13 replies to this topic

[Nathan]

Hello all.  I circle track race, and my track has revised tire rules for my class this year.  In a nutshell I can go one of two ways...

 

Tire A offers a 7.5" tread width (not to be confused with section width) but is 26.1" tall and would add 20 lbs of total unsprung weight because it also means moving to a larger diameter wheel. The size of the tire would put me at about 5000rpm when at peak velocity

 

Tire B offers a 6.1" tread width is 24.2" tall, allowing me to run the car 1" lower, and puts me around 5400rpm at peak velocity.

 

Both tires are Michelin Premier A/S, an all season , so over-heating the tires is always an issue

So the root of the question is, will 22% more tread area and assumed lower tire temps trump running a 1"lower CofG and the unsprung weight advantage and ability to simply accelerate the tires faster while putting less stress on my brakes.

 

Some quick info...

- Peak HP is at 5200rpm

- I cant change gear ratios

- wheels must be an OEM or some sort

- the car is a FWD V6, makes considerable torque relative to the competition and is also around 200lbs heavier

- the 1/3 mile race track is made up of 2, 1/8 mile straights, so getting from corner exit to the next corner entry, to me, is where you gain the most.

 

 

Thanks for any help!

Edited by Nathan, 27 June 2016 - 17:57.

[mariner]

How much weight do you have on the front end/

 

If the car is +200lb I would guess its substantial and might suggest the wider tyre to ease the overheating issue.

[MatsNorway]

Bigger tire would be my pick. Even in a 70hp corolla at trackday events im shredding the tires if go hard at it. 175/65 R14 the car also needs toe out i believe but thats off topic.

Edited by MatsNorway, 27 June 2016 - 20:14.

[Nathan]

I don't have an exact number, but I would think it is safe to suggest about two-thirds of the weight is on the front end.  Car currently weighs around 2400 lbs.

 

The one knock against the wider tire is due to silly camber restrictions. Judging by used up tires I may have half the rubber of the right front on the track. Lots of sidewall roll....

Edited by Nathan, 27 June 2016 - 21:54.

[gruntguru]

It would help if you gave all the same info to compare your current tyre/wheel to the new choices.

 

My gut feel is:

 - for your track, gearing is the first priority (Choose the smaller wheel)

 - over-heating next (Choose the larger wheel)

 - CGH next (Choose the smaller wheel)

 - Unsprung weight next (Choose the smaller wheel)

 

Smaller wheel wins!

Edited by gruntguru, 27 June 2016 - 23:48.

[Nathan]

The rule change has really changed tire choice.  Before they simply capped section width and treadwear.  This year they added minimum sidewall ratio.

 

My previous seasons I could run a low profile sidewall and run the max section width and got the best of both worlds. My Vmax was right at peak power.  The minimal sidewall rule tosses that out the window and it's now A or B.  With the exception of another (who is very competitive and experienced), everyone seems to want maximize tire width.  As I stated in my last post, none of us can use the whole width of the tire at the FR so I don't see the fuss.

 

I think running the smaller tire is the way to go, but you lot here are far brighter and more experienced on the matter.  I'm lucky in that my first set of tires is free via a sponsor, so it doesn't effect me too much to have to buy an extra FR or two.  My only stumbling blocks are I can't think of any race series where people don't use the widest tire they can, I have to be very easy on my tires (which is probably good as I tend not to over drive the car as a result) and being the heaviest car running on the skinny tire while everyone is on the fat one seems to defy some logic.

Edited by Nathan, 28 June 2016 - 02:02.

[gruntguru]

Forgot to mention - I assumed no gear-changes during a lap. In that situation optimum gearing normally puts Vmax much higher than peak power - even to the extent where Vmax is "on the rev-limiter" - ie compromised.

 

Plenty of series out there where tyre size is un-regulated. There is an optimum size.

 

All else being equal, the only benefits of a larger contact patch are to allow reduced tyre pressure and softer compound ie if a softer compound isn't available, a wider tread will probably not help.

[Lee Nicolle]

Smaller tyre is probably the go. IF it rolls on the rim less it is putting down more power over more of the tyre. And the unsprung weight will slow accelaration even more

I presume this is a bitumen track? You should not melt a road tyre on dirt.

How much more inside weight can you get? Moving weight and lowering the left side will take some load off the R/F. Tyre pressures too will be critical to limit r/f wear.

Wide tyres on narrow rims are a disaster, though if the narrow smaller tyres too are on a narrow rim probably worse. You need the tyre on the max size rim recomended by the tyre manufacturer.

 

Here in Oz we have a Modified sedan class using [ nominally] 8" wide tyres, on now 7" wide rims. Tyre manufacturers recomend min 8" wide rim. Guess what they tyres roll all over the rims and build up a LOT of heat and right side tyres fall off the rim, even with tubes. Then the cars crash, and great lumps of clay come over the fence. Been there, hit in the face from 30 feet away. By the car I was spannering on!

[thegforcemaybewithyou]

Nathan, what do you estimate is the time percentage where you are traction limited(braking, cornering, very first part of the acceleration phase)?

 

What is the estimated rev and corresponding power figure for the smaller and bigger tyre just at the point when you are no longer traction limited at the corner exit?

[Nathan]

Track is bitumen (asphalt)

 

Weight distribution...well I'm at the point where I have even moved the ECU from sitting on the passenger door kick panel to the center of the dash where the heater blower motor use to sit.  If it can't be removed, if possible it's been lowered, moved to the left and rearwards.

 

If I move to a wider tire (215) I will be using 16x6.5" Subaru wheels to match Michelins recommendation.

 

Time wise, 55% of a lap I am traction limited.  Over a lap approx 5 seconds I'm limited under acceleration, 4 seconds in the corners, 2 seconds under braking, 10 seconds full throttle under acceleration.  All corners are single line.  Lots of guys get pissed off if I'm ahead because I can't get back on throttle as fast as they can (thus often getting a tap from behind), but when I do I can pull on them down the straight.  I can really only be passed if I make a mistake, or they get a great run out of a corner and displays bravery under braking. Going around someone in the corners isn't an option here. For me to pass it's all about corner exit and motoring by them (I'm about +20hp + 60 lb·ft ) just before the next braking zone.

 

When I return to full traction on the large tires approx 3750rpm, with the smaller tires approx 4050.

 

Modification wise all we can do is OE part swapping and typical intake work from inlet to the throttle body, and free exhaust from ports to tail pipe.  Factory numbers are 160 hp (119 kW) at 5200 rpm and 185 lb·ft (251 N·m) at 4000 rpm.

Edited by Nathan, 30 June 2016 - 14:10.

[thegforcemaybewithyou]

With the rpms of 3750 and 4050 and the peak power and torque figures given, i'd estimate the power

 

at 3750 as 97hp and

at 4050 as 106hp.

 

This difference of ~9% should also result in a similar difference in acceleration at the start. If you would give me the estimated weight of your car (including you), the velocity when you are no longer traction limited and the cd figure of your car, i can estimate what the difference for the 10 seconds acceleration phase would be.

 

I also estimated the average power under acceleration for the smaller tyre as 113hp and 109hp for the bigger tyre because of the different rev ranges used.

[Kelpiecross]

Test both sizes?

[Lee Nicolle]

Test both sizes?

[thegforcemaybewithyou]

If within his budget, that of course would be a smart thing to do!