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Jesse OBrien

Wheel size for handling

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I'm certain this has been addressed before, and I did a lot of searching (on several forums) but have come up short. This past weekend, I attended a hillclimb race event and made a few observations that made me rethink wheel choices. I had always assumed the widest wheel possible would offer the most mechanical grip and amount to the best 'momentum' car possible. I saw a few Turner cars ripping up the track with plenty of power and plenty of mechanical grip, along with well-sorted suspension. They were unquestionably fast:

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However, a cobbled-together awd twin-turbo Audi broke the track record. It was unquestionably fast as well.

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The thing that got me thinking and questioning what I "knew" is that the Audi is running significantly less tire, weighs roughly the same, and can put down roughly the same amount of power. Both cars had excellent pilots. However, the car with narrower tires was putting down better times, going against everything I had previously believed. I asked the drivers about it afterward, and I got a brief primer on roll centers, kingpin angles, and terrifying steering vs smooth steering characteristics. I had always assumed that the hub should be lined up with the centerline of the wheel, but apparently I had it all wrong and the roll center should be lined up with the centerline of the wheel.

 

I'm working my way through some of the experienced racers' advice on here, and am not quite ready to invest in wheels/tires, but I'd like to get some research done ahead of time for this. I have to assume that there's a wheel/tire combo that's a good 'starting point' for the s30. Right now, I suppose I'm looking for 15x8 wheels since they have a reasonable selection of sticky DOT tires, but I wanted to see if I could get some reading material from you fine folks before I spend money and potentially ruin the car's handling capabilities.

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I had always assumed that the hub should be lined up with the centerline of the wheel, but apparently I had it all wrong and the roll center should be lined up with the centerline of the wheel.

 

 

This makes no sense.  You're comparing a physical suspension component with a calculated abstraction.

 

The main reson the Audi was faster up the hill was 4WD.  Hill climbs are generally low speed events (like an autocross) and thus reward accelerative traction.  Anything you can do to increase grip under acceleration will reduce your times.  Spreading grip around four contact patches gives you twice as much area for acceleration in a 4WD vehicle (given the same size tires).

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If you can fit 15x9s, the 225 extreme summer performance and DOT tires work well on a 9. They're really wide for 225.

 

You said roll center should somehow relate to the tire. I think you meant scrub radius. There are lots of race cars running a positive scrub radius. The most immediate effect in a Z is that a big scrub radius will make it harder to steer.

AWD is the traction king. Google "traction circles" and you should find some stuff about how much traction each tire has at any given point. Suffice it to say that in a high hp 2wd car the undriven wheels are usually not pushing to their max, and that's why AWD has an advantage. That thing really does look cobbled together though... LOL

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The answer to your question is probably more complex than you expect. Correctly sizing wheels/tires factors in a car's entire performance window, and you really need tire data to pick the right one. Good luck getting that from the manufacturers, unless you know somebody! You need to plan out maximum acceleration numbers for take-off, braking, changing direction, etc. and try to match up a tire to that without wasting material (e.g. too wide of a tire for your needs will increase unsprung weight and rotational inertia).

 

Tire size also affects gearing, so I'd start by choosing a diameter to work with that matches the gearing you'd like...

 

Ditto JC on the RC comment.

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This makes no sense.  You're comparing a physical suspension component with a calculated abstraction.

I agree, none of this makes any sense. I thought I had a moderate understanding of how things worked 'till these guys started explaining things to me. I may just be taking away the buzzwords from the conversation, and didn't absorb all the context. I feel like I'm starting over in suspension 101 again.

The main reson the Audi was faster up the hill was 4WD.  Hill climbs are generally low speed events (like an autocross) and thus reward accelerative traction.  Anything you can do to increase grip under acceleration will reduce your times.  Spreading grip around four contact patches gives you twice as much area for acceleration in a 4WD vehicle (given the same size tires).

That makes perfect sense, and is what I had always assumed. More mechanical grip means faster times. The Audi driver reported slower times with wider tires, which is what threw a wrench into my brain.

