vega Posted January 16, 2012 Share Posted January 16, 2012 Your contact patch will remain the same if you run a 175mm wide tire or a 235mm wide tire IF, and this is a big if, you use the same make and model tire and same tire pressure in this comparison. Now if you would run a wider tire with lower tire pressure, your contact patch would increase also increasing your static traction. Now before all of you either start arguing with me or go out and buy super wide rubbers and stuff them under your cars, hear me out further. Narrow tires will not roll as well as wider tires. Wider tires will distribute your vehicles weight across the wider contact patch, deforming the tire less. The more tire deformation will result in more rolling resistance of the tire. But, wider tires are less aerodynamically efficient at high speeds and add extra drag to the overall vehicle. Taller tires have more affect on tire traction than wider tires in pure lateral movement (i.e. drag racing). But as a result of running taller tires, you increase the torque on your entire suspension and drive train systems, causing potential for component failure. In the case of tires, although traction increases with load, it increases less than linearly, and coefficient of friction decreases with load. A larger contact area reduces the load per unit area, resulting in more grip. There is a point of diminishing returns, such as weight and aerodynamic drag. It's also true that a larger tire dissipates heat and with a lighter load involved, it wears less. Another reason for a larger contact area is to compensate for debris or track imperfections interfering with the tire and pavement contact. Note that load sensitivity is commonly used to adjust the understeer or oversteer of car. When a car turns, the down-force on the outside tires is increased and the down-force on the inside tires is decreased. The body of a car also rolls a bit, and the suspension can be used to unequally distribute the load between the front and rear tires. If the front end is relatively stiffer, then more of the down-force is exerted on the outside front tire, and the relative grip is reduced because of tire load sensitive. A stiffer front end causes the front end to lose some grip in turns, resulting in understeer. A relatively stiffer rear ends results in oversteer. Street cars are generally setup with understeer, while race cars are setup with a small amount of oversteer. For a variety of reasons, such as deformations, molecular bonding type reactions as well as surface roughness, real world friction isn't the simple thing described in physics books. Now to sum up everything rather than starting an endless rant… Going wider will not always give better traction. If you want an all sweeping statement that is roughly true. Wider contact patch sacrifices linear traction for lateral, narrow tires sacrifices lateral for longitudinal traction. And the most important thing about tires is not contact patch area but that they are at correct working temperature. Wider tires are not always better. They don't always give better traction. It depends on the car, the situation, the conditions. Eg. Rally cars use wider tires when on tarmac rallys, and use (surprisingly) very thin tires on ice rallys. F1 cars used to use narrow tires until aero began to be used in the 60's. Drag racers actually want tall tires, width is there to stop the tire being destroyed. An example I can think of is formula student cars, they used to use 8 inch tires but couldn't get them up to temperature. They switched to 6.5 inch and got more grip because they can get them up to temp. So before you go out and buy some super wide meats and cut the crap out of your fenders to stuff them in there…. Sit back and consider some factors. What temperatures are you going to be driving in? Are you only accelerating in straight lines like drag racing or are you autocross racing and need more lateral stability? How long do you want your tires to last? Obviously sticky tires don’t last as long as hard compound tires, but have a much higher coefficient of friction. And running tires with lower tire pressure can put too much heat into the tire. Lastly, if you are using your car as a DD, running 235mm wide tires when the stock size is 195mm, probably isn't helping you at all. Instead of modifying your car to fit these tires and paying higher prices for these wide tires, really isn't worth anything to you unless you plan on whipping it through your local neighbor hoods like a rally driver on a daily basis (which I will never encourage or think is “cool†to drive like an idiot on public roads)the point is though the mentality that wider always = better is wrong. There is an optimum limit even on drag racers. Basically though, there are just so many factors to consider when choosing a tire that one post cannot cover all of them. Gearing, suspension setup, driver preference, car classing rules, car style/power-train layout, car weight, wheelbase, wheel width/diameter, compound, heat generated, tire aspect size, power of car, overall width of setup, ...the list goes on. The theory all summed up sometimes a narrower tire will be faster than a wider tire for a variety of reasons. Quote Link to comment Share on other sites More sharing options...
