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Old 10-11-2011, 01:35 PM   #1
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Default +19 offset: how to quantify handling impact?

I have always liked the RPF1 in black. I noticed a set on sale recently and purchased them on impulse. Later, when my brain was plugged back in I realized that they are +19 offset. From what I have read, this might not be the optimum offset, handling-wise. The consensus seems to be that wider track is good, but scrub radius is bad.
How can I quantify that? Conceptually I can see how the steering might be a little "combative" as the wider track gives the tires more leverage. But how bad, really? There are tradeoffs everywhere -- lowering CG versus maximizing suspension travel, adding lightness versus adding stiffness, etc. In the larger scheme of things, how concerned should I be about the increased scrub radius?

I am thinking of using a 225/50-14 Toyo R888, if it fits without the use of a sawzall. This is strictly a dry weather street (canyon) toy.

TIA

Chris
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Old 10-11-2011, 02:40 PM   #2
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225s on a +19 will definitely not fit without a fender pull on both ends. I would run 185/195 Dunlop Star Specs, Kumho XSs, Bridgestone RE-11s, etc.
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Old 10-11-2011, 03:44 PM   #3
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uhh, well for a 7" I would listen to the guy above me. run a 185 will be better from what I have been reading as you want to take some slack out of the sidewall. plus, you will have an easier time with fitment.

Not sure if they will fit on an NA w/o a roll or not.
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Old 10-11-2011, 04:01 PM   #4
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Uhh. wat? I run 195's on my stock 5.5in wheels. If I extend your logic then I should see performance gains by moving to something like a 145-70. I don't buy it.
I understand the logic behind putting a wider wheel under a wide tire (thus the attraction of a 15x9 6UL). But I disagree that he reverse is true. Wide tire on a wider wheel might be better than wide tire on a narrower wheel, but I will argue that more tire (within reason) is always better than less tire.

As to fitment, I am fine with rolling and pulling. I will even retract my sawzall condition. It will help feed my obsession with lightness.

Chris
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Old 10-11-2011, 04:08 PM   #5
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more tire isn't always better than less tire. if thats the concept that we disagree on, then you can do plenty of reading on that as you like.

http://forum.miata.net/vb/showthread.php?t=357256

Generally, .5" stretch over the maximum manufacturer's suggested wheel width is the ideal tire. so for a 205/50R15, you want to run an 8", for a 9" 225/45, etc. etc. its a generalization, and subject to change depending on size. Emilio has plenty of posts about that, which you can read.

Its not a big problem in the end, I am running 205/50s on a 7" right now, but its only temporary.
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Old 10-11-2011, 04:14 PM   #6
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better yet, let me refer you to a thread here:
https://www.miataturbo.net/showthrea...t=49858&page=2

read both Sav's post on the first page and Hustler's post on the second page. a little bit of stretch is the ideal setup. note I said LITTLE.

We all learn the hard-way on some things. :/
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Old 10-11-2011, 04:33 PM   #7
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I AGREE that wider wheels are better. I AGREE that "a little bit of stretch" is ideal. So if the question is "what is the optimum wheel width for a 225-50", then we agree that a wider wheel is optimum.

My point is, you can't invert this logic and say "therefore, the optimum tire for a 7in wheel is a 195". Optimum in what sense? crisp turn-in? or maximum grip? I maintain that a wider tire (within reason) will always have more grip than a narrower tire (within reason).

Toyo recommends a rim width of 6 to 8 for the 225-50: toyo.com
For the 15x7 6UL, 959 endorses 195/50 , 205/50 , 215/45 , 225/45 , 225/50 tires: 949racing.com
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Old 10-11-2011, 04:50 PM   #8
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Do as you wish my friend.

As for scrub radius, sure it will be off. but its not going to kill you, just more strain on the bearings. I wouldn't be too concerned, but I would never really run a +19, so perhaps I am not the best person to ask on that particular one.
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Old 10-12-2011, 12:41 AM   #9
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For the record, I didn't say anything about optimum tire size for a given wheel width. I said a 225 at +19 won't fit under stock fenders without a pull. You said the sawzall was out of teh question, which I took to mean you were averse to permanent and externally visible body mods.

