Wider rear tires/wheels then front, Why?? Options

[Jetsetsurfshop]

Ive been running a "square" set up for years. 15x7 with 205/50/15 Hoosiers.
I'm now contemplating running wider rears then fronts and started to ask myself why? At the HSR races all the 914 guys are staggered. It looks like the PCA guys stagger too.
Whats the performance advantage here? Is it handling? Something to do with getting hard on the brakes? Putting the power down? Fitment on the front suspension?

Teach me something.... (IMG:style_emoticons/default/beerchug.gif)

[wes]

(IMG:style_emoticons/default/popcorn[1].gif)

[bretth]

I am no expert but I always thought it was for more grip from the driving wheels and smaller wheels at the front because it may make steering harder it fronts were too wide.

[GregAmy]

Teach me something.... (IMG:style_emoticons/default/beerchug.gif)

Balance and max grip.

How's your balance now? Oversteer? Understeer? What are your hot tires temperaturs and pressures; are you reaching the compound's max capability?

Just making rear tires bigger won't make you faster unless you've already maxxed out your rear grip through compound and setup and you have a steady-state oversteer situation that you can't tune out.

And if you currently have steady-state understeer then making front tires smaller and/or rear tires bigger will make it worse.

"Why" are they doing it? Probably because other people did it. Don't blindly follow the crowd. "Never follow anyone. Be your own hero." - Neil Peart

[Jetsetsurfshop]

Teach me something.... (IMG:style_emoticons/default/beerchug.gif)

Balance and max grip.

How's your balance now? Oversteer? Understeer? What are your hot tires temperaturs and pressures; are you reaching the compound's max capability?

Just making rear tires bigger won't make you faster unless you've already maxxed out your rear grip through compound and setup and you have a steady-state oversteer situation that you can't tune out.

And if you currently have steady-state understeer then making front tires smaller and/or rear tires bigger will make it worse.

"Why" are they doing it? Probably because other people did it. Don't blindly follow the crowd. "Never follow anyone. Be your own hero." - Neil Peart

Car has a slight push now with the 15x7 205/50.
I have never had a chance to take tire temps. I'm a one man show. None of my friends get the race car thing. I guess i'll have to pay some kid to help. Theres always some track rats hanging around.

If I remember correctly I start with my cold temps 26.5 front/28.5 in the rears. I think I'm trying to get the rears harder to help with the understeer. Again, need to get some trackside help.

If the front tire is narrower then the rear wouldn't it steer better? It has more weight per square inch. I did notice that cars with 7s in the front and 8s in the rear almost always have a spacer for the fronts. 1" to 1 1/4". IS there something to this?

Like the quote (IMG:style_emoticons/default/aktion035.gif)

[wndsrfr]

Such a great question....love it. I have 15 inch X 8's.
I've run square 225/50/15 radials, 225/45/15 radials to gear it down a bit now on staggered Hoosiers (SVRA req'd) which are 7.5/23 fronts (close to 225's) and 8.5/24 rears (close to 245's) so the rear is not only wider but also taller.
At my talent level it's hard to say anything definitive, but here's a few thoughts and then feelings/observations.

Your comment about more weight per square inch needs more thinking because these are pneumatic--35psi is the force per square inch, so the tire will have a contact patch of the number of square inches to equal the corner weight on that wheel. Sooo, a real narrow tire will have a long front to back and narrow side to side contact patch, a wide tire will have a shorter but wider contact patch, see that picture? Staggered setup will give a more uniform patch in the direction of travel. I think steering effort is more influenced by camber/caster than tire width within these limits.

Do a corner balance-- on my car it works out to about 56%rear 44% front so the rear tires carry more of the weight. My fronts are like 460 pounds each and rears are like 600 pounds each. A little math comes up to have contact patch be 13.1 square inches on fronts and 17.1 square inches on rears at 35psi.

Braking and cornering have more effect than acceleration in our cars--we can corner at 1g putting much more weight on the outside wheels than steady state would lead you to think. Hard braking lifts the rear--we want as much contact patch as we can get there to avoid getting squirrely.

My lap times are pretty similar between radials and bias Hoosiers. The biggest difference that I notice is (other than the ridiculous cost) that the bias tires will hold the same cornering force when they slide than they did just before breakaway--the radials get more slippery after breakaway so harder to catch. So the Hoosiers are more "forgiving" or provide more time at the limit than I can get with radials. That said, radials have less rolling resistance so will give higher top speeds on long straights and better acceleration times throughout the lap. Pressures are radically different--radials start at 29 to finish at 36psi in a 30 minute sprint session, bias start at 16-17 to finish at 22psi. That means larger contact patch for the bias ply tires all around and when sticky give more cornering force.

