When are tires too wide......

[Grant 302]

The problem with this argument (for both sides) is there is so much theoretical positioning about this that it all amounts to nothing. No real examples even when talking about featherweight cars.

Nobody is really mentioning the added rim/wheel weight to support a larger tire. Incrementally, that might not seem like much when you add 1/2" or 1", but if you are talking about a small/light car that 'fits' a 6" or 7" wide rim and bump it to say 10", 11"+ in addition to the tire weight gain, there will be a *huge* penalty with regard to acceleration.

Nobody is mentioning track specific issues with small cars and *any* added wheel weight. Go uphill at say Laguna Seca in a small/low powered car and you *will* feel the difference when adding wheel/tire weight. Conversely, I know I felt the difference dropping 4 lbs./corner in a 400hp Mustang. I'm sure you would feel a gain of 2 lbs.+ per corner in a car with half the weight and HP. Going form say a 225 to 295 could easily add 6 to 8 lbs./corner in both rim and tire weight. Tell me that sort of difference doesn't create an issue with lap times at *some point*...

 

[racer47]

Nobody is really mentioning the added rim/wheel weight to support a larger tire. ..
Nobody is mentioning track specific issues with small cars and *any* added wheel weight. Go uphill at say Laguna Seca in a small/low powered car and you *will* feel the difference when adding wheel/tire weight. Conversely, I know I felt the difference dropping 4 lbs./corner in a 400hp Mustang.

......The only downside I see is a bit of extra tire and wheel weight that needs to be spun up down the straights. But I've never seen anyone pick up 5 seconds a lap with light weight wheels. But everyone goes faster with bigger tires. I don't doubt that there are diminishing returns and probably some hypothetical maximum size. But thats likely to be way bigger than the practical size limit.

I brought up wheel weight as a negative. You may think you felt a difference with 4 lbs per wheel but did it make a significant lap time improvement?

So to sum all the stipulations.....Say you're racing a 800lb car, with a 30hp motor, on a road course that is mostly comprised of uphill straights, in 40F ambient temps and the only tire available is a super hard tread compound........then maybe a 175 width is the hot setup.

 

[Grant 302]

No. Don't put words in my mouth.

 

[racer47]

Not meant to be directly at you. I just combined all the restrictions everyone said (including your uphill) into something that I thought was funny / maybe a bit smartassy.

This whole things seems crazy to me. I've been going to race tracks since I was 5. My Dad raced USAC, then drag. I started in motocross as a kid, street stocks, gt1, gto, super lates and now back down to street car stuff. In all that time I've never heard anyone ever say that they needed less tire to go faster.

 

[WinterSucks]

^^ I also feel it is situational. I race an 87 Prelude in Lemons, class B, and we have used wider tires and it definitely hurt the corner exit acceleration in my opinion. I think the owner was trying 235's last year. We had to add ghetto scrap metal fender flares to what little fender was actually left. And the suspension is too Lemony to really bomb around turns and take advantage of the extra grip. We might be just as fast or faster than thinner lighter rubber if it was a time trial, but this is wheel-to-wheel with A LOT of inexperienced drivers. Quite often we end up bunched up behind a slow driver since the tracks here in NE are narrow with lots of turns. So being able to accelerate quicker out of the turn is more important than taking it faster, in my opinion. While in the turn I generally have no idea where a marginal driver will go. They take turns slow and are all over the place. It does not matter if you can take the turn faster on the line or being creative off the line to try and pass mid-turn, they will end up pushing you off the track or forcing you to brake hard with some random nonsense. But the one thing I do know is that if I can keep the line behind them and out-accelerate them on the exit then I can forget about them for a few laps. If I can't get out in front by the time we exit I just end up behind them again at the next turn since the one thing they can do is mash a pedal and my whopping two-digit ft-lbs of Honda torque is not going to save me in a straight line. If I can't get around them by the second turn then it gets really hairy since the class A or faster B cars behind/next to me start getting impatient and out-accelerate me easily when the rolling road block in front does finally leave an opening. It can get bad when you have 120 cars on the track at NHMS. The class C guys know how to drive slow properly it seems. They are slow but they stick to one side of the track so that everyone else can get by.

 

[Grant 302]

This whole things seems crazy to me. I've been going to race tracks since I was 5. My Dad raced USAC, then drag. I started in motocross as a kid, street stocks, gt1, gto, super lates and now back down to street car stuff. In all that time I've never heard anyone ever say that they needed less tire to go faster.

I brought up wheel weight as a negative. You may think you felt a difference with 4 lbs per wheel but did it make a significant lap time improvement?

I tried to bring the discussion about smaller or lightweight examples into context. When you mentioned "a bit" of extra wheel weight, you make it sound trivial. 6 to 8 lbs. per corner on a sub-200 HP car is not trivial on track. More so when you add elevation changes.

But back to the 4 lb. in my car, that was with a compound and other changes, so I can't say what the weight reduction was worth for the incremental change.

<snip>...something that I thought was funny / maybe a bit smartassy.

I got the latter, not the former. <- smilies let people know when you're trying to be.

 

[Busdriver]

For whatever it's worth based on my previous example it requires in the neighborhood (depends on what baseline acceleration value you use) of a 20% +/- 5% reduction in average acceleration for the big tired car to not get to the end of the straight before the small tired car. The less acceleration available to the small tired car, the larger the percentage reduction in acceleration value is required to get them to the next corner at the same time.

