I currently have 18x10 CCWs on all 4 running 285/30r18s. Lately Ive been thinking about getting 2 18x11.5s from CCW for the rear and running a 315/30r18. I only wanted to go to a 305/30 but there are very limited tire choics in that size. Anyway, I just wanted to get some more input on what offset to go with. I would like to keep the fender as stock as possible. I know I will have to roll it a good bit, and possibly start to push it out, but I have an eastwood fender tool and should be able to do that no problem. So heres what I think, my 18x10 +50s are now very close to rubbing the trailing arm, so I dont want to push it inward any further at all. I was thinking an 18x11.5 with a +30mm offset should put it as far inward as can be. Any suggestions?

Also, I will email or call CCW soon, but would I be able to send in my current wheels and just have them swap out the barrel for the different width and offset I need, or will I need entire new wheels?

 

Track down Kevin Wyum. He has 17 x 11's w/ 315 on the back. I do know he had to bend the trailing arms to allow them to fit. I've been under his car a few times and its a tight fit. He tried I believe a 5mm spacer on one side and then it hit the outer fender. Last I remember he was going to run the spacers and give the outer fender a "Slight" flair or someting like that. 18's might fit a little different, but the end result would be hte tyre width right? It was tight. There are pictures of his back tyres on this site somewhere.

 

Try a search for Gene's tire sizing info (gfelber). He was running an 18x11 in the back -- he had to CUT the rear fender lips off (rolling was not enough). I don't think there's any chance in hell of fitting 11.5s without fender "surgery".

 

^^^^^^^^ this is true.

Gene had 11" +48's... and maybe a little room to spare towards the trailing arm, but none towards the fender. You'll be officially in "uncharted territory." Good luck!

 

I'm just curious why you guys run such big tyres. I mean a 500bhp Ferrari F430 only needs a 285/35/19.

In all honesty, are you guys making that much power? If so, is it just a drag car? If its also a road/track car, do your tyres even reach optimal temperature with such a big section width?

Seems to me, for open track/racing you guys are over sizing the tyres. if you still need that grip, why not 285 with a better compound? Also, for the front, I see some of you guys running as big as 285 or more up there. WTF. I say the same thing, i think a GOOD compound 255 would be more than you could ask for.





If I'm missing something, fill me in. Or is the "cool" thing to do around here stuff as much tyre you possibly can no matter what?







Also, i've noticed quite a lot of hate for altering Mazda's stock suspension geometry. Hm. Its stock street car, not a track car. You could put flares on, a lower offset, a much wider track (making a more centralized center of mass in relation to the width of the track), and run a narrower tyre (and because of the wider track, would give you as much/more grip than the HUGE things you are fitting now). Also the widebody+wider track would allow you more room to fit these huge tyres if you did in fact have a need for them.




Anticipating your responses, I figure you'll tell me not every car wants to "look" like a track car with wide fenders, but still wants the performance and grip of the widest tyres ever.
To which I will respond right now: You can't have everything. The width of an FD is 1750mm. Keeping with the Ferrari F430 example, that car is 1923mm. So that would be a 100mm EACH SIDE widebody on the RX7. I'm not saying that the cars will be a direct comparison, and I know one is MR and the other is FR, but I'm using the F430 as a benchmark for dimensions, power, weight, and performance. I mean... out performing one of those would certainly be an accomplishment at the local track day, eh? And for as highly as you guys talk about FDs, something tells me outperforming those "ricey" other imports like EVOs and STis or a handful of modified Nissans doesn't stroke your goat as much as TRUE sports cars.

Most of you would start to point out how that $150,000 Ferrari has little relevance to our discussion. Bear with me... That car, with its 1923mm width, has 285/35 tyres. If you're trying to run 315s, i will for now assume you NEED that grip. Why not go wide body and space out your track/run a smaller tyre like I said previously?

^^That's my argument/question/suggestion/opinion.

