Guest bastaad525 Posted December 15, 2003 Share Posted December 15, 2003 Now that I'm on a mission to learn everything I can about how to make my car handle well... been going thru old magazines, SCC and others, looking for tips and stuff.... came across an fairly large article in an old issue of Sport Compact Car magazine that was very imformative, dedicated to suspension set up and how to tweak your car for a great handling ride. In one paragraph the writer makes a big deal about how even seemingly trivial things, like your toe adjustment/alignment of the front tires, and the tire pressures you are running, can have a HUGE effect on a cars ability to grip. They tested this on a project car of theirs... only adjusting those two things, and were able to get the car from pulling only .79g's to .84g's... a larger improvement than the numbers might suggest. So I got to thinking... changing tire pressures is free... why not? Now... reading further I gleaned a couple other tidbits... according to them, a tire inflated to a higher pressure, will provide better grip. This one threw me right there... because I know for drag racing you lower the pressure to get better grip at launch... does this apply differently for carving corners? They then made a suggestion of raising the front tire pressure while lowering the rear... however, when reading the article as a whole it becomes very clear that it is biased towards front drivers, which means this tip is probably also specific to a front wheel drive setup... everyone knows front drivers are very understeer prone... so taking the first factlet (higher pressure = more grip, lower pressure = less grip) adding the second (higher pressure in front, better grip for the steering wheels, lower in back, easier to make the tail step out) hence, a car with much less oversteer. Is this correct so far? Do you guys agree with all this? So my question is... what would the optimal balance be for a rear drive, oversteer prone car be (talkin about my Z here)? I'm thinking lower pressure in front and higher in back, or maybe just high all around? Up to maybe 40psi? I'm running plain-jane cheapy street tires, 205/70-R14's, on ZX rims, recommended hot pressure is 32-35psi. I'm not expecting huge improvements in my car's handling abilities from doing this, but the article did show some improvement, so I figure it's worth doing. What do you guys think would work better on a Z? Quote Link to comment Share on other sites More sharing options...
johnc Posted December 15, 2003 Share Posted December 15, 2003 Now... reading further I gleaned a couple other tidbits... according to them, a tire inflated to a higher pressure, will provide better grip. As exepcted from SCC - more mis-information. They have a few smart guys working there so I wonder how BS like this gets out. Without getting into a deep discussion about tires here's some basic information. Proper tire pressures for a street car and street tires are different then for a race car on race tires. Street Car on Street Tires You need to be concerned about load capacity, impact resistance, and then handling. Assuming you are dirving a street Z including you, a passenger, full fuel, and the other crap that ends up in your car, you need to do some basic math. Gross weight: 3,000lbs Weight dist: 52/48 Tire size: 185/70-14 (tires on my 810 as an example) Tire max load: 1,235lbs @ 44psi. How much weight can each psi in the tire support? 1,235 / 44 = 28lbs How much weight on each front tire? (3,000 * .52) / 2 = 780lbs How much air pressure to support the 780lbs each front tire holds up? 780 / 28 = 28psi So, we know that the MINIMUM tire pressure for each front tire is 28 psi assuming a static weight distribution. Under hard braking we can see up to 70% of the weight on the front tires so that means each tire needs to be able to support 1050lbs, which requires 38 psi. That can now be our MAXIMUM tire pressure for the front tires (You guys do the match for the rear tires.) We now have a range of pressure to work with to adjust the handling of our car (28psi to 38psi). What you'll want to do is start at the high number and keep an eye on tire rollover. You want to see the tire scuffed to the top of the little wear triangles marked on the side of the tread. Don't scuff the tire any farther onto the sidewall. Lower the front tires pressures until you see the scuffing to the point mentioned above. In some extreme cases you may have to go above the high number of the range due to suspension issues or driver problems, but normally this shouldn't be the case. Quote Link to comment Share on other sites More sharing options...
