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Opeler
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382 Posts
Discussion Starter #1
I've been doing some autocrossing with my GT and have been a bit frustrated with the lack of grip. I'm looking at buying better performance tires and am trying to weigh my options. There is plenty of information out there for comparing tire models, but my question is what wheel and tire size combinations work well on a GT? Is there such a thing as too wide? For example right now I'm running 225/45-17 with front and rear swaybars and the front end lowered about 2" and the rear about 1.5" (I think). The front end likes to plow straight when I tell it to turn. Does the GT handle better with a smaller diameter or narrower tire? Perhaps a staggered setup with narrower in the front and wider in the rear?

(If this has already been answered in another thread please point it out to me. I've never had much success finding what I'm looking for with the search utility.)
 

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I have similar in 16s but 215 in front & 225 on the rear. .The front is 1 inch narrower than the back. May want align them by adding half inch spacers up front. Helps with understeer. I added 15mm with great results.
 

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Opeler
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844 Posts
Try removing the front sway bar. If that has too great an effect and you get oversteer, you could try softer bushings in the front sway bar, such as rubber vs. the polyurethane you are probably using. As far as tires, you would be better off with a smaller diameter. If you are in a street tire class, then a 15" setup will allow you a good selection of performance tires. If you are willing to commit to race rubber, then 13" is even better and there are autocross tires available yet.

Bill
 

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Opeler
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Front wheel alignment has strong influence on GT's behaviour. To decrease understeer, you should adjust the wheel camber to "0" or even go to slightly negative camber.

Also, let's face it, the street tires (even "performance" street tires) will not fully satisfy you when autocrossing. If you are actively involved in autocrossing, you should consider investing in the second set of wheels with the specialty tires.
 

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Just Some Dude in Jersey
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The front end likes to plow straight when I tell it to turn. Does the GT handle better with a smaller diameter or narrower tire? Perhaps a staggered setup with narrower in the front and wider in the rear?

Well, understeer is a GT's claim to fame. Everything is cool as you push your cornering speed faster and faster and then all of a sudden: Nothing. On the bright side, the point at which the understeer occurs is absolutely predictable. Take your favorite turn at 43mph and everything is cool, but at 44mph you got nuthin' and you're off the course.


Lightening the front end, which seems counter-intuitive to me, is supposed to be the cure for understeer. You got no need for headlights on an autocrosser, so you could toss them and save about 50-60lbs, and moving your battery to somewhere behind the seats is supposed to help.


I've always wondered about the skinny vs. wide tire traction thing. I remember seeing those old European movies where you'd see little boxy Fiats with 4" wide tires taking turns insanely fast and wondering how the heck those bicycle tires held the road.

:veryhappy
 

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Opelitis afflicted
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530 Posts
I've always wondered about the skinny vs. wide tire traction thing. I remember seeing those old European movies where you'd see little boxy Fiats with 4" wide tires taking turns insanely fast and wondering how the heck those bicycle tires held the road.

:veryhappy
Because they leaned into the corner?:D
 

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Living in the past
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2,336 Posts
Cornering in a GT

The best set of tires I ever had as far a handling was a set of BF Goodrich Comp T/A's on 6 inch 13" rims, they were 205/50/13's. If you take the front sway bar off and leave the rear that will make it handle almost neutral, moving the battery or going to a lighter dry cell will also help. I ran Bilstein adjustable shocks set on firm and they worked out pretty well.
 

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You didn't elaborate much on your suspension setup.

Spring rates?

Shocks?

Bushings?

Sway bar sizes?

Alignment specs?

Wheel widths?

As I recall you swapped the rear axle. Anti squat specs? Roll center height?

If you are using a leaf spring up front and are lowered 2", how much did you clip off the front bump stops? If you said "none" then this is your problem. You would be on the bump stops while cornering, meaning the front spring rate would be nearly infinite. That'll make it push like a pig.
 

