jt1 Posted January 4, 2008 Share Posted January 4, 2008 Pretty much a bench racing type question, but I would like to hear your thoughts. Let's consider a V8 road race S30, about 2500# and 400hp, caged, with a modified stock style suspension, say the Arizona Z setup, running on a two mile road course with two long straights and varying medium radius turns. You have a choice of two tire/wheel combos to run, different sizes with the same compound: Hoosier R6, 275/30/15, 23" tall, mounted on a 14# wheel, with a total tire wheel weight of 34#. Hoosier R6, 335/35/17, 25.6" tall, mounted on a 22# wheel, with a total tire wheel weight of 51#. You can vary camber, caster, and toe to suit each tire, but you must keep the vertical angle of the LCA's the same. The 335's will raise ride height 1.3", and we will keep the same suspension geometry for each tire. So which tire/wheel combo will give the faster lap times? The 335's offer greater contact patch, but increase total weight 68#, increase rotating unsprung weight the same amount, and raise the CG 1.3". What do you think, and why? John Quote Link to comment Share on other sites More sharing options...
JMortensen Posted January 4, 2008 Share Posted January 4, 2008 I'd be inclined to try the smaller ones first because of the type of course you are running. If it was a really tight course you might make use of the wider tire, but with a fairly wide open track with high speed corners I don't know that the traction benefit in the corners would outweigh the detriment in acceleration and braking and aero everywhere else. I don't think you'd really want to keep the same alignment settings for both tires either. And spring choice will vary as well, figure on stiffer springs with the wider tire. Neither is a small tire so it's not really a black and white thing, I'll be interested to hear what other people have to say. I assume this is all in prep for the running the IMSA flared car? I can't imagine fitting those tire sizes under much less flare than that... Quote Link to comment Share on other sites More sharing options...
mark Posted January 4, 2008 Share Posted January 4, 2008 I'm sure John has some very technical things in mind when he asks a question. Jt is very detail oriented and it shws on the track and in his car preparation. I'll give you my scattered less detail oriented thoughts. I've ran the yellow Z numerous times, mostly on victor racers. 245 17s rear and 225 17s front. Spring rates from 225 to 350 at varoius times. Struts are double adjustable konies. No matter the alinment settings, springs or shock adjustments, I steer with the throttle. Turn in is crisp and predictable. The car will rotate how and whenI want it to based on throttle input. I love it but I want more grip. My yet to be proved solution for this is 315 17 rears and 285 fronts. Different shocks and springs. Lighter car with a little less camber in the rear and more caster in the front with a little less toe out in the front. My goal is to have a car with similar handeling characterists but at a higher limit. I*'ll start the process of debugging the car soon. The general alingment specs on both cars follow John COffeys recomendations that is in a sticky somewhere. I did end up geting the big tire car about a half an inch lower at the rockers than the yellow car. I've yet to scale the car but I bet weight distribution will be very similar between the two. If I rambel or am incoherent it's 10 mg of valium speaking:biggrin: John, What are you working on? Quote Link to comment Share on other sites More sharing options...
johnc Posted January 4, 2008 Share Posted January 4, 2008 Ran a similar kind of tire test on a K Prepared/DSP E36 BMW swapping from 245s to 285s. My calculations said it would be a draw. Actual real world testing showed the 285's had a 1 to 2.5% improvement in lap times with less falloff in grip towards the end of the session. The 285s had a larger OD and were a heavier wheel and tire setup but the gain in corner exit speed more then made up for any loss in the rate of acceleration down the straight. The car was easier to drive at 10/10ths and had more consistent lap times throughout a 30 minute race session. Go with the bigger tires if you don't have to compromise alignment, roll centers, etc. Quote Link to comment Share on other sites More sharing options...