 

It's kind of funny, my whiteboard in the garage has a 'johnc prep list' on it with starting recommendations straight from you. I think that should be standard reading for anyone who buys an s30 (http://forums.hybridz.org/topic/91330-240z-chassis-prep-reinforcements/?do=findComment&comment=864737). I think we all end up asking that question at some point in our ownership.

 

If you can fit 15x9s, the 225 extreme summer performance and DOT tires work well on a 9. They're really wide for 225.

 

You said roll center should somehow relate to the tire. I think you meant scrub radius. There are lots of race cars running a positive scrub radius. The most immediate effect in a Z is that a big scrub radius will make it harder to steer.

 

AWD is the traction king. Google "traction circles" and you should find some stuff about how much traction each tire has at any given point. Suffice it to say that in a high hp 2wd car the undriven wheels are usually not pushing to their max, and that's why AWD has an advantage. That thing really does look cobbled together though... LOL

It's very carefully cobbled together. Scrub radius is what they were explaining to me, but I'm not sure anyone used that term.

 

I want 0 offset on the 15x9's correct? I'm really just looking for a solid starting point, it doesn't need to be the end-all best possible wheel. I've ogled Braap's old yellow z with its massively wide tires many, many times and would love to be able to justify running wide tires like that. I'm trying not to put the cart before the horse though. 

 

The answer to your question is probably more complex than you expect. Correctly sizing wheels/tires factors in a car's entire performance window, and you really need tire data to pick the right one. Good luck getting that from the manufacturers, unless you know somebody! You need to plan out maximum acceleration numbers for take-off, braking, changing direction, etc. and try to match up a tire to that without wasting material (e.g. too wide of a tire for your needs will increase unsprung weight and rotational inertia).

 

Tire size also affects gearing, so I'd start by choosing a diameter to work with that matches the gearing you'd like...

 

Ditto JC on the RC comment.

I have a lot of math to do, but for right now I'm just looking for a starting point. I'm trying to balance this to be street driveable as well, since I don't have a truck-trailer to haul it on. The main problem is that I don't know what the car's performance window actually is. I don't have dyno results for the engine, haven't changed any suspension components (yet, since I don't know exactly what the car needs yet). The only real determination I could make is gearing, and even with that, I'd like to swap up to an LSD (STI, perhaps) which will change my gearing again. For now, a starting point is all I need.

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If you look at the cobbled Audi, he wasn't exactly running 145's. So a wider contact patch helps. But also, that thing doesn't look like it weighs 4000lbs either, so that also helps with acceleration as well. And since it looks to be a tube framed car, I'd imagine he isn't using a stock suspension either; I'd bet its some form of adjustable coilover shock . So as Everyone has said, handling comes down to more than just tires. It is tires yes, but good tires on a 4000lb car with mushy suspension still isn't going to handle all that much better than before. 

 

But tires, I feel the widest and stickiest you can run in your application will give you the best handling. Along with other mods of course; suspension etc. 

Edited by R3VO 3VOM

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If you look at the cobbled Audi, he wasn't exactly running 145's. So a wider contact patch helps. But also, that thing doesn't look like it weighs 4000lbs either, so that also helps with acceleration as well. And since it looks to be a tube framed car, I'd imagine he isn't using a stock suspension either; I'd bet its some form of adjustable coilover shock . So as Everyone has said, handling comes down to more than just tires. It is tires yes, but good tires on a 4000lb car with mushy suspension still isn't going to handle all that much better than before. 

 

But tires, I feel the widest and stickiest you can run in your application will give you the best handling. Along with other mods of course; suspension etc. 

Right, but he was only running 225's, where the Turner was running something like 355's. Both had well-tuned suspensions for the hill and carried incredible speed through every corner. Both are ~1500lb cars plus fuel/driver. I guess I'm just surprised that AWD can bridge that much of a tire size gap.