ModernS30 Posted January 16, 2012 Share Posted January 16, 2012 Your contact patch will remain the same if you run a 175mm wide tire or a 235mm wide tire IF, and this is a big if, you use the same make and model tire and same tire pressure in this comparison. Now my only problem with this statement is that you left out wheels. I see that the same sized wheel would cause a wider tire to bulge making the contact patch more skinny, but if you have a properly size tire to wheel, the the tire should always be the size as advertised on the sticker/sidewall. Is this not correct? Quote Link to comment Share on other sites More sharing options...
johnc Posted January 16, 2012 Share Posted January 16, 2012 Taller tires have more affect on tire traction than wider tires in pure lateral movement (i.e. drag racing). ...longitudinal... Quote Link to comment Share on other sites More sharing options...
JMortensen Posted January 16, 2012 Share Posted January 16, 2012 (edited) Look at a given class for a particular type of racing, the class regulations for the max wheel and tire size and the size tire that was run by the champion in that series, and then show me the car that won a championship by running narrower tires than are allowed. There are VERY few examples where a tire less than the max allowed size will be faster, but they are out there. A couple of examples come to mind: 1. Land speed racing or circuits where the top speed is very high and there aren't many turns; places where aerodynamic efficiency trumps traction. 2. Power and/or weight limited cars like your FSAE example. If they were allowed unlimited power or set a minimum weight it's a pretty safe bet that they would run bigger tires. But they're maximizing to a particular rule set and they have optimized designs for weight and limited power, both of which dictate against large wheels. 3. Using bigger wheels at one end to balance a car that is not able to be balanced within the ruleset or by adjusting suspension settings. I used to autox with a guy who ran 8's on the front of his EP Ford Fiesta and 6's in the rear and this was faster because it balanced the handling. Likewise you might see a 911 running bigger tires in the back. I know Coffey has mentioned 350Z cars in stock classes running their slightly wider rears in the front. 4. As you mentioned, ice racing. Skinny tires with studs means more pressure per stud, so they're really trying to get the best balance of weight per stud to maximize traction. 5. Some classes of mud racing where you want a tall tractor tire that gets to the ground under the mud rather than trying to drive over it. I'm sure there are more, but for the vast majority of racing classes, more tire is better. If you're going to play monkey-see monkey-do with tire sizing though, I think the best place to start is the car that is most directly related to your own. For most of us that means looking at someone like John Thomas who runs a 16x10 which is the largest allowable tire and wheel combo in FP. He could choose a 16x7 or a 16x8, but he doesn't. Or Greg Ira, who runs a cantilevered slick on his EP car. If skinnier was better, one assumes he would run a regular sidewall. Those are the two most successful Z racers at present, but it's important to keep in mind that they are very limited in power compared to a lot of the cars on this site. Even at the power levels they run, they have opted for the max wheel and tire size. For those of us who are road racing and autoxing in a world of far from perfect suspension designs and not so limited horsepower, I think the best way to play is to get the widest tire you can fit to the car. We're shooting for the Glenn Bunch model, not the FSAE model. If you haven't seen it, Glenn has a 2500 lb Challenger running 15x14's on front and 15x17's on the back. http://www.youtube.c...ennbunch/videos Edited January 16, 2012 by JMortensen Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted January 16, 2012 Share Posted January 16, 2012 I find this topic confusing and potentially misleading. I'd just like to point out that anyone interested should take the initial post with a grain of salt as I think it glosses over much complexity, and go and read more authoritative topics if they're interested: http://www.amazon.com/Race-Car-Vehicle-Dynamics-R146/dp/1560915269 http://www.amazon.com/Competition-Car-Suspension-Design-Construction/dp/185960644X http://www.amazon.com/Competition-Car-Suspension-Practical-Handbook/dp/1844253287 The first couple of chars of RCVD (the only ones I've actually read), cover a lot of tyre basics and are well worth it IMHO. Dave Quote Link to comment Share on other sites More sharing options...