jt1 Posted January 5, 2008 Author Share Posted January 5, 2008 Well, if I post something incoherent, it's probably the tequila. It's been a tough week, and I'm glad it's almost over, despite the fact I'm working tomorrow. Despite making very little actual progress on the track car, I keep thinking about it. The contractor in me requires that I have a pretty complete plan in place, and the engineer demands that any solution be fairly optimized as part of a complete system, not a pile of ill fitting parts. The basic goals for the track car are to be considerably safer, and at least some faster. The white car is pretty quick, but 160 mph in a 35 yr old car with a no cage, stock seats, five point belts, no fuel cell, and no fire system is just a risky proposition. I'm a very conservative driver, but you can't eliminate mechanical failure or the actions of another car, such as oil or antifreeze all over the track. I try to do about 20 days on track per year, and eventually the odds are going to run out on me. So....... The plan is cage a series one tub, put a good suspension on it, and transfer the motor, clutch, trans and diff from the white car. I might tune the motor up a little. Fuel cell, fire system, good seats and belts along with the cage will be a lot safer, and it should be as light as possible. Lighter is both safer and faster. I'm even trying to lighten up the driver a little, but he is an obstinate ass. So, what tires? I went from a 17# wheel to a 14# wheel on the white car, and was amazed at the difference, mostly under braking and off. So, a 17# difference in rotating unsprung weight would probably be akin to towing some crackheads taurus entering or getting off a turn. However, the whole point is to gain corner speed, and I suspect that right thru the center of a turn, the 335's will show an advantage. Since the 275's and 335's are the same compound, there is at least some possibility you might not get the 335's up to optimum temp. Full race slicks aren't in the program. I've proved with the white car too low is bad. No matter how many holes you drill in the crossmember, or spacers under the spindle, you need to keep the thing gaining negative camber in roll. When it goes positive, all you do is roll off the outside of the tire and slow down. The rear isn't as bad, but it's still there, and raising the rear LCA pivots is a bad day. So, when you put on the big tires, you raise the CG in order to maintain some sensible camber curve. Ultimately, the mass of the car, the height of the CG, the track width, and the amount of lateral grip determine how fast the car corners. So with the big tires you gain grip, lose CG height, and increase weight. The smaller tires have less grip, but get in quicker, off faster, and transfer less weight, so you use them all more equally. Damn if I know which is better. I know I won't be as fast as Mark, but I want to at least be able to keep him in sight. Cost is also a factor. I spend a good bit on the car, but again, it has to be optimized. 335's will run about $1300 a set, 275's about $1000. I usually buy 3 or 4 sets a year, so that's not insignificant. The 335 wheels will run $2500, 275's probably $2K, and you need a couple of sets. More $$$$$. I guess the answer is to buy two sets of wheels and try the two options. Or i could have another drink and shoot my big toes off. That ought to put the tire decision on the back burner. This has got to be the most rambling buch of crap I've ever posted. jt Quote Link to comment Share on other sites More sharing options...
mark Posted January 5, 2008 Share Posted January 5, 2008 I read this about 20 minutes ago and I'm still laughing. You bring the tequila, I have the valium. Together we can both shoot our toes off and not care. Well, if I post something incoherent, it's probably the tequila. It's been a tough week, and I'm glad it's almost over, despite the fact I'm working tomorrow. Despite making very little actual progress on the track car, I keep thinking about it. The contractor in me requires that I have a pretty complete plan in place, and the engineer demands that any solution be fairly optimized as part of a complete system, not a pile of ill fitting parts. The basic goals for the track car are to be considerably safer, and at least some faster. The white car is pretty quick, but 160 mph in a 35 yr old car with a no cage, stock seats, five point belts, no fuel cell, and no fire system is just a risky proposition. I'm a very conservative driver, but you can't eliminate mechanical failure or the actions of another car, such as oil or antifreeze all over the track. I try to do about 20 days on track per year, and eventually the odds are going to run out on me. So....... The plan is cage a series one tub, put a good suspension on it, and transfer the motor, clutch, trans and diff from the white car. I might tune the motor up a little. Fuel cell, fire system, good seats and belts along with the cage will be a lot safer, and it should be as light as possible. Lighter is both safer and faster. I'm even trying to lighten up the driver a little, but he is an obstinate ass. So, what tires? I went from a 17# wheel to a 14# wheel on the white car, and was amazed at the difference, mostly under braking and off. So, a 17# difference in rotating unsprung weight would probably be akin to towing