Different F/R wheel/tire sizing

Suspension and other discussion

Different F/R wheel/tire sizing

Postby lumbercis » Tue Apr 10, 2007 7:33 am

Okay, here's a deep question for whoever styles themselves the board wheel/tire/handling guru. The ultimate question is about how to get the best handling out of our cars with the way our AWD system is setup. But first, a few pieces of evidence:

Image

We've seen this car posted quite a few times, but mostly because it looks great. But not a peep about the funky wheelspecs: Rims are apparently both 17x9 +38 with a PCD changer. But note the tires, 245/40F, 225/45R. And Hideki told us that the owner had previously run the more conventional 255/40 Front and Rear. So he presumably thought this setup handled better.

Exhibit 2: From our own Handling FAQ http://www.alltrac.net/phpBB2/viewtopic.php?t=8799&postdays=0&postorder=asc&start=15
Note the first two pages discussing front and rear track and esp. CMS-GT4's observations about how keeping a wider front track seemed to improve his handling. Also related is how the article from whiteline mentions that the AWD design of the GT4 is basically adapted from a FWD design. In other words, the GT4 is more or less a FWD car with a differential putting power to the rears as well.

With that in mind check out the specs of this Integra Type R from the "Touge Showdown 2" video (It laps Tsukuba circuit at 1:04 with an almost stock engine):
http://video.google.com/videoplay?docid=-6277203205548987907&q=touge+showdown+2&hl=en

Hows this for odd wheel/tire specs: 225/45 R16 Front, 195/55 R15 Rear

If you then compare a road race car with an advanced AWD system like the Mine's or MCR Skyline GTR, you will notice they run the same size wheels/tires front and rear.

So what does all this tell us? My theory is that, because of our primitive AWD system, our cars behave much more like a FWD car handling wise and should probaby be set up like one. That is, wider up front, skinnier in back. I have no clue what effect different sidewall sizes have, but it must play a role as well. Problem is, Im just making an argument based on what I have observed. I know jack about handling and what makes a car handle better or worse. And thus have no real way to suggest specific setups for our cars that might work. But I thought I would throw this out there and see what you all think. Maybe some of the board autocrossers might be willing to experiment and let us know the results?

J.
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Postby turbo4wd » Tue Apr 10, 2007 1:23 pm

"Wider up front, skinnier in back" is something I've always thought was the way to go with FWD cars in general - and I have that Best Motoring DVD with that particular Touge Showdown - nice little battle :)

The 185 (like 99% of other cars) is dialed in with a moderate amount of understeer from the factory. The car also has a wider track in the front relative to the rear, which would imply that Toyota had to resort to this measure to gain more front end grip. That being said, my opinion is that a person who goes the opposite direction and gets wider rubber in the rear or increases the rear track by installing spacers without changing the fronts accordingly is going backwards in terms of obtaining the best handling possible for the 185.

I'm not at all surprised that Hideki said the owner of the 185 posted above said he had better handling with a wider tire up front. I'm assuming that the owner used the same width PCD changers on all 4 corners to maintain the front to rear track ratio.

The only thing that would concern me regarding different tire widths are the differences in rolling diameters, because AWD cars (and their diffs) in general don't like that - the setup on that particular 185 has a 1.0% difference, so maybe our diffs can tolerate it without worry. A better combination wouldve been to run 255/40/17 up front with the same 225/45/17 in the rears because that comes to a 0.2% difference.
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Postby CMS-GT4 » Tue Apr 10, 2007 2:34 pm

You forgot to meantion another gtfour with different width front and rears. The red Garage cruise car. It used 255 in the front. I don't remember the rest of the specs. Rolling dia. will be your largest concern.

This might be ideal in some racing events, but also look at Martin's setup. By changing the rear spring rate he created the same effect while keeping all the tires the same size all around.

I think there are many factors to examine when tuning the suspension of an alltrac.

• Its handling behavior is not consistant:
Meaning it does act like a fwd car on corner entry but by apex and corner exit the rear really plays a large part of the cars handling. Even with an open rear lsd my cars apex and corner exit are much more rear biases.
Where a normal car might have a fairly consistant arch on a turn, the alltracs arch would be larger on entry but tighter on exit if you kept the wheel the same and the speed the same. Its center VC is slow to react and once its working you are gaining grip.

