johnc Posted December 10, 2008 Share Posted December 10, 2008 Having experienced something similar I say yes, for road race tracks with long straights, a wider tire can actually be slower then a narrower tire due to aero drag and the additional MOI. From the latest Mark Ortiz chassis newsletter: IS THERE SUCH A THING AS TOO BIG (IN TIRES, THAT IS)? My question this time is regarding slicks and the width thereof. I have an MG Midget set up for road racing and at present run 8in slicks. The front is the usual double wishbone and the rear is live axle. All are converted to telescopic shocks. Given the same suspension system, is there a point where tyres can be too wide? Another midget runs 7in slicks and the comment was made that 7in was as wide as was needed but the previous owner went to 8in as the tyres were easier to get at the time. Would there be any advantage in going to 10in wide? Would the setup need to be changed a lot to get any advantage? I was going to offset the rims so that the extra width added to the overall track. If a road course were a skidpad, the answer would be fairly simple. In general, wider tires give greater lateral acceleration, within any practical limits, on practically any car, provided that setup and inflation pressure are optimized. However, most road courses have significant straights. In many cases, road races are won on the straightaways. In most of these cases, the straights are at least partially won in the turns: faster cornering lets us start each straightaway at a higher speed, and brake later at the end. It may be that there are cases where a narrower tire actually will give more cornering power due to higher temperature. Some tread compounds – most, in fact – have a threshold temperature, below which they do not make good grip. However, hotter is not better without limit. Above a certain point, higher temperature hurts grip. It may also cause more severe heat cycling, which makes the tire go off more as a run progresses. If we do have a case where a narrower tire grips better due to temperature, we can say with great confidence that the effect will be highly weather-sensitive: the colder the day, the narrower the tire should be. That makes it difficult to state categorically what tire size we should put on a particular car. In general, we can say that bigger is better for grip within any practical limits, especially if we have some freedom in choosing compounds. The main drawbacks to big tires are that they weigh more, have more rolling resistance, and make the car wider. Greater car width compromises the line we can take, and it adds aerodynamic drag. The line issue is more important for autocross or hillclimbs on narrow roads than on higher-speed road courses. In general, for road course work, the issue mainly comes down to grip versus drag. I mentioned the possibility of winning the straightaway in the turn. Perhaps an example will help illustrate. Suppose you are on wide tires and are racing another car on narrow tires, and consequently you have more grip than your competitor, but he has less weight and drag, and therefore has better top speed and top-end acceleration. Suppose your grip advantage is good for a 1mph advantage in a particular turn. Suppose that this competitor is right on your tail approaching the turn. You will be able to brake later, partly because your car can slow at a greater rate (not only because of greater grip; drag actually helps you here), and partly because you are able to take the turn faster. So you will start pulling out some distance, or at least some time, as soon as your competitor begins braking. You will continue to build your lead through the turn. At exit, you are still 1mph faster, and still building your lead. Now your competitor can build speed faster than you can, at least if the turn was fast enough so that both cars are now power-limited rather than grip-limited. Is your competitor gaining time on you? Not yet. His drag and weight advantage has to get him up to your speed before he even starts cutting your lead. Once he starts cutting your lead, he has to catch you. Once he catches you, he has to pass you. He has to at least get up even with you before the next braking zone to be able to even have his nose alongside your tail entering the next turn. Remember, he has to brake earlier than you. If the straight is long enough, he can get by. But the length required is greater than might be imagined. It will be apparent that in at least some cases, the choice will depend on the course. Long straights and high speeds favor the narrow tires more. A track where most of the lap is spent cornering will favor wide tires more. The amount of engine power influences the choice as well. Ample power favors wide tires. Meager power favors narrower ones. Quote Link to comment Share on other sites More sharing options...
datsun40146 Posted December 10, 2008 Share Posted December 10, 2008 Very informative, thank you. Quote Link to comment Share on other sites More sharing options...
cockerstar Posted December 10, 2008 Share Posted December 10, 2008 Does a great job at going in depth while maintaining readability and clarity So, in an all out race car the consensus is bigger is better most of the time due to 'winning the straight in the corners'? For the narrow tire to be better, the straight would have to be ridiculously long, correct? Quote Link to comment Share on other sites More sharing options...
aziza z Posted December 10, 2008 Share Posted December 10, 2008 Having experienced something similar I say yes, for road race tracks with long straights, a wider tire can actually be slower then a narrower tire due to aero drag and the additional MOI. When you experienced something similar to that in the letter, was it in a z car? Mind posting some specs (weight, engine/power, rims size, tire size, course etc)? Just for education purposes anyway. Mind be good to know. Quote Link to comment Share on other sites More sharing options...
stony Posted December 10, 2008 Share Posted December 10, 2008 drag cars run skinny tire for a couple reasons. small contact patch, less weight and of course less drag. I noticed a pretty substantial difference in my track times just switching to a skinny front drag tire. I know this doesn't apply to road racing but thats my 2 cents Quote Link to comment Share on other sites More sharing options...