some crackheads taurus entering or getting off a turn. However, the whole point is to gain corner speed, and I suspect that right thru the center of a turn, the 335's will show an advantage. Since the 275's and 335's are the same compound, there is at least some possibility you might not get the 335's up to optimum temp. Full race slicks aren't in the program. I've proved with the white car too low is bad. No matter how many holes you drill in the crossmember, or spacers under the spindle, you need to keep the thing gaining negative camber in roll. When it goes positive, all you do is roll off the outside of the tire and slow down. The rear isn't as bad, but it's still there, and raising the rear LCA pivots is a bad day. So, when you put on the big tires, you raise the CG in order to maintain some sensible camber curve. Ultimately, the mass of the car, the height of the CG, the track width, and the amount of lateral grip determine how fast the car corners. So with the big tires you gain grip, lose CG height, and increase weight. The smaller tires have less grip, but get in quicker, off faster, and transfer less weight, so you use them all more equally. Damn if I know which is better. I know I won't be as fast as Mark, but I want to at least be able to keep him in sight. Cost is also a factor. I spend a good bit on the car, but again, it has to be optimized. 335's will run about $1300 a set, 275's about $1000. I usually buy 3 or 4 sets a year, so that's not insignificant. The 335 wheels will run $2500, 275's probably $2K, and you need a couple of sets. More $$$$$. I guess the answer is to buy two sets of wheels and try the two options. Or i could have another drink and shoot my big toes off. That ought to put the tire decision on the back burner. This has got to be the most rambling buch of crap I've ever posted. jt Quote Link to comment Share on other sites More sharing options...
Mayolives Posted January 5, 2008 Share Posted January 5, 2008 John and Mark.........After carefully reading all this......I really can't add anything intelligent, so I'll just continue to laugh out loud...all by my self. Quote Link to comment Share on other sites More sharing options...
johnc Posted January 5, 2008 Share Posted January 5, 2008 There have been some issues with the Hoosier 275/30-15 in A6 compound - a lot of excess outer shoulder wear even in cars running as much as 4 to 5 degrees of negative camber. I've only seen the issue on a CSP Miata (the championship winning one this year) and a DSP E30 BMW. It shortens the tire life somewhat but its kind of a toss-up because the A6 Hoosiers don't last long anyway. I would only run that tire in car weighing under 2,500 lbs with driver (even in R compund). Anything heavier will not last. Given that advice, I would seriously look at the Kumho V710 in 285/30-18 on a 10.5" wide wheel. 24.7" tall, fairly light, and lots of wheel choices. Also look at the BFG gForce R1. I've been running them under my 350Z and am pretty impressed. Not as sticky at the start as the V710 but they last longer and have more left at the end of the race. Quote Link to comment Share on other sites More sharing options...
Flexicoker Posted January 5, 2008 Share Posted January 5, 2008 Heres my take: Say you have three different width tires; small, medium and large. The plan is to test the capability of these tires to produce a lateral force under varying normal loads. I beleive that under a light normal load, the smallest of the tires is going to produce the most force. As the load is steadily increased the medium tire will surpass the small tire as the one producing the most normal load. As the load is increased further the largest of the tires will begin to produce the most normal load. This theory neglects the affects of temperatures and just about everything else. What it means is that there is an 'optimum' tire for a given normal load, you can be under-tired and you can also be over-tired. This theory states that the size of your tire is directly related to how much weight is on it. There is a reason Porsche and other rear or mid-rear engined cars run wider tires in the back, and its not because those are the driving wheels. When you take temperature into account things get alot trickier. It is possible that the larger tires won't get up to temperature, but its also quite possible that the smaller ones will overheat. So... I know this doesn't tell you which tires are the best, but its very possibly that the smaller ones could be better in terms of grip, as well as being significantly lighter, and keeping the car lower. I also disagree with your statement about the whole point being to increase corner speed. If corner speed were the most important factor that the 'classic line' of straight line brake, constant radius corner, and then accelerate would be the fastest way around the corner. Trailbraking however, is faster. This means that the goal is not to be at the highest speed possible at the apex... the goal is instead to accelerate your car from one direction to another as quickly as possible, and this involves braking, turning, and accelerating. I think the smaller tires are the better choice. All of my real-world experience is limited to autocross though, so take my advice with a grain of salt. Quote Link to comment Share on other sites More sharing options...