 

I cant speak for everyone, but Ill respond to a few of your quesitons and points. A corvette Z06 runs a 325/30r19 from the factory and makes ~500BHP, and it will still roast the tires if you are throttle happy. Not the point though. Im not claiming to be an expert, but I do have alot of experience running alot of different wheel/tire combinations on rx-7s. 1st off, while I do run track evens for fun every once in a while, my car is primarily a street car. So with that, I am limited to a certain amount of grip from non-R compound tires. Also I am running an LS1 engine that will be making 400+RWTQ coming from a 2krpm up. For this reason I could really use a wider tire out back. I dont want to add flares, just because personally I think they look terrible on an FD. Also for your wider track argument, that may be true in the back, but out front running a super-low offset with flared fenders will totally screw your scrub radius. With my 285/30 up front with the factory 50mm offset, I can have all the advantages of that larger tire with not much drawbacks. Im not arguing with any of your statements, as you do make valid points, but the 285/30r18 on all 4 corners with the 18x10+50s is a track proven setup that performs very well. In my case, i would like to squeeze a little more out back if possible.

 

Gotcha, that's what I figured. Running a torque monster LS1 and needing all that tyre makes much more sense, especially to put the traction down in a straight line. I kinda just assumed you still had a 13B (opposite of torque monster), so that's what my post was based on.


As for the scrub radius thing... MAN i hate it when people think a larger scrub radius is bad. I talked to a couple of engineers about this (couple from Volvo, one from BMW, one from Daimler Chrysler). There is a range of desirable scrub radius. Scrub radius is part of the suspension geometry that gives feedback directly to the driver via the steering wheel (resistance). The more scrub radius, the more the driver can feel the road. Of course TOO much is detrimental (mostly because of the inherent bump steer though). So while it may make sense that scrub radius is BAAAADDD because your mental picture of the car and suspentions static (stationary, not moving, stopped) fighting the resitence of asphalt while trying to turn the wheels, think about the actual situation where it matters: while cornering. That resitance is now rolling resistance which is much easier to overcome, and like I said, translates back to the driver as feedback.


If your car had 0 scrub radius, it would feel like you were on ice whilst trying to turn. So yah, sorry, but I don't like the "stock scrub radius" thing. But that's because I prefer brutal track cars that barely idle, scrape frame rails, and shoot fire.

 

Hi,
Can you please give me some advice on my suspension. My car is Mazda RX7 twin turbo 1993 model.

I have at present alloy wheels fitted 17inch with 8.5 JX ET42 on the front with tyres 235/45/17 & 9.5 JX17 ET45 on the rear with 255/40/17.

I have just bought a new set of ADR wheels from USA fronts are 18inches 8.5J Offset 35 & rear is 9.5J Offset 35.
The tyres i was going to buy are 225/40/18 for front & 255/35/18 for rear.

Now my question is will these new wheels i've just bought fit my car due to smaller offset of 35. ADR manufacturer say they will & i will need 3mm spacer for front wheels, but i have had email saying the rears are to wide.
Appreciate any help you can give, please put me in the right direction.

Regards
Dave

 

Concerning scrub radius, have you ever driven a car with super-low front offsets or large (~1"+) spacers? I have, and in a turn at close to full lock, you are just pushing the tire. That being said, I dont know the exact point to where this starts becoming a problem, but I do know too much is a bad thing.

 

Keep in mind that the F430:

A) weighs a little more than the FD
B) Is a mid-engined car, so the majority of the weight sits in the middle/rear of the car
C) is geared taller than the FD
D) has lots of little computers to control the traction

All of those things help it keep the tires sticking under acceleration. Bearing that in mind, I was talking to a few F430 owners this weekend, and they told me that unlike the F360, the 430 can break the tires loose when accelerating in a straight line (with the traction control off, of course).

It's nice to have traction. My car has only 350bhp with 285s in the rear and I can make *** end will slide out in first and second gear on cold mornings. Given, the tires I have are not IDEAL for straight line acceleration (SP sport 9000s seem to have relatively thin contact patches for some reason?).

 

Yeah. I agree that too much is a bad thing (of course). My car right now (FC) has 17x9.5 +2 on the front. Quite a bit different than stock, I would say. I love it. I have so much confidence in the car because it feels like i can tell the diameter of every pebble I hit, the depth of every crack in the asphalt, etc.