You can do your own tire temps. I do it and I'm 75 years old for gods sake. Get an inexpensive probe type unit, hop out of the car and what you're looking for is the temperature profile across the fronts and rears to decide on camber & toe setting. Push the probe in 1/8 inch or so. Doesn't matter that it's instantaneous, just fairly quickly after the session, best if pitting without a cool down lap. Profile should be close to uniform, say within 10-15 degrees from outside to center to inside of the tread.

Wow--sure got going on that one, eh? Need to get a life....

[Jetsetsurfshop]

Such a great question....love it. I have 15 inch X 8's.
I've run square 225/50/15 radials, 225/45/15 radials to gear it down a bit now on staggered Hoosiers (SVRA req'd) which are 7.5/23 fronts (close to 225's) and 8.5/24 rears (close to 245's) so the rear is not only wider but also taller.
At my talent level it's hard to say anything definitive, but here's a few thoughts and then feelings/observations.

Your comment about more weight per square inch needs more thinking because these are pneumatic--35psi is the force per square inch, so the tire will have a contact patch of the number of square inches to equal the corner weight on that wheel. Sooo, a real narrow tire will have a long front to back and narrow side to side contact patch, a wide tire will have a shorter but wider contact patch, see that picture? Staggered setup will give a more uniform patch in the direction of travel. I think steering effort is more influenced by camber/caster than tire width within these limits.

Do a corner balance-- on my car it works out to about 56%rear 44% front so the rear tires carry more of the weight. My fronts are like 460 pounds each and rears are like 600 pounds each. A little math comes up to have contact patch be 13.1 square inches on fronts and 17.1 square inches on rears at 35psi.

Braking and cornering have more effect than acceleration in our cars--we can corner at 1g putting much more weight on the outside wheels than steady state would lead you to think. Hard braking lifts the rear--we want as much contact patch as we can get there to avoid getting squirrely.

My lap times are pretty similar between radials and bias Hoosiers. The biggest difference that I notice is (other than the ridiculous cost) that the bias tires will hold the same cornering force when they slide than they did just before breakaway--the radials get more slippery after breakaway so harder to catch. So the Hoosiers are more "forgiving" or provide more time at the limit than I can get with radials. That said, radials have less rolling resistance so will give higher top speeds on long straights and better acceleration times throughout the lap. Pressures are radically different--radials start at 29 to finish at 36psi in a 30 minute sprint session, bias start at 16-17 to finish at 22psi. That means larger contact patch for the bias ply tires all around and when sticky give more cornering force.

You can do your own tire temps. I do it and I'm 75 years old for gods sake. Get an inexpensive probe type unit, hop out of the car and what you're looking for is the temperature profile across the fronts and rears to decide on camber & toe setting. Push the probe in 1/8 inch or so. Doesn't matter that it's instantaneous, just fairly quickly after the session, best if pitting without a cool down lap. Profile should be close to uniform, say within 10-15 degrees from outside to center to inside of the tread.

Wow--sure got going on that one, eh? Need to get a life....

We have a life....at the track!! (IMG:style_emoticons/default/driving.gif)

I have an event next month. I'm going to do tire temps then. I always wanted to get them hot off the track. I guess an extra minute won't be the end of the world. All data is good data, right?

I looked up my corner balance info. I'm 52%rear 48%front. I have that huge fuel cell for endurance racing up front. I bet my car was more like yours before I added that tank.

I'm also working on the fact that my car tends to push. Its there with a full tank and settles down quit a bit once she's empty. Little push is fine. I was thinking that maybe the staggered set-up was to help these mid engine car with that. My car tends to corner awesome until I'm backwards looking at on coming traffic. (IMG:style_emoticons/default/driving.gif) My thought is a little oversteer could be better.

This all started because I flared my car. Maybe I should have waited till the -6 is ready. But I had the parts and the free time so... (IMG:style_emoticons/default/blink.gif) ].

So a square set-up it is. 15x8 with 225/45. Im only going to increase my footprint and my track width. I'll play with pressure there and try to dial it in. I really want to get into the mid 30s at Sebring with a -4. (IMG:style_emoticons/default/first.gif)

[GregAmy]

Smaller tires up front are counterintuitive to improving front grip. However, it's important to take tire temps to know what they're doing. If, for example, you're not getting the front 205s up to temp (unlikely) then a smaller tire "may" help that.

Example: I run non-square on my 2008 Honda Civic Si Super Touring car. 245/40-17 up front and 225/45-17 out back. FWD cars have systemic understeer so it helps the balance. Oversteers like a bitch when cold (I spend the pace lap working on the rear temps).