TimZ Posted December 16, 2003 Share Posted December 16, 2003 Now... reading further I gleaned a couple other tidbits... according to them, a tire inflated to a higher pressure, will provide better grip. As exepcted from SCC - more mis-information. They have a few smart guys working there so I wonder how BS like this gets out. I guess I don't see this as being that far off - yes, it's a bit over-simplified, but assuming that you are talking about lateral grip specifically, and that you are starting out with a reasonable tire pressure (say 32psi), this holds up pretty well as a rule of thumb. You need to be concerned about load capacity, impact resistance, and then handling. Assuming you are dirving a street Z including you, a passenger, full fuel, and the other crap that ends up in your car, you need to do some basic math. agreed Gross weight: 3,000lbs Weight dist: 52/48 Tire size: 185/70-14 (tires on my 810 as an example) Tire max load: 1,235lbs @ 44psi. How much weight can each psi in the tire support? 1,235 / 44 = 28lbs How much weight on each front tire? (3,000 * .52) / 2 = 780lbs How much air pressure to support the 780lbs each front tire holds up? 780 / 28 = 28psi Now, I hate to do this, but I have to disagree here - this math does not hold up (sorry John ). The psi units refer to the amount of pressure being exerted on all points on the inside of the tire - while there is a relationship, you cannot just say that each psi supports x amount of weight - the units don't match. PSI is in the units of force per area, so for the tire to exert a force on the ground in order to support the weight of the car, it must do so through the patch of tire that contacts the ground. So, I believe that what happens here is that the pressure applied across the area of the contact patch is what exerts the force on the ground. For instance, using the numbers above, 780lbs being supported by the tire inflated to 28psi would result in a contact patch of: 780lb/(28lb/in²)= 27.8in² So, for a given tire, increasing the inflation pressure should decrease the contact patch size accordingly, and vice versa. So how does increasing tire pressure result in better grip? Well, some other things are going on in the tire as well. In general, as the pressure goes up, the shape of the contact patch changes. This is a result of the way that the tire is constructed - the sidewalls generally get stiffer, and the contact patch gets wider (side to side) and not as long (front to rear). While this is helpful for lateral grip, it doesn't do much for longitudinal grip, which is why decreasing the pressure on the driven wheels can help you get traction when drag racing. Think of it more as adjusting the size and shape of the contact patch. Remember that the tire generates force in more than one direction, and the shape of the contact patch dictates how much force can be generated in any given direction. From this, should be fairly clear that the rule of thumb from SCC that started this thread is just a rule of thumb - increasing the tire's pressure will get you more lateral grip for a while, but at some point the decreasing contact patch size will start to dominate, and you will lose grip. Conversely, if you let too much pressure out, at some point the sidewalls will not support the tire sufficiently and bad things will happen. Yes, there is still more to it than that, but hopefully this will give you a better picture of what is going on... Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted December 16, 2003 Share Posted December 16, 2003 well it was a good thing the article got me interested in 'tweaking' my tire pressures... I just found out why my car has been feeling so floaty and soft in the turns lately... all four tires were inflated to between 15-20psi!! I feel like such a newb.... So I filled them up to 35 and now the car feels like I just put a whole brand new suspension on it again... the body roll is back to being all but gone in sharp turns, the steering is much lighter... the car feels like a 'handling machine' again, and is no longer frightening on the freeway The downside is I'm now remembering just how stiff the ride was after putting in the eibachs and tokicos... with that in mind I dont think I'll follow the magazines advice of pumping them up any more in the name of grip.... at least I dont think I will... for now Quote Link to comment Share on other sites More sharing options...
johnc Posted December 16, 2003 Share Posted December 16, 2003 The psi units refer to the amount of pressure being exerted on all points on the inside of the tire - while there is a relationship, you cannot just say that each psi supports x amount of weight - the units don't match. While I can't argue pneumatic theory (if the is such a thing) I think the relationship is pretty close because the math matches the recommended inflation pressures for my 1977 Datsun 810, 2000 F350, 1964 Lincoln Continental, and my 1970 240Z. PSI is in the units of force per area, so for the tire to exert a force on the ground in order to support the weight of the car, it must do so through the patch of tire that contacts the ground. So, I believe that what happens here is that the pressure applied across the area of the contact patch is what exerts the force on the ground. While I agree that the 780 lbs is pushing down on the ground through the contact patch, the vehicle is being supported by the air in the tire acting on all surfaces within the tire. In essence, you have 780 lbs pushing down through the contact patch and an equal and opposite force of 780 lbs pushing back up through the air chamber in the tire. That pushing up force is spread out over a large area so the force per square inch of tire is pretty small. That's why you get a contact patch to begin with. Increasing air pressure in the tire increases the force per square inch and reduces the contact patch size (rubber in contact with the road). I think that's just a rephrase of what you said. Given the above, more tread rubber in the contact patch increases traction until we get into tire overheating, contact patch distortion (from excessively low air pressures), or exceed the optimal slip angle. I don't have Miliken's book here at the shop, but they have lots of math to backup this statement. Now, there are exceptions to this - 335 tires on a 1500 lb vehicle would not stick very well because the tires would never get warm. If we are going to make general statements (as SCC does) then lower tires pressures increase grip until the tire starts to roll over. Due to sidewall flex you might see a decrease in responsiveness but now we're getting into a more detailed statement that must take into account the type of tire: DOT-R tires have stiff sidewalls to counter the reduction in responsiveness from lower tire pressures. I guess I'll have to remember to get Miliken's book from the house to respond in a more intelligent manner. Quote Link to comment Share on other sites More sharing options...
zguy95135 Posted December 17, 2003 Share Posted December 17, 2003 i have a great book about handling at home. its called Performance Handling: How to Make Your Car Handle, Techniques for the 1990's by Don Alexander . they have all kinds of info on tire pressure, camber, toe ect. they got a geo prism to do .91 from .81 just by changing camber and toe. they even have a 1.0g Shogun (hopped up festiva), and a GMC S-15 truck that pulls .93. amazing stuff Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted December 17, 2003 Share Posted December 17, 2003 So the question still remains, will a car handle and/or stick better with stiffer tires, or with lower pressure in them, using the factory recommended pressure as the starting point? I know you want less air pressure in the rears for better grip in drag racing, but what about for cornering? The car definately FEELS better at 35psi than it did when all the tires were at 20... Quote Link to comment Share on other sites More sharing options...