Check out Le Mans cars (especially older ones like Porsche 908/3) there is always a trend, smaller wheels up front (less frontal area, more room in the wheel well) and super low offsets. The front fender just aft of the front wheel is usually scalloped away b/c the scrub radius is such that the wheel moves so far fore and aft that it swings into that area while turning. Current cars do the same, but the ACO regulations state that that area needs to be covered on the outboard edge of the car (various reasons).

 

I'd be curious to see what you say trying a 265 size Advan AD-07 Neova, or Bridgestone Potenza RE-01R.

 

Most of these guys are just doing track days, not wheel to wheel racing, so tire life is an important consideration. Going with a wider, harder tire should give the same grip as a narrower, softer one, but should last longer, lowering the need for replacement tires, helping to lower the cost.

Low offsets are detrimental, they place a lot more stress on the wheel bearings and axles and will lead to decreased lifespans of the bearings and inrease the risk of axle failure.

Keep in mind when looking at those old racecars that back in the day most cars had much lower offsets, trending towards zero or even negative offsets, so having the low offset wheels isn't a reflection on what's best, it's a reflection on what was correct for the car. The Porsches are also very heavy in the back, so a stagger is needed, the RX-7's benefit from a more balanced tire package, with the fronts being big too. I'll bet that a lot of the time on the Le Mans cars the wheels are so far out not just with offset and spacers, but with loger control arms for more ideal suspension behaviour. Increasing track width with wheels alone isn't that great.

I've got 17x8's 30mm offset wheels on my FC and I wish I could go with an offset closer to stock (40mm) to get rid of some of the tramlining and to lighten up the manual steering a bit, but the strut and spring is in the way.

 

My best friend did a wirte-up about this ages ago on Ziptied. I'll quote that response for you, sicne my explaining it would just lead to people wanting to see explanation and I would end up quoting it anyways.

Just think about it for a bit... Not in "common sense" but in Physics. One of those has the ability to be wrong, and its not the laws of physics...

 

Curious.

Something looks funny about the way he's expressing those cross products.

...

I did some tinkering and it appears works out as the entire cross product rotates counterclockwise (viewing from the front of the car) as you pull more lateral g's, thus increasing the moment at full load.

 

Yes and no on that explanation.

That assumes that the bearing is contained at the wheel mounting face when it's actually a significant distance away (in relative terms, we're talking about mm here). There's the brake rotor hat, the hub face and then the bearing is somewhere behind all that. It's also got a length, so figure the moments about the halfway point. With a 50mm offset I bet you're getting close having the contact patch inm line with the bearing, so going to an offset of 30mm will increase that moment. A 2mm offset like your wheels will definetely stress the bearings more.

 

To play into your scenario that the bearing is actually 50mm away from the mounting surface (i don't think it really is that much), my +2 offset will have a higher stress load while static, yes. But not while loaded.

If we are to assume that your +50 will act like the 0 offset in Kookz's model, then my (lets just say 0 offset for ease of argument) will act like the -50 offset in Kookz's model.



So factoring in YOUR model:

If we figured in a +50 offset, we would have a moment of 0 N/m at rest, and a moment of 2400 N/m in a 1g turn with the entire load on the outside wheels.

With a +/-0mm offset, the moment at rest is now -200 N/m according to how we defined the positive moment. Since a bearing is symmetrical, it doesn't care which direction the moment occurs, so this -200 N/m moment is no more stressful than the +200 N/m moment. And, when in that same turn, the moment has been reduced to -200 N/m + 2400 N/m = 2200 N/m. Even lower.





Again, while it is 200N/m moment at rest for my +0 vs. your 0 N/m at rest for the +50, we have a 2200N/m for my +0 while loaded, and a 2400 N/M for yours.

Like I said, sure the moment is higher while static for my lower offset, it is 200N/m. That's nothing when compared to the car seeing 2200N/m in a corner.

 

The point remains, the model he used is fundamentally flawed, and there will be higher stress on the bearings with lower offsets. The car spends WAY more time not cornering than it does cornering, it's also making a very big assumption about weight distrobution, the inside wheels are loaded and as such the moments will add. Bearings don't wear out from occasional high loads, they wear out from sustained loading.