jt1 Posted January 5, 2008 Author Share Posted January 5, 2008 John, thanks for sharing your experience with different sizes tire. Most of us around here are running the whatever we can fit on our car, and don't have exposure to varying setups. Some of your comments got me looking at the 275 to start with. It would be sweet, but I wonder how long Hoosier will build it. The 335 & 315's are corvette sizes and I figure they will be around forever. Flexi, I should have said the goal is to improve lap times, not just corner speed. The white car is fast on the straight and fast into the turn, but where I give up speed is thru the middle and off, so that's why I've got the big tires on my mind. I've consistently noticed that the cars I have a hard time running with, Z06's and GT3's, have a lot more tire on them than my skinny 225's or 245's. A medium tire may be best. We have a guy who tracks a 914/6 with a strong motor, big flares, and 16" wide tires, and the thing is only medium quick, not fast like you would expect. Jon, the track car's gonna have YZ flares, so with the right backspace anything up to a 12" rim should fit. My current car has stock fenders and I'm using a 16x8 rim. I know Tom is laughing at me, but that's OK I'm used to it. jt Quote Link to comment Share on other sites More sharing options...
Mikelly Posted January 5, 2008 Share Posted January 5, 2008 John, In a recent email traffic about my fuel issues, you commented that your current fuel setup is "great". I bet by adding that wider contact patch, you just gave up that "great" fuel delivery. I pulled up your lap times from last april at VIR North and comparing your times to Jim M.'s GT3, you were only 4tenths off... Those tires you're thinking about will solve your "grip" and corner exit issues, but I bet you're going to upset what you're used to in the fuel department, as well as in where you've been used to braking. You just added a bunch of rotating mass to each corner... Once you get the fuel delivery, brake bias, and chassis setup sorted out, you'll be a lot faster. But I bet the first couple of events are gonna be "fun"! Mark, Tom and I won't be the only guys cussin' and throwing tools in the pits! Mike Quote Link to comment Share on other sites More sharing options...
jt1 Posted January 5, 2008 Author Share Posted January 5, 2008 Mike, that's very true. A car is a big, long, complex equation. It's impossible to change one thing, you usually wind up changing several things. Getting it all to work together is the key. I don't know if my current setup is great( a flying toilet would be great), but it's simple and works pretty well with the rest of my car. Bigger tires will change the whole deal and I will probably need to make some improvements in the fuel system on the track car. It's a big ol' long slippery slope. jt Quote Link to comment Share on other sites More sharing options...
Mayolives Posted January 5, 2008 Share Posted January 5, 2008 John, Having no experience whatsoever with the 335 R6 tire, I can only base this response on my seat of the pants feelings that "wider is normally better" when it comes to road racing our old Dutsuns. Knowing that your setup has gobs of HP and torque, would lead me to believe that you would benefit from the taller gear ratio provided by the larger diameter tire/wheel combination and the added traction of more rubber on the ground. Sure, the wider tire would create more drag but that shouldn't be a big problem for you. We have discussed ways to lower the rear end our cars by raising the mounting points of the differential. Now would be a good time to consider making these changes to the rear. Logic would dictate that you would improve lap times by being faster through the middle and getting off the corners. I believe more tire would accomplish this for you. Also, I would agree with you about Hoosier making the wide boys for a long time to come but I have also read about how short lived these fat boys are. Coast is always a factor for me. And John, I would NEVER laugh at you only with you. You crack me up and have provided many good moments. You may take this post with a grain of salt because I rarely know which way to turn. Quote Link to comment Share on other sites More sharing options...
tube80z Posted January 5, 2008 Share Posted January 5, 2008 Let's consider a V8 road race S30, about 2500# and 400hp, caged, with a modified stock style suspension, say the Arizona Z setup, running on a two mile road course with two long straights and varying medium radius turns. You have a choice of two tire/wheel combos to run, different sizes with the same compound: Hoosier R6, 275/30/15, 23" tall, mounted on a 14# wheel, with a total tire wheel weight of 34#. Hoosier R6, 335/35/17, 25.6" tall, mounted on a 22# wheel, with a total tire wheel weight of 51#. You can vary camber, caster, and toe to suit each tire, but you must keep the vertical angle of the LCA's the same. The 335's will raise ride height 1.3", and we will keep the same suspension geometry for each tire. So which tire/wheel combo will give the faster lap times? The 335's offer greater contact patch, but increase total weight 68#, increase rotating unsprung weight the same amount, and raise the CG 1.3". What do you think, and why? First a couple of questions. How camber sensitive are these tires? And are you stuck with running radials? If the wider tire is more camber sensitive you'll probably have issues with a stock S30 chassis. While these may work for a BMW our chassis isn't as sophisticated. Have you considered running used bias ply slicks? These are a much better match in my opinion and a lot cheaper than the prices you mention for running the radials (perhaps a class issue). There are three primary factors for cornering speed. Weight, CG height, and track width. Your losing two of those for sure going to the bigger tire combo. If you have some basic data you may want to think about downloading bosch lapsim and trying it out. It may help you make the choice as to which is better. Cary Quote Link to comment Share on other sites More sharing options...