Personally, I hate tires that are stretch at all. There is a valid argument for turn in feel and predictability on stretchy tires, but on a street car I think it looks retarded.


To answer your original question, I don't think there's a way to quantify the effect that scrub radius has without letting you drive the same car back to back with different offset wheels on it. Drastically reducing the scrub radius will make the car darty and unpredictable. It plays hell with the effect that king pin inclination, caster and Ackermann have on wheel movement as the wheel turns. If you're really interested in it, I'd suggest reading Millikan and Millikan's Race Car Vehicle Dynamics. If you just want more of an idea of how all this stuff works, try the MotoIQ Suspension guide articles.

If you just want some encouragement to do it, I think they look bitchin, and I'd do it in a heart beat on a street car.
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Old 10-12-2011, 08:17 PM   #10
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I also love the 14" rpf1s on an NA. I'd rock them on my NB if my brakes would fit.

Quote:
I would run 185/195 Dunlop Star Specs, Kumho XSs, Bridgestone RE-11s, etc.
+1
R888s are a great tire, but you would be replacing them pretty often if you drove them somewhat regularly on the roads since they are a R comp and they are not cheap.
Quote:
Later, when my brain was plugged back in I realized that they are +19 offset. From what I have read, this might not be the optimum offset, handling-wise. The consensus seems to be that wider track is good, but scrub radius is bad.
I may be wrong, but I was under the impression that a very similar offset is proving to be fastest in spec.
Quote:
YES, this aggressive fitment requires a substantial fender lip roll for Miata fitment...but worth it for the same FAST offset that has proven fastest in Spec Miata.
This is what good-win-racing says about the konig wideopens which are 25 offset. 6 mm isn't a huge difference imo. http://www.good-win-racing.com/Mazda...t/21-1013.html

I also don't think you be pulling the fender as much as you think. I ran a 15x8 +0 XXRs with 205/50s on my old NA during my ricer phase (lol... shut up I learned my lesson), and the fenders needed a decent amount of pulling (mostly in the back). With 20 mm less offset on a 7" wheel, I think you'll definitely need to roll, but I doubt there will be much pulling if any, and if you do need to pull your fenders a little, hit up your local forum and see if anyone has a proper fender roller. They are simple to use, and as long as you take your time and do it right(heat gun), it'll be fine. Check out this website too: http://www.rimsntires.com/specs.jsp
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Old 10-13-2011, 01:55 PM   #11
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With a fender roll, they will fit easily and fine. You won't need a pull.
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Old 10-13-2011, 04:18 PM   #12
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I needed a LOT of roll to run 225/45 R1Rs on 15x8 +32 Kosei K1s. Toyo sticky tires run REALLY wide. I actually pulled mine slightly, and another 1/2" out would still be all up in the fenders.

Last edited by vehicular; 10-13-2011 at 08:40 PM.
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Old 10-13-2011, 06:12 PM   #13
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http://www.rimsntires.com/specs.jsp

Found the link @ 949Racing

Get some measurements and compare to what you have now, should get you pretty close to an answer.
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Old 10-13-2011, 11:30 PM   #14
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Quote:
Originally Posted by RedemPhex View Post
more tire isn't always better than less tire. if thats the concept that we disagree on, then you can do plenty of reading on that as you like.
It's usually better. There are stock-class autocross guys that shoehorn 225-wide tires onto 6" wheels. It's faster than a 205 on the same wheel so they do it.

On very high-speed courses with low power, a 205 can be faster than a 225, but beyond that the wider tire is pretty much always faster - especially in autocross situations.
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Old 10-14-2011, 12:27 AM   #15
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Quote:
Originally Posted by Savington View Post
It's usually better. There are stock-class autocross guys that shoehorn 225-wide tires onto 6" wheels. It's faster than a 205 on the same wheel so they do it.