The bearings are only symmetrical about one axis, they're conical, to say that they're symmetrical is misleading. The moments can be added and subtracted, but the stresses aren't going to be equal from end to end.

 

Well if a scientific equation won't change your views on low offsets, I guess there is no hope. I'll continue to live in the real world, and you can be my guest to stay in fantasy-land.


At one point in this thread I was involved in a discussion about getting grip out of RX7s and then it lead to low offsets are useless.


Lets get back on track... I recently saw a post with someone putting down 270-ish rwhp and running 285s all around!!! I'm going to venture out and say that's over-sized. I understand the high treadwear daily driver+preformance car need to contain LS1 torque above 2k, but I'm lost on why you'd run such over-sized tyres on anything with 300-400 hp.

And for a car i took to the track more than... 6 times a year, I'd definitely have a set of track wheels with dedicated R-compounds or grippy grippy tyres.




Dare I say this big tyre business for *some* people is a "my dick is bigger" contest?

 

I already explained why your freinds reasoning is faulty. BEARINGS WEAR OUT FROM SUSTAINED LOADING, NOT JUST PEAK LOADS. That's a fact. If you want I can find a formula from my Mech class where we learnt about bearings. Using actual bearing to wheel centerline distances, lower offsets (less than say 30mm) will increase the mean loading, leading to reduced bearing life.

I find it funny that you advocate small wheels and tires, yet you run 9.5's up front (although you probably use a 215 or something rediculous like that).

I'm pretty sure the wider tires do give more grip and do get up to proper operating temperatures. I know many of the guys who run those 285's started out on much narrower tires and moved up and found it to be better. Race cars with less than 400hp often run wider race tires. Narrower tires will give less grip despite a marginally wider track, whose effects will be minimal if any (only increasing by a few mm). Power oversteer will be a much bigger issue on the smaller tires too, making the car harder to control, making it harder to get out of the corners quickly which will hurt your speed on the straights. You didn't even mention the effects of the increased rolling resistance when going to larger tires, this suggests an incomplete understanding of the situation. To properly asses the effects of the wider tires this needs to be taken into account. Many stock power level Miatas are faster on 205's than 225's due to this.

I think the name says it all, Drifto = get a low offset to get a huge mad pimpin lip!

 

Moving up in tire width isn't always a good thing. At some point, you won't be able to turn the car very well.

After autocrossing the Rx7 on 235's and 275's, the 275's definitely are better in the lateral grip department, however, I have given up some steering response and the wheels bind up more often. It used to be easy to slip around cones by slightly overdriving the car with early turn in. Now, I have to take straighter lines.

I also autocrossed the Rx8 with 225's and 275's. I am thoroughly convinced that the 245's are faster overall on course than 275's or even 285's.

I have been considering running 275's and 315's in the rear for the 2007 season, but after testing some various R-compound setups, I believe I will be sticking with 275's.

 

As to the question, "why run such big tires".... well, because they do produce more grip. I suspect there is a point at which going wider would go beyond the car's ability to use it and produce excessive rolling resistance and drag, but I don't belive a 285/30/18 is at or beyond that point. You'd need to conduct back to back tests keeping all things (such as wheel offsets and brands, and tire compounds) the same and compare lap times.

I do know that with 365 rwhp on the street, my car will be a little squirmy or even break the rears loose if it isn't hot out, and my streets are 275/35's. I've matched those w/ 255/35 fronts and the car feels ballanced to me for street driving/windy road cornering that is typically not at 10/10th's, and the smaller front tire rides a little better, clears a little better, and are easier to turn w/out power steering.

For the track, I knew I wanted no less than a 275 rear given the experience on the street w/ regard to rear traction, and since i'd tracked the street set a few times, I knew that the ballance at 7/10'ths w/ the 275/255 combo dissappears at 10/10th's in SOME areas, particularly long slow corners like they have at Summit and VIR... you really want a matching front tire size. Using the same size all around also allows you to rotate for tire life, which is more important for track tires in my experience that street tires.