 

[VoodooBoss]

Most TMO members are going from 33 lbs wheels with 255 tires to 23 pound wheels with 305’s. That single change will improve your lap times better than just about anything else you can do to a street car. It’s a huge win. Hard to go much wider without modifying the car. If you can stuff wider tires on your Mustang I say go for it!

 

[Mad Hatter]

We all know that a 315 is a successful upgrade! But my question was more for the massive sizes like 335 o 345's I have seen several cars now that have been faster with 315's then 335's.

 

[VoodooBoss]

That’s my way of saying I agree with the below. Vorshlag @modernbeat showed how far you can go with wider wheels and tires on a S197. There’s always limits and compromises the key is to find the max width to optimize your situation. There are lots of variables so I don’t think there’s one answer to your question.

I can't see any drag penalty. The rolling resistance difference is trivial. The tire aero drag difference is trivial on full bodied car. The only downside I see is a bit of extra tire and wheel weight that needs to be spun up down the straights. But I've never seen anyone pick up 5 seconds a lap with light weight wheels. But everyone goes faster with bigger tires. I don't doubt that there are diminishing returns and probably some hypothetical maximum size. But thats likely to be way bigger than the practical size limit.

 

[Norm Peterson]

I have seen several cars now that have been faster with 315's then 335's.

Same diameter? Same compound? Same number of heat cycles?


Norm

 

[Mad Hatter]

Both cars were with new rubber and they actually ended up being slower then older smaller R7's. I suspect its more complicated then just tire width, as mentioned by several others, suspension changes or limitations as well as changes in setup surely have a big part to play..... and both cars are over 500rwhp. Hope to find out if I get 18x11 tires and 315s soon.

If I remember right the diameter stays pretty much the same between 295, 315 and 335 Hoosiers for 18 and 19" sizes in "30" series.

 

[Norm Peterson]

Going up from 315/30-18 to 335/30-18 gives up about 2% acceleration.

Agreed that there are plenty of other factors . . . that a 335/30 is likely to be a tiny bit 'softer' on the same rim width being yet another possibility.


Norm

 

[Mad Hatter]

Going up from 315/30-18 to 335/30-18 gives up about 2% acceleration.

Agreed that there are plenty of other factors . . . that a 335/30 is likely to be a tiny bit 'softer' on the same rim width being yet another possibility.


Norm

Is the 2% calculated from friction? In R7's in 18" with either 315s and 335, they are the same diameter. 25.6"

 

[Grant 302]

Going up from 315/30-18 to 335/30-18 gives up about 2% acceleration.

Agreed that there are plenty of other factors . . . that a 335/30 is likely to be a tiny bit 'softer' on the same rim width being yet another possibility.


Norm

Yeah, wider tire on the same rim effects haven't been mentioned earlier. Doesn't always pay off, or has some undesirable side effects. I like the responsiveness of a 275 on a 10" rim, but the 305s were sloppy and mushy on the same rim.

If the 2% you're talking about is for the change in gearing, I'd agree. People do seem to forget/ignore that a taller tire is one way to get a larger contact patch and/or have a slightly longer lasting tire.

One example to ponder...if a Mustang on 305 Pirelli DH can lift two inside tires on a turn, how much faster is it going to be on 315 or 325 DH? Versus the same theoretical size in R7 or A7 compound?

 

[Norm Peterson]

That 2% is from the difference in diameter, the 335/30 being about 2% taller (calculated from 335 mm wide x 30-profile since I didn't have any exact dimensions). Revs per mile data might find the difference to be somewhere between 1.x% and 2.x%, but I figured that 2% was a good enough answer.


Norm

 

[Mad Hatter]

That 2% is from the difference in diameter, the 335/30 being about 2% taller (calculated from 335 mm wide x 30-profile since I didn't have any exact dimensions). Revs per mile data might find the difference to be somewhere between 1.x% and 2.x%, but I figured that 2% was a good enough answer.


Norm

Have the Hooser R7 specs pretty much memorized and as mentioned before......... the R7/A7 in 315/30-18 and the 335/30-18 are 25.6" tall. Guess it would be better if Hoosier called them 315 and 335 /650-18 like a race tire. Even the 295/30-18 tire is really a 295/650-18. Imagine its for people to have a easier time seeing what tire fits their car! But the size nomenclature they use does not compute accurately like a typical street tire would...

 

[Norm Peterson]

Fair enough - I wasn't expecting that much difference in nominal size to end up at exactly the same OD.

But there is a 4 load index difference, which might be affecting the revs/mile once the car's weight is placed on them through differences in tire vertical stiffness (load index is mainly an air volume inside the tire thing, and I'm pretty sure that tires are actually rated based on deflection). I'm not even going to try to guess at any % here.

My reference to 'softer' in a previous post was with respect to lateral stiffness rather than vertical.


Norm

 

[Grant 302]

My reference to 'softer' in a previous post was with respect to lateral stiffness rather than vertical.

I think that was clear.

 

[stuntman]

305/18 R7s on a spec Miata will be slower than 205/15 R7s. There is always a point of diminishing or worsening returns. Whilena 225 will probably be faster, at some point, going wider on a tire will not make it faster. I'd venture to say anything beyond a 275 would be slower in terms of rotational and unsprung mass, and that a spec Miata can't generate enough acceleration or corenering load to benefit from a much wider tire than a 275.

Again, if your paradigm is limited to just heavy and powerful Mustangs, you'll probably not ever get to the point where you will lose time from a wider wheel/tire combo, unless you're losing efficiency by putting say a 335 on a small 10" wheel.