JMortensen Posted December 17, 2003 Share Posted December 17, 2003 The real answer is that there is no generic answer to your question. Each tire is going to react differently. I think the cheapest easiest way to see how your tires are reacting is to put white shoe polish over the edge of the treads and up onto the sidewall, and take some hard corners. See how much of the polish gets worn off, and adjust the pressure accordingly. As John C stated before, you want the wear to go over the edge to the top of the sidewall. Any more than that and they are rolling too far, any less and you aren't using all of the available traction. Pressure can only adjust this so much, so next step would be adjustable camber. Jon Quote Link to comment Share on other sites More sharing options...
TimZ Posted December 17, 2003 Share Posted December 17, 2003 In essence, you have 780 lbs pushing down through the contact patch and an equal and opposite force of 780 lbs pushing back up through the air chamber in the tire. That pushing up force is spread out over a large area so the force per square inch of tire is pretty small. Because the entire 780 lbs is transferred through the contact patch, the amount of force per square inch will by definition be the same as the inflation pressure. It doesn't have a choice. If we are going to make general statements (as SCC does) then lower tires pressures increase grip until the tire starts to roll over. Due to sidewall flex you might see a decrease in responsiveness but now we're getting into a more detailed statement that must take into account the type of tire: DOT-R tires have stiff sidewalls to counter the reduction in responsiveness from lower tire pressures. It sounds like you are making the assumption that the contact patch is always the same shape and just gets bigger and smaller as you inflate and deflate the tire. I am pretty certain that this is not the case. Due to the construction of the tire (I'll assume a DOT-legal radial tire here), the contact patch will change its shape as the inflation pressure goes up or down. In general, lateral adhesion goes up with increasing pressure, longitudinal adhesion goes down with increasing pressure. This has been bourne out through years of practical experience, by both road racers and drag racers alike. In case you are wondering, yes I have done this particular exercise, btw. To answer Bryan's question, you shouldn't really think of using tire pressure as a way to increase your total cornering grip. Yes, the car will pull more lateral g's with the tires at 35psi than it will at 20psi, but you should never try to do limit handling with the tires at 20psi, so the point is moot. The difference in ultimate lateral g capability between 35 and 40psi is probably not going to be that dramatic. You should use it more as a way to fine-tune your handling balance. For starters, I would use the advice that others have already mentioned as far as using chalk marks on the sidewall to determine a good base pressure to use for all four tires. You can then use the tire pressures to fine tune your handling balance - for instance, if you think the car understeers a bit too much, you could increase the pressure in the front and/or decrease the pressure in the rear. Start with small pressure differentials (say 1psi at a time) and see what difference it makes. It helps to not leave lots of time between your evaluation tests - it's really easy to forget exactly how much the car was oversteering or understeering if you don't evaluate back-to back at the same speeds over the same course. Do your tests in a safe place, like a race track or autocross course (legal disclaimer ). Finally, I would not recommend lowering your tire pressure below the manufacturer's recommended pressure for limit handling work. Reinforced sidewall or not, lower pressures will increase the risk of de-beading the tire during hard cornering, which would not be a fun thing. Quote Link to comment Share on other sites More sharing options...
johnc Posted December 17, 2003 Share Posted December 17, 2003 It sounds like you are making the assumption that the contact patch is always the same shape and just gets bigger and smaller as you inflate and deflate the tire. I am pretty certain that this is not the case. I'm not a trained engineer, more a monkey-see, monkey-do engineer if there is such a thing so take my statements for what they are worth... I think the shape and orientation of the static contact patch remains basically the same (an oval or an elipse, wider laterally for a low profile tire, wider longitudinally for a high profile tire) and that basic shape is determined by tire construction. An exception to this is when the tire is severely underinflated and has run out of tread width. The total square inches (the size) of the contact patch varies by inflation pressure and load but the overall shape remains an oval or an elipse. Dynamic contact patch shape, pressure differentials across the patch area, etc. are a whole 'nuther thing. In general, lateral adhesion goes up with increasing pressure, longitudinal adhesion goes down with increasing pressure. This has been bourne out through years of practical experience, by both road racers and drag racers alike. My experience has been different when racing on shaved street tires, DOT-R and slicks. On Hoosier R3S03s I'm running tire pressures 5 psi below what Hoosier recommends and the lap times, temps, and life have improved considerably. Same is true when running Kumho V700s and Hoosier 45 slicks. My example is different because I can get whatever camber I want and I adjust to get good tire temps across the tire. For a street car that is limited in camber adjustments you can't drop the pressures anywhere near as much. But, you DO want to drop (or increase) the pressures until you get scuffing to the tops of the triangle shaped wear indicators. In Brian's example where he was running 20 psi, the tire was underinflated. So, SCCs recommendation is true because he needs to increase his pressures. If the tires on his Z were inflated to the recommended 28 psi they are probably still low for best handling on his car. I guess if SCC is assuming that everyone is driving around underinflated, then their statment is true. Quote Link to comment Share on other sites More sharing options...
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