Secondary note: you're looking for 38 psi hot on Hoosier R7/A7 radials (and I shot for the same when running the radial slicks). 40 is pushing too high; 36 is too low. Adjust as needed to get there. My starting point for any weekend on the 2600# Civic is 28F/32R and then bleed down as needed to get to 38. I run 205/50-15 A7s on the 914 for vintage but the sessions are short; I'll typically start the weekend at around 32.

[stownsen914]

My car is set up a bit different, but I've run 10.5" slicks on the front and 12" on the rear for years and have been happy with the balance. I agree that most 914s run staggered when setup and rules permit. In general, rear and mid engined cars will have wider tires in the rear to support the extra weight out back.

You mention that your car understeers. As suggested, tire temps will give you some info. about what your tires are doing. I'm thinking that understeer in a 914 with a square setup is unusual. Maybe something in your setup is causing it?

[Jetsetsurfshop]

Tire temps will be priority one at the track. The guys I race with have the tool that has memory built in. Should be pretty easy. They are always pretty stressed about the corvette they run. I try to stay clear when that car acts up but if I ask for help they will be glad to do it.
It looks like I might be the only one in my run group again so I can use then weekend to tune tire pressures. I just hope the camber isn't off. I feel that I have that close from what everyone else uses. I could alter my front easy but I don't want to mess with the rears until I can get on a alignment rack. Plus, I don't want to do anything with my alignment at the track. I'm sure I'd mess it up or miss races while figuring it out.

About the slight push I have.
With a full take of gas I go where the fuel goes. (Only fill it up for endurance races) Its kinda funny at first but once you burn down to 1/2 tank it gets to be fun. I might even be looking for some sort of excuse for my sub par racing skills.

I found this chart on another forum. I downloaded it but cant find the forum again. I have no idea if this sheet is legit or not. If it is there are thing I can do on this sheet like soften my front sway-bar.
Is all this stuff known racing knowledge?

[stownsen914]

The chart is generally in line with my understanding, though a bit more detailed than the things I usually think about. Swaybars and tire pressures are the easiest things to change at the track, so I'd start there. Tire temps may be telling too. Lots of variables there depending on what the temps are.

For swaybar adjustment, to fix a push I'd either loosen the front sway (move the adjuster closer to the end of the arm) by one position if your setup had the multi-hole adjustment style, or by 1" if you have the slider style adjustment. Or stiffen the rear bar by a similar amount. Whichever you choose, just make sure you do both sides by the same amount (IMG:style_emoticons/default/smile.gif)

[Jetsetsurfshop]

The chart is generally in line with my understanding, though a bit more detailed than the things I usually think about. Swaybars and tire pressures are the easiest things to change at the track, so I'd start there. Tire temps may be telling too. Lots of variables there depending on what the temps are.

For swaybar adjustment, to fix a push I'd either loosen the front sway (move the adjuster closer to the end of the arm) by one position if your setup had the multi-hole adjustment style, or by 1" if you have the slider style adjustment. Or stiffen the rear bar by a similar amount. Whichever you choose, just make sure you do both sides by the same amount (IMG:style_emoticons/default/smile.gif)

This is what i was thinking. I'll start with tire temps and pressures first. See where I'm at. We have a friend that used to crew for an IMSA team that always comes with us. I can lean on him for help. I'll just have to buy dinner. (IMG:style_emoticons/default/chowtime.gif)
My front sway bar would be easy to adjust in-between sessions. Should have plenty of time to get that done if needed.
Thanks!!
(IMG:style_emoticons/default/driving.gif)

[ThePaintedMan]

Shane,
This is a really cool site which explains what's happening with the tires very well. Maybe even a little too detailed in some ways.

http://racingcardynamics.com/racing-tires-lateral-force/

Here's what's important, in my understanding - remember that the tire reacts in three dimensions - X,Y,Z in side forces (yaw), acceleration and in vertical loading. If you look at a tire in terms of contact patch, it's never just a rectangle on the ground unless you're sitting in the pits. It's constantly deforming in the X and Y planes, and x4 for each corner. When chasing grip, you're always trying to maximize that contact patch, or the amount of tire which is interfacing with the racing surface.

The fastest way to influence that is by changing the characteristics of the tire itself - i.e., air pressure. But what you do with the suspension of the car influences the forces placed on each tire of the car under given scenarios. So if you need more grip at say, the front, you want to transfer more weight onto that tire (i.e. braking before turning). What the suspension does is allow you to control how much weight transfers when you do hit the brakes (or turn, or accelerate, etc).

As others have stated, by increasing rough tire width, you are instantly giving more static contact patch. Logically, putting more tire on the rear with an already understeering car is probably not going to improve the understeer. I think when you see staggered setups it's because they have crossed into the threshold where they have enough engine that they are constantly breaking the tires loose, particularly on corner exit, so they need more contact patch where it counts.