vega Posted January 17, 2012 Author Share Posted January 17, 2012 Look at a given class for a particular type of racing, the class regulations for the max wheel and tire size and the size tire that was run by the champion in that series, and then show me the car that won a championship by running narrower tires than are allowed. There are VERY few examples where a tire less than the max allowed size will be faster, but they are out there. A couple of examples come to mind: 1. Land speed racing or circuits where the top speed is very high and there aren't many turns; places where aerodynamic efficiency trumps traction. 2. Power and/or weight limited cars like your FSAE example. If they were allowed unlimited power or set a minimum weight it's a pretty safe bet that they would run bigger tires. But they're maximizing to a particular rule set and they have optimized designs for weight and limited power, both of which dictate against large wheels. 3. Using bigger wheels at one end to balance a car that is not able to be balanced within the ruleset or by adjusting suspension settings. I used to autox with a guy who ran 8's on the front of his EP Ford Fiesta and 6's in the rear and this was faster because it balanced the handling. Likewise you might see a 911 running bigger tires in the back. I know Coffey has mentioned 350Z cars in stock classes running their slightly wider rears in the front. 4. As you mentioned, ice racing. Skinny tires with studs means more pressure per stud, so they're really trying to get the best balance of weight per stud to maximize traction. 5. Some classes of mud racing where you want a tall tractor tire that gets to the ground under the mud rather than trying to drive over it. I'm sure there are more, but for the vast majority of racing classes, more tire is better. If you're going to play monkey-see monkey-do with tire sizing though, I think the best place to start is the car that is most directly related to your own. For most of us that means looking at someone like John Thomas who runs a 16x10 which is the largest allowable tire and wheel combo in FP. He could choose a 16x7 or a 16x8, but he doesn't. Or Greg Ira, who runs a cantilevered slick on his EP car. If skinnier was better, one assumes he would run a regular sidewall. Those are the two most successful Z racers at present, but it's important to keep in mind that they are very limited in power compared to a lot of the cars on this site. Even at the power levels they run, they have opted for the max wheel and tire size. For those of us who are road racing and autoxing in a world of far from perfect suspension designs and not so limited horsepower, I think the best way to play is to get the widest tire you can fit to the car. We're shooting for the Glenn Bunch model, not the FSAE model. If you haven't seen it, Glenn has a 2500 lb Challenger running 15x14's on front and 15x17's on the back. http://www.youtube.c...ennbunch/videos How does one calculate how much tire is needed for a given amount of power (from a dig) then? If you have a 80hp fwd crx and you have 305s (modded to fit of course )on it i am betting this isn't going to help. So how much is needed based on power? Quote Link to comment Share on other sites More sharing options...
JMortensen Posted January 17, 2012 Share Posted January 17, 2012 I don't have a formula for you, probably just have to keep adding tire until you go slower, then go back to what was fastest. Lots of guys in OR run FA tires which are 15x14 rears and 15x10 fronts, and I don't think any of them has gone back to 10s in the rear. I'm going to try running the rears on all 4, so I'll have something to report in 2042 when my car is running... Quote Link to comment Share on other sites More sharing options...
vega Posted January 17, 2012 Author Share Posted January 17, 2012 how do manufactures figure it out then? Quote Link to comment Share on other sites More sharing options...
JMortensen Posted January 17, 2012 Share Posted January 17, 2012 Never really thought about i, but if I were to hazard a guess I would suppose that they use a set of criterion that does not include "dry road course handling with slicks with several hundred lbs less and several hundred hp more than originally designed" (which is what is important to me) and that style, cost, safety, ride comfort, and tire wear are very high on the list for most car manufacturers. Quote Link to comment Share on other sites More sharing options...
jt1 Posted January 17, 2012 Share Posted January 17, 2012 You've got to throw compound in the mix also. Sometimes a smaller softer tire might be faster than a larger harder tire. jt Quote Link to comment Share on other sites More sharing options...
vega Posted January 17, 2012 Author Share Posted January 17, 2012 Never really thought about i, but if I were to hazard a guess I would suppose that they use a set of criterion that does not include "dry road course handling with slicks with several hundred lbs less and several hundred hp more than originally designed" (which is what is important to me) and that style, cost, safety, ride comfort, and tire wear are very high on the list for most car manufacturers. I would agree, if one were to stay the same weight same suspension setup, same (comparable) compound as factory. would the 130 (whp for a z) and the 195mm be a way to try to factor that in? 130/195=0.666666667. So every 1hp needs 0.666666667mmm of width for the tire. so a 450whp z would need a 300mm tire- etc. Quote Link to comment Share on other sites More sharing options...
Leon Posted January 17, 2012 Share Posted January 17, 2012 I would agree, if one were to stay the same weight same suspension setup, same (comparable) compound as factory. would the 130 (whp for a z) and the 195mm be a way to try to factor that in? 130/195=0.666666667. So every 1hp needs 0.666666667mmm of width for the tire. so a 450whp z would need a 300mm tire- etc. This would require an assumption that the relationship between tire size and maximum grip is a linear one, and that all equally-sized tires are equal. There are a ton of variables that play into this, so I would have serious trouble making these assumptions. IMO, there are two ways to choose a tire, either experimentally or from the manufacturer's tire data. 99.9% of the time, you don't have access to tire data which means you should go about it experientally. The right tire is the fastest one. Quote Link to comment Share on other sites More sharing options...
johnc Posted January 17, 2012 Share Posted January 17, 2012 Vehicle and tire manufacturers are primarily concerned with four things: 1. A tire's load capacity. 2. Cost. 3. Fuel economy. 4. Noise. With modern traction and skid control systems they are less concerned with grip then they used to be, except in the higher performance cars. I actually have a chassis engineering book at home that discusses this kind of calculation for chassis engineers and the above items are the primary concerns in that book. Quote Link to comment Share on other sites More sharing options...