 

Don't know on the offset, but there are a bunch of threads on that subject. You might try the Z transmission calculator for tire diameter and gear selection.

 

http://webspace.webring.com/people/cz/z_design_studio/

That's my plan as well, I just feel like I should ask for some input from more experienced people before going crazy with the widest tires I can get, and slapping overfenders under them. I want a car that I can drive quickly and effectively, not one that's bored waiting for me to figure out how to wrestle with it.

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One of the things not addressed was where the specific tires on the Audi, combined with his gearing put him in his engine RPM range.  If the the narrower tires were the same series as the fatter ones then the narrow tires would have been a little smaller in diameter and raised his engine RPM at any given speed.  That, combined with the total greater contact area for power transmission would have helped him in the acceleration/maximum speed department.

I ran a hill climb recently in my 280Z and was running 225/50-16 Dunlop Direzza's.  I was faster up the hill than the twin turbo Z32's that were running wider tires.  The basic reasons were:

1. It was a hill climb and the need for power is amplified.

2. I was probably 5-600 lbs lighter than the Z32's

3. I was running an LS1/T56 combo so I was always in the right gear to maintain maximum torque/HP and my torque range was huge.

I was also a little crazy too so maybe that helped. :-)

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Trying to fit a wheel wider than 7" on an S30 is a pain in the ask. If you are looking to run a 225/50/15, make your life easier and limit your wheel width to 7". Zero offset or, 4.5" back spacing. Bolt it on and go. Johnc gave me this advice years ago and it has proven true. Even going to 8" has been very painful without flares.

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Lots and lots of variables to make a conclusion on tire size especially for hillclimbs. It been my experience that hillclimb times are HIGHLY dependant on how hard the driver is willing to push. For example, my first hillclimb I beat a former Daytone 24 winner with him in his purpose built hillclimb special complete with big wings, low weight, turbo, huge slicks, etc and me in my LS1 Z on 245 STREET tires. Im not that great a driver either.

 

Then you need to pick a tire and venue. Hillclimb need tires that work now so if your tire needs a lot of heat then smaller can be better to get them up to temp faster.

 

If you want to put some ZG flares on, I've had much success with 245 17's. RS3's

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Tire compound is more important then tire width (a softer compound provides more grip). Tire temp is more important then tire width (a 205 width Hoosier R6 at 190F grips better then a 285 Hoosier R6 at 80F). Tire heat cycles are more important then tire width (a new set of scrubbed 205 Hoosier R6s grip better the a 285 Hoosier R6 with a years worth of races)... See the trend?

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Johnc, if EVERYTHING else is optimized: ( dry, clean pavement, sticker new, perfect temperature of the perfect tire compound, perfect inflation, perfect shocks and spring rate, unlimited funds, infinitely stiff chassis, infinite power and torque, perfect alignment for the venue, perfect driving ), is there a down side to trying to achieve the largest contact patch that you can? I know this is a super complex question, so I'm really asking it for the original poster in order to get him to a simple answer to a complicated question.

Edited by RebekahsZ

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All else being equal never is. Another example:

 

B Stock Solo2 RX8. You're limited to stock rim width. Same car, same driver. On a 18 x 8" rim the car was quicker on 245/35-18s then 285, 275, 265, 255, and 225 width tires. All the tires had an OD within 5% of each other. All testing done in the same place on the same day with retest to reduce variability.

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A different test:

 

STX Solo 2 E36 BMW. Car was tested on 245s on a 8.5" rim, 255s on a 9" rim, and 285s on a 10.5" rim. All 18s. The 285s were significantly faster then the others except for the morning test runs. The 245s were faster when it was cool.

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I'm learning a lot just watching this discussion go back and forth.

Johnc, if EVERYTHING else is optimized: ( dry, clean pavement, sticker new, perfect temperature of the perfect tire compound, perfect inflation, perfect shocks and spring rate, unlimited funds, infinitely stiff chassis, infinite power and torque, perfect alignment for the venue, perfect driving ), is there a down side to trying to achieve the largest contact patch that you can? I know this is a super complex question, so I'm really asking it for the original poster in order to get him to a simple answer to a complicated question.