What fascinates me is how *tiny* the actual part of the tire is that interacts with the road at any given time. It's a fraction of the size of the whole tire, but even with a street tire, it's enough to corner over 1G in most cases.

Having been the only other person here that's driven you car (I think), my impression is that you still have plenty of rear tire for your current setup. Note - I've also only driven it with DOT radials, not slicks. If anything, I always felt like it suffered more from lack of initial turn-in. When we had a full tank of gas, which is much closer to your front axle-centerline due to the fuel cell, I could get a lot of weight up front, which is ideal for turn-in. The problem I saw was that I was locking up the wheels, which means the brakes were overpowering the tires, at least with a full tank of fuel. That's a good problem to have, IMHO- you never want to have more tire than brakes available. In my mind, what would improve it would be to make the whole front more compliant, which would mean loosening the sway bar, maybe dropping front pressures a little to allow you to trail-brake a little more (keeping weight on the front longer) without locking up the wheel.

The real fun part, as you mentioned is when some of the gas burns off, will those adjustments make the car oversteer like crazy? I personally like some oversteer, and as is often said "loose is fast."

Take all my thoughts with a grain of salt - I don't have near the experience as most of these guys. (IMG:style_emoticons/default/smile.gif)

[Jetsetsurfshop]

Shane,
This is a really cool site which explains what's happening with the tires very well. Maybe even a little too detailed in some ways.

http://racingcardynamics.com/racing-tires-lateral-force/

Here's what's important, in my understanding - remember that the tire reacts in three dimensions - X,Y,Z in side forces (yaw), acceleration and in vertical loading. If you look at a tire in terms of contact patch, it's never just a rectangle on the ground unless you're sitting in the pits. It's constantly deforming in the X and Y planes, and x4 for each corner. When chasing grip, you're always trying to maximize that contact patch, or the amount of tire which is interfacing with the racing surface.

The fastest way to influence that is by changing the characteristics of the tire itself - i.e., air pressure. But what you do with the suspension of the car influences the forces placed on each tire of the car under given scenarios. So if you need more grip at say, the front, you want to transfer more weight onto that tire (i.e. braking before turning). What the suspension does is allow you to control how much weight transfers when you do hit the brakes (or turn, or accelerate, etc).

As others have stated, by increasing rough tire width, you are instantly giving more static contact patch. Logically, putting more tire on the rear with an already understeering car is probably not going to improve the understeer. I think when you see staggered setups it's because they have crossed into the threshold where they have enough engine that they are constantly breaking the tires loose, particularly on corner exit, so they need more contact patch where it counts.

What fascinates me is how *tiny* the actual part of the tire is that interacts with the road at any given time. It's a fraction of the size of the whole tire, but even with a street tire, it's enough to corner over 1G in most cases.

Having been the only other person here that's driven you car (I think), my impression is that you still have plenty of rear tire for your current setup. Note - I've also only driven it with DOT radials, not slicks. If anything, I always felt like it suffered more from lack of initial turn-in. When we had a full tank of gas, which is much closer to your front axle-centerline due to the fuel cell, I could get a lot of weight up front, which is ideal for turn-in. The problem I saw was that I was locking up the wheels, which means the brakes were overpowering the tires, at least with a full tank of fuel. That's a good problem to have, IMHO- you never want to have more tire than brakes available. In my mind, what would improve it would be to make the whole front more compliant, which would mean loosening the sway bar, maybe dropping front pressures a little to allow you to trail-brake a little more (keeping weight on the front longer) without locking up the wheel.

The real fun part, as you mentioned is when some of the gas burns off, will those adjustments make the car oversteer like crazy? I personally like some oversteer, and as is often said "loose is fast."

Take all my thoughts with a grain of salt - I don't have near the experience as most of these guys. (IMG:style_emoticons/default/smile.gif)

We need to share the car for a test day. Dial this car in. (IMG:style_emoticons/default/smash.gif)
I'm going to losing the sway bar this event. A little at a time. I believe the the push got worst after I pulled the stick torsion bars to those monster 911 ones (23mm). I used to be able to hang around 78-80mph though turn 1. Now I don't feel comfortable over 75ish. Maybe loosing the sway bar will help. All I can do is try.
I think Ill order some smaller bars too.

Be safe...... (IMG:style_emoticons/default/flag.gif)

[Justinp71]

When my car was a 4 cyl running on 4 205's it could get snap over-steer. Since I have been running 225s in the rear with a 6 I have not had that problem in 15 years :fingerscrossed: although there are many other things that have changed about the car, but the suspension has been similar over the years.