspect most people tend to overlook (or never knew?) and that is tire temperatures.

 

Good race rubber generates it's best grip at temperatures that would surprise the casual observer (pushing 200F). Our very light Seven's, (unless they have ridiculous amounts of power) are generally "over Tire-ed" by their owners, and the benefits of wider contact patches and stiffer sidewalls are more than lost due to tires than run well below their optimum temps because they are so lightly stressed; (again, though, huge power needs bigger tires, at least at the rear).

 

Many years ago, Car&Driver did an exhaustive study of an SCCA-spec race CRX and proved 13" tires were far superior on it to the larger "tuner" styles; imagine for our cars that are 700 pounds lighter....