On very high-speed courses with low power, a 205 can be faster than a 225, but beyond that the wider tire is pretty much always faster - especially in autocross situations.
good to know. I am quite confused on this as I normally assumed that wider is faster, but quite a few guys at m.net have been swearing that emilio says a 205/50 on 15x8 is faster than the 225/45. I am taking it with a grain of salt, thats why I am not really arguing about it here. So in your experience, what is faster? and has anyone fitted a 245/40 Avon CR500 on the 15x9 and posted about it?

I am personally looking at this from a track prespective, I don't give a **** about autocross really.

Btw, here is the thread in particular:
http://forum.miata.net/vb/showthread...=426884&page=2

Last edited by Seefo; 10-14-2011 at 12:44 AM.
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Old 10-14-2011, 10:19 AM   #16
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Sorry about the thread jack here, but I figured I would share.

well, I have just found out the CR500 is a full track tire. 20 treadwear rating, so nevermind that. I guess we will just keep waiting for till there is a 245/255 street tire.

I would still like to hear what more Sav has to say on tire width though!
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Old 10-14-2011, 05:12 PM   #17
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Quote:
Originally Posted by RedemPhex View Post
I would still like to hear what more Sav has to say on tire width though!
Quote:
Originally Posted by Savington
On very high-speed courses with low power, a 205 can be faster than a 225
I'm talking like 115-120whp. Emilio's enduro cars have ~140whp and they both run 225/45s on 9" wheels.
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Old 10-14-2011, 05:25 PM   #18
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Some say 205 is too much stretch on x9 also, but Emilio said he got feedback that 205 RA1s (I think) felt better on x9s than x8.

As for offset, I ran 15x8 et36 + 20mm spacer for a while, effectively +16mm. I did not notice anything not to be expected... steering effort may increase (dpowered rack), steering feedback slightly less (but still plenty), and the typical effects of extra unsprung weight (my x9 wheel @ 12.7~.8 lbs rides smoother than my old OEM x5.5 @ 14lbs).

FWIW: I don't really need a x9, but I wanted one, and sold the x8 + spacer setup to get 9s. It weighs the same, gives the tire more support, maybe more drag but I guess I'll find out on the next track day if it was detrimental.
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Old 10-14-2011, 06:29 PM   #19
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I'm running a 15x8 +25, which (if my math is right) is about 7mm farther out than a 14x7 +19. With a mild fender roll I'm not having any rubbing issues on a 225/50/15 tire. On a 225 you will probably need to roll the fenders but I don't think you're anywhere close to clubroadster status. Personally, I think it's a great size and offset for the car.
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Old 10-19-2011, 09:20 PM   #20
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Quote:
Originally Posted by greeenteeee View Post
.. Emilio said he got feedback that 205 RA1s (I think) felt better on x9s than x8.
Nope, not something I said or wrote. Hoosier 205's are almost an inch wider that any other 205 race tire so they alone will be faster on a 9" wheel. For any other 205 tire, I'd recommend an 8" wheel.

As far as which is "faster" between 205 on 8 and 225/45 on 9, it depends on the car and course. The 225 will always put the power down better and generate more braking grip simply due to it's greater surface area. The 225 take more power to keep at optimum slip angle and push through the air. An extreme example might be T2 at WSIR entered at about 105mph in a Spec Miata. On 225's in certain track conditions that spec will scrub off speed until the car can no longer maintain optimum slip angle (grip) of the tires. With 205's, if you are careful and enter it with a lot of mo, you can just hold all four tires at optimum slip angle. Down the front straight at WSIR with a 120hp Spec, the one of with 205's would have a 2-5mph advantage due to reduced aero drag. In that extreme case, 205's are faster. Few tracks on the planet are as fast as WSIR so take that with a grain of salt. At SOW, I'd take 225's even with 90whp.

Oh and to the OP, sorry for the drift
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