johnc Posted January 5, 2008 Share Posted January 5, 2008 you were only 4tenths off... Which equates to 8 seconds at the end of a 20 lap race. Tire management is a critical aspect of running on a race track. A very good driver can keep some tire in reserve by the closing laps of a race to make the pass for the win - in the last turn on the last lap. A tire with a larger heat capacity makes this a little bit easier. Assuming same compounds and proper suspension setup, a physically larger tire will have a greater heat capacity. Quote Link to comment Share on other sites More sharing options...
mark Posted January 5, 2008 Share Posted January 5, 2008 Have you considered running used bias ply slicks? These are a much better match in my opinion and a lot cheaper than the prices you mention for running the radials (perhaps a class issue). Cary Cary where do you get bias ply slicks for much less than 1000 to 1300 per set? I would love to run full slicks but usually just go with victor racers. They are a pertty good value for track days. As for the class issue, I assure you there is no class at all when this group is at the track. Quote Link to comment Share on other sites More sharing options...
Mikelly Posted January 5, 2008 Share Posted January 5, 2008 As for the class issue, I assure you there is no class at all when this group is at the track. :lmao: Quote Link to comment Share on other sites More sharing options...
heavy85 Posted January 5, 2008 Share Posted January 5, 2008 Cary where do you get bias ply slicks for much less than 1000 to 1300 per set? I would love to run full slicks but usually just go with victor racers. They are a pertty good value for track days. As for the class issue, I assure you there is no class at all when this group is at the track. http://jbracingtires.net/index.htm I got a set of Hoosiers for $75 per tire. These were 15" x 9.5" x I think 23.5 but can't remember and am too lazy to go look. They were used once in practice at the run-offs and had 100% tread left. I'm not kidding as they were coated in a thick layer of rubber. Once scraped off the molding marks were still there. I could not have been happier with them. Cameron Quote Link to comment Share on other sites More sharing options...
JMortensen Posted January 6, 2008 Share Posted January 6, 2008 Used slicks is definitely the way to go fast cheap. If the Formula Atlantic or Toyota Atlantic series run in your area you might check when they are at the track practicing. I was running TA take offs, I only used the fronts which are a 250/45/15 or so. The rears are a 350/30/15 I'm guessing. HUUUUGE tread width, but the Toyota Atlantics have a spec tire and it's roughly like a Kuhmo Victoracer in terms of compound, meaning fairly hard. I always heat cycled them to death before I wore out the tread. The Formula Atlantics run slightly different sizes, but the Goodyears and Hoosiers are supposed to be fast. They are a 22 x 9.5 x 15 front and the rears are a 22.5 x 11.5 x 15. Quote Link to comment Share on other sites More sharing options...
majik16106 Posted January 6, 2008 Share Posted January 6, 2008 can i ask a seemingly related but a little different situation just to get some more insight from you guys. maybe get some cool info for some of the guys who build dual purpose cars. On a car that is a street/ track car. there seems to be two popular ways of going with wheel tire combos.. a problem i had. old school vs race car styling. i like to look at what im driving, but i also love being competitive at a track. So some of us would go with Zg flares and 16inch wheels (IE John C's track set up) and then some of us may go Imsa/yz flares and run 17s. so a 16x9/16x10 with say 245 and 265 v700s or... 17x10 and 17x12 with 275/335 v700s.... with suspension being the same,except that with the 16inches the car may sit a little lower because of the smaller wheels, your saying that for my track set up... the 17inch car would in theory be faster? so street guys who are a little more track oriented may be better off going bigger? Quote Link to comment Share on other sites More sharing options...
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