RB240zDET Posted January 17, 2012 Share Posted January 17, 2012 This is a lot to take in. Never thought there were so many factors in choosing a tire. Good info though none the less. Quote Link to comment Share on other sites More sharing options...
vega Posted January 18, 2012 Author Share Posted January 18, 2012 (edited) JohnC what books? I would really like to read them. I have been studying these things for a while and it is difficult to find definitive answers. Edited January 18, 2012 by vega Quote Link to comment Share on other sites More sharing options...
johnc Posted January 18, 2012 Share Posted January 18, 2012 JohnC what books? I would really like to read them. I have been studying these things for a while and it is difficult to find definitive answers. http://www.amazon.com/Automotive-Engineering-Powertrain-Chassis-Vehicle/dp/1856175774/ref=sr_1_6?ie=UTF8&qid=1326904823&sr=8-6 http://www.amazon.com/Automotive-Chassis-Engineering-Jornsen-Reimpell/dp/0750650540/ref=sr_1_5?ie=UTF8&qid=1326904823&sr=8-5 Quote Link to comment Share on other sites More sharing options...
Leon Posted January 18, 2012 Share Posted January 18, 2012 Race Car Vehicle Dynamics ("the Milliken book") has a nice section on tire physics and is overall a great book. You'll likely have to read each section multiple times and take some time for it to sink in, but it has some great information. Quote Link to comment Share on other sites More sharing options...
vega Posted January 18, 2012 Author Share Posted January 18, 2012 Thank you for the recommendations fellas, without buying the books yet have we figured out a general idea of what a z car NEEDS for tire to hook with 400whp (2800 lbs)? I can't really run slicks on a road course (i know there are auto x tires) and I would like to drive the car on the street as well. Quote Link to comment Share on other sites More sharing options...
Leon Posted January 18, 2012 Share Posted January 18, 2012 (edited) Thank you for the recommendations fellas, without buying the books yet have we figured out a general idea of what a z car NEEDS for tire to hook with 400whp (2800 lbs)? I can't really run slicks on a road course (i know there are auto x tires) and I would like to drive the car on the street as well. Generally, it needs a tire that can provide enough traction (essentially friction force) at the wheels when 400hp is applied (assume 200whp per wheel), at a normal load of 700lb per wheel (assuming no weight transfer on inital take-off, and symmetrical weight distribution). Power is Torque X Speed, so depending on what the roadwheel speed is when you're making 400whp (depends on what gear you're in), you can calculate the torque at the wheel. Knowing the rolling radius of the wheel, you can then calculate the force that the tire must support in order to sustain that power. The hard part is finding the tire data to match up to your calculations. You're just not going to get it unless you work for a big-time race team. Hence, I gave the recommendation (paraphrased) of "put tire on, press down pedal. Traction or no traction?" There are just simply too many factors involved in making these generalizations. If you are truly that concerned, then it comes down to testing different sizes and compounds on the surface that you plan to run on. Experienced racers may be able to give you a loose suggestion based on what they've done but it's hard to say that XXX tire will be perfect for you, while sitting behind a computer. My general recommendation is fit the biggest tire as you realistically can on the rim and get the stickiest compound that you can, and that will be the best that you can do to "hook up" at 400hp. Any less, and you're taking away performance. If you want to talk road course, then you need a sufficiently stiff sidewall (not a big, bulging tire trying to swallow the rim). EDIT: Clarified a statement. Edited January 18, 2012 by Leon Quote Link to comment Share on other sites More sharing options...
johnc Posted January 18, 2012 Share Posted January 18, 2012 There is no tire, other then a purpose built drag racing slick, that will give you "no spin" levels of traction with the horsepower you're talking about. My 325hp, 2,100 lb. 240Z could spin its 275/45-16 Hoosier R4s at will in 1st and most of 2nd gear if I tried. Build your engine with good throttle response, make sure your throttle linkage has plenty of travel and is smooth, run a good LSD, and learn how to drive the car. Quote Link to comment Share on other sites More sharing options...
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