Since I have had my alltrac there has always been a real racer in the background saying the same thing. Slow in, fast out.
If you can master corner entry in this car, you can get a hell of a lot more speed out of it on exit. I even applied this at my last autox ~3years ago and got a better time then plowing thru the turns. If you have some traction to spare going into the turn its easier to set the car up for a full blast exit.
I experiemented with this some last night with pleasant results.
However, if you can increase that corner enry speed all the better.

I plan on posting results when my lsd is installed, then when I get my alignment. After installing my coilovers, I never got a proper alignment. Now I see that I have a lot of negative camber on the rear and the fronts look like they don't have hardly any camber. I think a decent alignment might be the largest factor yet.

• The alltracs suspension is heavy. There are not a whole lot of options for lightening the suspension w/o custom fabrication. I still think this is one of the most important aspects to getting a more responsive front suspension. I really do not think the car itself is that heavy. There are plenty of cars that handle better that weight as much if not more. However less weight does help, especially on the suspension.

• The alltrac is nose heavy. You can strip as much weight as you want, but it will always weight more in the front driver side, thanks to transmission placement. Corner weighting your suspension may be a big factor here. Having a height adjustable suspension and getting it corner weigted will help in this respect. This might have something to do with TEIN HA specs making the rear lower than the front. Maybe not. A proffessional alignment specialist should be able to do this for you. Expect to pay 120-200 bucks for a race alignment. Might be good to invest in your own equipment and learn how to do it yourself. We have a few porche racing shops (not like tuner shops) around here that build winning race cars for real race teams and I have picked upa few tips from them. One of them employs one of SCCAs national champions as their alignment pro.

• Rear end slop. Already a cure for this. Solid dif mounts and poly bushings. By reducing or removing the give in the rear the rear lsd will work more.

• Steering feels like ass. Why, cause of the turning radius. I have to turn the wheel a lot more to get the same turn as I do in my Z. I am going to see how much a small steering wheel helps this. I think our steering inputs might not be turning the wheel as far as we need to to get proper turning. Then we have to turn the damn wheel all the way the other way. By not turning the wheel far enough we are creating the illusion of more understeer.
I altered my techniqe and was happier with my turn in. I place my hands on the bottom of the wheel so I can turn the wheel more for each turn.

• Spring rates. Off the shelf springs rates are not hte way to go. Adrian has done a lot of work on his stuff. Even though most of us use the spring rates Tein provides they do custom spring rates for the same suspensions if you special order the coilovers. Martin increased his rear spring rate now the car is tail happy if you look in his latest video. Alltrac guys take whats offered cause half the time we don't know better. The subaru guys have been doing higher rear spring rates for a while to make the tail swing around easier. This may be a solution for us as well. Rather than the smaller rear tire size.
Essentily with less grip on the rear is what makes it come out. Not as much as the cars power bias. Increasing the rear dampning and spring rate is the same idea.

So. This tire idea might work. Its hard to say. Its an expensive idea to test and not be happy with it. I do think the front needs more grip, and more tire is how its going to get it. How you get the rear to come out is up to you.
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Postby turbo4wd » Tue Apr 10, 2007 8:14 pm

Our steering radius isn't great because of the AWD setup - you're limiited physically because of this - any increase in the steering angle would ending up causing the front axles to bind up with the hubs. Your 350z doesnt have to worry about this because it doesnt have any drive axles feeding into the front hubs so the steering angle can be greater and the radius smaller..

When it comes to spring rates and increasing the rear stiffness over the stiffness of the fronts, this may work to help bring the car's rear end around in tight turns in a slow speed short course environment like the one you encounter in autox. But on a large, high speed road course, that "ease" of bringing the rear around in autox is greatly exagerated and can be dangerous as the car becomes more influenced by the characteristics of the track (bumps, undulations, etc).
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Postby CMS-GT4 » Tue Apr 10, 2007 10:04 pm

Good point about the higher speeds.

Even though the angle for steering can only be so great it can still be improved over stock.
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Postby b00staholic » Wed Apr 11, 2007 6:23 am

Interesting thread...I love threads like these :D

So...for the majority of us who want to keep the same size rims and tires all around for the sake of our differentials and the ability to rotate tires, what does this all mean?

I guess it all goes back to what's the best way of improving the car's handling by manipulating the front and rear track?

a) Adding spacers to the front (what size? 10mm, 15mm, 25mm?) and no spacers in the rear.

b) Adding spacers to the front AND rear (what size?)?

I'm really curious what size spacers you can run in the front without rubbing the fender or negatively affecting the steering effort / geometry and turning radius too much.
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Postby lumbercis » Wed Apr 11, 2007 6:33 am

Lets assume for the sake of argument that we are setting the car up for a fairly tight road course (like the touge videos). So not as tight as an autocross track, but not a super-fast open road course either.