In terms of offsets, you'll want that +50 up front for clearance sake, and you'll also want to run as close to the factory wheel center to spindle distance as possible to maintain the geometry. I wouldn't over look this aspect. The more offset your front wheel has, the less it rotates around the spindle when you turn the wheel, and the more it orbits the spindle.... hence my comment in the *other* thread about that car looking like an R/C car. Those old, cheap Rad Shack R/C cars just had a solid front axle that rotated on a central pivot point and as such the front wheels swung dramatically from front to rear as you turned the wheels. This is what happens to a lesser degree when you put a high offset wheel up front. I don't claim to be a suspension engineer, so I don't have the mathematicl formula for it, but it's an easy phenomenon to visualize. When Alain Prost arived at Williams in 1993, one of the first things he requested after testing was that they change the offset of the front wheels. Given the precise width/track requirements in F1, this required modifying more than a few suspension bits to compensate. It apparently made a significant difference to lap times and front tire wear. I point this out to illustrate that it's most certainly not something that "doesn't matter". Given that we're all on a budget, and don't necessarily have the funding for custom made wheels, we all shoot to get as close as we can given what's available on wheels in our price range.

 

Rolling resistance? We're talking 285 vs 315!!!! I'm not advocating a 215 vs. a 315, jesus. The difference in rolling resistance will be minimal, especially with a 400hp+ car, and a difference in compound would likely have just as big of an effect.

Show me all the formulas you have, and I'll show you my bearings, after years of "abuse" from low offsets. I haven't had a wheel bearing fail on me EVER, and when I have seen it, its due to rust and neglect not god damn wheel offset!


You say you pointed out flaws in my buddy's formula, so I went along with YOUR word and plugged our numbers into that, and then all of a sudden your argument is "Oh yeah well.. static loading of the bearing kills it... murmurmurmurmur." While that may be the case, which I agree with, I'm telling you that my low offset wheels aren't gonna have a hig enough moment to actually have a big effect.



And perhaps, if you can open your eyes to the fact that there are other forms of driving (drifting being one of them), you'd actually start considering how (like I've showed you) a lower offset wheel will decrease the maximum moment of lateral acceleration. Perhaps this is actually beneficial while spending hours on end at higher G loads than you encounter during normal racing.

I NEVER mentioned drifting at all in my posts, you brought it up in an attack meant to discredit me. Jesus ******* Christ, I'm not 17 with an GXL FC, cut springs and a TII hood (for cooooling, dawg!). If you can't get over yourself and have some ingrained hate for people who choose to drive their cars differently than you, so be it. Maybe looking past a damn forum name and realizing that a single person might actually enjoy MULTIPLE motorsports, including grip, drift, kart, offroad, etc.





Whatever, next your argument will be that narrower tyres (285) will make his car oversteer into a bus full of children and kill them.




I still maintain that if you are having trouble fitting 315s, it MAY BE WORTHWHILE TO TRY A BETTER COMPOUND TYRE IN 285 OR 295!!!

 

Yes, thank you! Much more clear, I appologize for assuming everyone is on an all-out full bore budget. My question stems from the recent Time Attack competition up at Buttonwillow. I know a lot of the drivers, but more importantly, the team members for the tyre manufacturers. A couple of the teams (even in the same tyre brand) were running WAY too wide of tyres on their FD (295/315) and turned much slower times than the other FD that was running 255/285. I know the cars weren't IDENTICAL, but the tyres were the same RT615s.


As for that "R/C car" phenomenon, that's scrub radius, what we were talking about earlier. Especially with Long A-arms like most current sports cars (and FDs) the offset for the front wheels is higher than the rears. I just said a slightly lower offset will increase scrub radius, which most people feel is horrible, but imo for certain drivers and certain car setups, this might be beneficial. Without knowing specifics, its impossible to say, but you would def. gain more inboard wheel clearance (assuming you have the proper wide fenders or modifications to contain the outboard edge of the wheel), and that increased feedback to the driver MAY increase his driving abilities, DEPENDING ON THE WHOLE SETUP. Of course, it COULD also be worse, but either way it goes, its not absolute... It depends on the car setup. Like your Prost example. What are the specifics, he might have just asked for a 0 offset instead of a -50. You don't really know. There's just as big of a chance of that happening as him asking for a +50 from a 0. Who knows. But your point does illustrate that TOO much scrub radius is not good.