I was wary to ask this question, because I know exactly how I'd answer it if I were the one with more experience.

 

All else being equal never is.

... just like that.

 

So there's no simple answer, no formula that fits every car. Because our runs are ~4minutes (absolute max, that's a stock Daewoo ambling up the longest hill in the area), I think I need to focus on cool-weather temps. For now, I think my question has been answered: On a mostly-stock s30, 15x8 0 offset 225-[proper-gearing-height]-15 is acceptable. I'll have to do my own testing and fine-tuning from there.

 

Heavy, I'm curious as to why you'd choose 17" over 15" if the widths are the same, and brakes don't demand it. I'm trying to focus on wheel width more than wheel diameter, but what's one more wrench in the equation?

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Thanks for the examples, johnc. The answers come with testing. That is why we do qualifying races for bracket racing-to see where the car is running TODAY. As the temperature and humidity drop into the night, everything changes. And testing is expensive. CobraMatt recently talked with me and described how he wound up totalling out his Corvette in testing (not at a race) just to get the angle optimized on his rear wing. Had to reconstruct car to get back to testing.

 

Jesse-my argument for 17" wheels is brake clearance and availability of used race tires (Hoosiers). I'm working my way into 275/40?/17s for that very reason-lots of those available from the S2000s and Mustangs. The most readily available 15" race tire is the 205/50/15 from the Spec Miata bunch. There are always lots of R6s available in that size for CHEAP. Everytime you change diameters you gotta spend $1000-ish on wheels. I'd go straight to 17s so you don't have a bunch of wheels and tires you can't get rid of when you upgrade your brakes next year.

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I too, have read that some 15" wheels will fit. Unfortunately, mine do not. A set of AZC brakes are in the mail to me right now to be installed in November/December. I will be willing to let folks send me any wheel for test fitting. Dave test fit a wheel that I sent him and it took him like six weeks to get to it and two months before I got my wheel back, with me calling him every week to remind him-super nice guy, though. My 15" wheels will require a 1" spacer to fit, which will push the tire out of the wheel well. So, I'm going to have to order $400 worth of new rear wheels with a 1" larger back spacing to keep the tire in the wheel well. I plan to do a write-up. Unfortunately, I haven't found anyone who has catalogued WHICH 15" wheels will fit. I will try to do that based on any wheels folks chose to send me.

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I too, have read that some 15" wheels will fit. Unfortunately, mine do not. A set of AZC brakes are in the mail to me right now to be installed in November/December. I will be willing to let folks send me any wheel for test fitting. Dave test fit a wheel that I sent him and it took him like six weeks to get to it and two months before I got my wheel back, with me calling him every week to remind him-super nice guy, though. My 15" wheels will require a 1" spacer to fit, which will push the tire out of the wheel well. So, I'm going to have to order $400 worth of new rear wheels with a 1" larger back spacing to keep the tire in the wheel well. I plan to do a write-up. Unfortunately, I haven't found anyone who has catalogued WHICH 15" wheels will fit. I will try to do that based on any wheels folks chose to send me.

That sounds like a great project, 15" wheels are more budget-friendly on both wheels AND tires than 17" (I've had a very hard time finding inexpensive steel race wheels in 17", but there are piles of manufacturers making quality 15" wheels). Your point about Spec Miata (and spec e30, if I remember correctly) tires lends even more value to the inexpensive wheels. While I don't hate the idea of overfenders or spacers, it'd be nice to know what would fit on which brakes without any modifications. It'd also be nice to know which wheels interfere with suspension components.

 

The PROBLEM I see with 15" wheels is that they're really only common for 205 and 225 widths. Anything wider and 17" is really necessary. Because I'm running a stock l28 on SU's, mostly-stock suspension, and this will be my first season racing, available grip isn't going to be the limiting factor and I don't expect to bump up against the limits of those tires.

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