CMS-
I see that you are looking at the car's handling holistically, and of course, that's ultimately how you judge the car's overall handling performance. But I was wondering if one possible limitation of this approach is that, since the overall system is very complex and almost infinitely adjustable, could you not be using one particular modification or setting change to mask/band-aid a less than optimal setting in another element of the car? I think what I am getting at is, isn't there an optimal setting for both spring rate and F/R tire/rim?

Spring rates have been talked about quite a bit on here, but im interested if we can get even better handling, over and above what we already get by tweaking spring rates, by playing with the F/R tire/rim setup?

Getting down to brass tacks, if you had the money to try this, what kind of wheel/tire combo might you try first?

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Postby lumbercis » Wed Apr 11, 2007 6:40 am

@vincent

Im certainly no expert, but i would think that if you wanted to keep the same size rim and tire all around, the best thing to do would simply be to NOT change the nature of the F/R track. From what i understood in the handling thread, the problem was that people were running rear spacers only for appearance reasons, which made the track more even. This might be great if we had an EVO or WRX AWD system, but with ours, its better to keep the rear track slightly smaller.

Thats my theory anyway :shrug:

The question Im asking here is what would be optimal if one were willing to go all out and have staggered wheels/tires.

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Postby syko says » Wed Apr 11, 2007 7:14 am

in another thread, this was broguht up

b00staholic wrote:What are the F/R tracks of the actual rally Celicas?

From http://www.tte.de/st185.html,

F: 1510mm = 59.45 in.
R: 1510mm = 59.45 in.


sorry to bring it up again, but maybe we can spark up something and get the real idea behind it. :shrug:


*EDIT* link doesnt work
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Postby turbo4wd » Wed Apr 11, 2007 1:13 pm

syko says wrote:in another thread, this was broguht up

b00staholic wrote:What are the F/R tracks of the actual rally Celicas?

From http://www.tte.de/st185.html,

F: 1510mm = 59.45 in.
R: 1510mm = 59.45 in.


sorry to bring it up again, but maybe we can spark up something and get the real idea behind it. :shrug:


*EDIT* link doesnt work


If you want the real idea, email TTE.. They're really the only ones that can give you their reason on why they did what they did. I have no clue as to why they set up the 185 rally cars with equal f/r tracks. It probably goes without saying that they found it was optimal for the loose stuff (dirt, gravel, & snow).. tarmac stages play a small percentage in rallys overall.
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Postby turbo4wd » Wed Apr 11, 2007 1:21 pm

b00staholic wrote:I'm really curious what size spacers you can run in the front without rubbing the fender or negatively affecting the steering effort / geometry and turning radius too much.


This is going to rely on a few things:

- the offset/width of the wheel you run.
- the tire size (both width & sidewall ratio).
- the ride height of you car.
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Postby CMS-GT4 » Wed Apr 11, 2007 2:35 pm

turbo4wd wrote:
b00staholic wrote:I'm really curious what size spacers you can run in the front without rubbing the fender or negatively affecting the steering effort / geometry and turning radius too much.


This is going to rely on a few things:

- the offset/width of the wheel you run.
- the tire size (both width & sidewall ratio).
- the ride height of you car.


+1. I could run those spacers on the front w/ a 17x7 or even a 17x8 wheel. Even with the fenders slightly wider, I am going to get some rubbing trying to do it on my current 17x9 255/40 setup.

Here is something else to concider, as was mentioned a long time ago on the old board. The adjustment of the offset w/ spacers may have not been the factor in the improved handling. When I tested this, I had only installed the coilovers. I didn't get an alignment. I have a lot of negative camber in the rear and the fronts look fairly neuetral. By increasing the front offset, I am also changing its camber. So it is possible, it was alignment that changed the handling and not so much offset.
If the offset does prove true, I am concidering trying to trade someone my rear 25mm spacers for some 20mm spacers since I really don't want ot add a front spacer at this point.

• The only real way to test this is for someone to have th ecar set where they want. Run the car w/o rear spacers. Then run it with and compare the handling characteristics.