Silver Mine Motors Posted December 10, 2008 Share Posted December 10, 2008 This might be a bit off topic Quote Link to comment Share on other sites More sharing options...
ZWOLF Posted December 10, 2008 Share Posted December 10, 2008 With a slight twist the Tyrrell P34 is always an interesting example in the Aero/Tire discussions. http://en.wikipedia.org/wiki/Tyrrell_P34 In Indy and F1 cars the front tires create huge Aero/Drag issues. Neat car! I heard from the fellas that would know, that the braking capabilities of these cars were astonishing. Quote Link to comment Share on other sites More sharing options...
Guest Gr8White Posted December 10, 2008 Share Posted December 10, 2008 drag cars run skinny tire for a couple reasons. I noticed a pretty substantial difference in my track times just switching to a skinny front drag tire. Ditto, I dropped .10 seconds in the 1/8th with just going from 245 wide on a 15 x 8 to 165 metric radials on a 15 x 4...... Quote Link to comment Share on other sites More sharing options...
johnc Posted December 10, 2008 Author Share Posted December 10, 2008 When you experienced something similar to that in the letter, was it in a z car? Mind posting some specs (weight, engine/power, rims size, tire size, course etc)? Nissan 350Z, stock engine, some suspension mods, aftermatket LSD... California Speedway Roval course: This course puts you onto the front straight just past the exit of turn 4 and you're on the oval until about 1/3 the way down the back straight - probably 40 seconds with your foot to the floor. Last year I ran the event on 245/40-18 BFG gForce R1 tires on 18 x 8.5" wide wheels. Erik Messley was my co-driver and he's a pro racer having class wins at the Rolex 24 Hours, Paul Revere 250, Sebring, etc. His best time was a 1.56.9xx. This year, same car with some addition suspension mods (more camber in front) and 285/30-18 Hankook 214 C50s. Erik ran a best of 1.57.7xx and noticed the car was down 5 mph at the end of the front straight. Some of this speed/time difference could be due to the tires, weather, etc. but we both think the increase in tread width and the additional weight of the wheel tire combo contributed to the reduction in lap times. Much of a fast lap on the Roval depends on how fast you can run with your foot to the floor on the oval. Quote Link to comment Share on other sites More sharing options...
junglist Posted December 10, 2008 Share Posted December 10, 2008 Nissan 350Z, stock engine, some suspension mods, aftermatket LSD... Were you able to carry more speed into/through corners with the wider tires? On my 350Z I noticed a big difference in the cars ability to accelerate out of a corner earlier when I switched from 235/45/17-FR - 245/45/17 RR to 245/40/18 FR - 255/40/18 RR. Both sets of tires were falken azenis. The 17"s were the '03 enthusiast wheels and the 18"s were the '03 rays track wheels. Quote Link to comment Share on other sites More sharing options...
johnc Posted December 10, 2008 Author Share Posted December 10, 2008 Were you able to carry more speed into/through corners with the wider tires? In the Cal Speedway Roval example above you're (at least Erik is) foot to the floor from the point you get onto the front straight until you brake for the 90 degree left into the infield off the back straight. As Erik said, he was 5 mph slower entering turn 1 (with his foot to the floor) on the 285s then he was on the 245s. The difference was 140 mph vs. 145 mph. Tire scrub in turns 1 and 2 limit speed gain. Getting on to the front straight was about the same as indicated by the shift point from 3rd to 4th - right when the car transitions onto the front straight banking from the pit entry. Quote Link to comment Share on other sites More sharing options...
TheNick Posted December 10, 2008 Share Posted December 10, 2008 John I assume your 5mph difference accounts for the different tire diameters too? The 285 would reduce your top speed in each gear by about 4% Quote Link to comment Share on other sites More sharing options...
johnc Posted December 10, 2008 Author Share Posted December 10, 2008 Maybe not... I forgot about the OD difference. The BFG 245/40-18 is 25.4" OD and the Hankook 285/30-18 is 25" OD. Stock tire OD is 25" so the BFG tire overstated speed by 1.6% which would be about 1/2 the 5 mph speed difference. Quote Link to comment Share on other sites More sharing options...
TheNick Posted December 10, 2008 Share Posted December 10, 2008 Makes sense - I got my 4% assuming the the 245 was 25.7" tall and the 285 was 24.7" (just worked the numbers without mfg data) Quote Link to comment Share on other sites More sharing options...
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