• On another note, TTE ran beefy struts. Even with my HAs, and my current tire setup, I might not clear the rear strut w/o the spacers. This may have been the logic behind an even track. TTE was also running an advanced awd system. There is a chance that their awd was helped with a wider rear track. Is hard to say. I have seen very few awd cars with high front offsets and one was an evo. There are a lot of varibles that effect each other so there is much to take into concideration when tuning the suspension.

lumbercis wrote:Lets assume for the sake of argument that we are setting the car up for a fairly tight road course (like the touge videos). So not as tight as an autocross track, but not a super-fast open road course either.


Frankly, I think you should attempt that course stock or with your current setup. I think every car is different when tuning suspension I do not think that any one solution will yeild the same amount of conclusion in every car. I also think you should look into how serious you want to race the car and if you stlll plan on using it as a street car.
Some of these mods are not really safe for the street. And most of the really good stuff is expensive.

lumbercis wrote:CMS-
since the overall system is very complex and almost infinitely adjustable, could you not be using one particular modification or setting change to mask/band-aid a less than optimal setting in another element of the car? I think what I am getting at is, isn't there an optimal setting for both spring rate and F/R tire/rim?


I think the optimal rates are those provided by aftermarket suspension companies. Teins rates and DMS's setup would seem most optimal in my eyes. I think they made their system general enough to cover most of your needs. They did some research and found a good general setup.

As far as tires go, there is a lot of debate about size. I think a 235 or 245 tire would meet most peoples needs. I would not go less than a 235 myself for racing, concidering the weight of even a lightened alltrac.

Another part of handling and tire choice that hardly gets talked about is gearing. I am currently happy with my higher gearing from my conciderably larger tires. It may hurt my acceleration some, but I can ride 1st and 2nd a lot longer on a short tight course. I think gearing is a hard factor to determine. Because its doubtful you will run the same course every time. Which may be benifited from differnt gearing.

Another thing I have not talked about, is lsd. Even though our system is old, it is still in practice today. WRXs and certain models of the neww evo still use a VC center and a torsen. Look at the y2k celica that fensport used to get some of the highest awd times on that course. And it used a standard st205 setup. (Not sure the rear lsd type [clutch or torsen]).
They showed with a proper suspenion setup and low weight, the car can do well.

But if I was going to race hardcore, here is how I would start.
Rear lsd
Solid diff mount
Upgrade bushings.
stock alignment with aggressive camber in the front. (As much as stock bolts will allow)
Smaller steering wheel.
New tierod ends
upgraded pads and fluid. I would not do lines for the sake of upgrading to different brakes in the future.
And an extra set of wheels that are just for racing. I would et the lightest wheel you could afford in a 8" or 8.5" width and a real racing tire. On the note of wheels though, you might just concider getting really good tires for the stock wheels till you really know what you want.

Then once you can 1) drive the car to it full potential and 2) learn its characteristics then you can determine what you need to change about it to get it handling how you like.
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Postby smog7 » Wed Apr 11, 2007 8:27 pm

Would longer wheel studs be necessary for spacers in the front? Does anyone here use spacers in the front? Which companies make front spacers for our cars? Anyone have pictures of how they would look?
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Postby turbo4wd » Wed Apr 11, 2007 8:36 pm

smog7 wrote:Would longer wheel studs be necessary for spacers in the front? Does anyone here use spacers in the front? Which companies make front spacers for our cars? Anyone have pictures of how they would look?


You'd use hub centric adapters like these:
http://cgi.ebay.com/ebaymotors/5x100-15mm-WHEEL-ADAPTERS-SPACERS-IMPREZA-WRX-LEGACY_W0QQcmdZViewItemQQcategoryZ42614QQihZ016QQitemZ260105787272QQrdZ1QQsspagenameZWDVW#ebayphotohosting
They're usually available in a number of different thicknesses.
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Postby smog7 » Wed Apr 11, 2007 8:54 pm

turbo4wd wrote:
smog7 wrote:Would longer wheel studs be necessary for spacers in the front? Does anyone here use spacers in the front? Which companies make front spacers for our cars? Anyone have pictures of how they would look?


You'd use hub centric adapters like these:
http://cgi.ebay.com/ebaymotors/5x100-15mm-WHEEL-ADAPTERS-SPACERS-IMPREZA-WRX-LEGACY_W0QQcmdZViewItemQQcategoryZ42614QQihZ016QQitemZ260105787272QQrdZ1QQsspagenameZWDVW#ebayphotohosting
They're usually available in a number of different thicknesses.


wow my mind really went blank. I already knew know about hub centric spacers. What got me mixed up is that I have never seen any used for the front, I just thought they would require longer studs.... Thanks for the reply turbo4wd. Anyone have any pics of how spacers in the front would look?
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