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Opeler
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382 Posts
Discussion Starter #10
Thanks for the input guys. This really is the best car forum I've used.
I'll go from the top down on responses.
  1. Yes I do have the G80 locker in the rearend which could add to the understeer when a wheel tries to break loose, but I think I'm having the issue equally on and off the throttle.
  2. I think my track width is pretty even. I have 1" wheel spacers up front to make it the same as the wider volvo rearend. (fwd offset wheels still fit under fenders) Dale, are you saying that a wider front track will help with understeer?
  3. I'm using a vw beetle rear sway bar with poly bushings (bolted to the bottom of the engine crossmember) for my front bar. (opel bar wouldn't fit with engine swap) I have the endlinks a little loose to allow some body roll before the bushings come into play. I guess I can try disconnecting the bar all together. Is smaller tire diameter better because of lower inertia?
  4. P.J. hit the nail on the head. I've come to the realization that I really need another set of wheels and tires just for autocross. Since they're a bit of an investment I want to select something that works well. I'd need to stick with at least 15" wheels to clear the brake calipers. I currently have two sets of 17" wheels but I also have an old Datsun roadster that will share the autocross tires so I'm considering buying another set of wheels. Just need to decide on size.
  5. Since this is not a track only car I have to retain the headlights, but I have thought about relocating the battery or switching to a lawn tractor battery that is about half the weight.
 

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Opeler
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382 Posts
Discussion Starter #11
Rally Bob I'll get to your response next time I have a work break
 
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Front wheel alignment has strong influence on GT's behaviour. To decrease understeer, you should adjust the wheel camber to "0" or even go to slightly negative camber.

Also, let's face it, the street tires (even "performance" street tires) will not fully satisfy you when autocrossing. If you are actively involved in autocrossing, you should consider investing in the second set of wheels with the specialty tires.
That is why i love this old pics in the net:yup:
Here we can see what P.J. mean:cool:

scoll down for the diffrent tires or camber

Google-Ergebnis für http://opelgtworld.de/rally.jpg

http://faimg1.forum-auto.com/mesimages/7202/Greder%2072.jpg
 

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Opeler
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844 Posts
[*]I'm using a vw beetle rear sway bar with poly bushings (bolted to the bottom of the engine crossmember) for my front bar. (opel bar wouldn't fit with engine swap) I have the endlinks a little loose to allow some body roll before the bushings come into play. I guess I can try disconnecting the bar all together. Is smaller tire diameter better because of lower inertia?
-Lower inertia is nice, every bit helps.
-Smaller wheels will gear the car lower, which will improve acceleration, but could be bad if they cause you to over-rev or makes a shift to third necessary.
-Low profile tires on small wheels will lower the car while increasing fender clearance, with no suspension geometry penalty.
-Smaller tires turn in faster.
-Smaller tires are generally less expensive than the equivalent larger sizes.

The drawbacks are:
-They will throw off your speedometer and odometer, but it sounds like you are leaning toward a dedicated race setup, so who cares.
-Few choices in max performance tires, but 15" is still fairly popular so options exist.

You really need to decide on what class you can be competitive in (and eligible), which may dictate what tires you can run.

Bill

p.s. Keep in mind this trend towards big hoops was started by the need of true high performance cars to fit large brake rotors. There is also a handling advantage to low-profile tires, to an extent. The rest is fashion.
 

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Opeler
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382 Posts
Discussion Starter #14
You didn't elaborate much on your suspension setup.
Spring rates? - I'm not sure. The car was lowered by a previous owner via front leaf modification and cut rear springs

Shocks? - orange Koni adjustables

Bushings? - original rubber

Sway bar sizes? - rear Addco 5/8", front bar 3/4"

Alignment specs? - I turned the upper ball joints 180 deg to add negative camber

Wheel widths? - current wheels are 17x7. If I get 15" wheels they'd likely be 15x6.5

As I recall you swapped the rear axle. Anti squat specs? Roll center height? - Yes I swapped the rear axle but retained all the original suspension components. I adjusted the pan hard rod mount on the axle so that the rear axle is centered at the current ride height and fabbed a torque arm which has the forward mount point in the same longitudinal location as the original torque tube center mount. I'm not sure how to best determine the roll center height.

If you are using a leaf spring up front and are lowered 2", how much did you clip off the front bump stops? - When I installed the front sway bar I cut the bump stops down by about half.
 

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11,747 Posts
With my skinny butt inside ..1/2 tank of fuel ~six gallons US
LF 563 RF 547
LR 529 RR 465
That's with a relocated battery in the right rear. And no hood.
 

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Über Genius
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9,314 Posts
With my skinny butt inside ..1/2 tank of fuel ~six gallons US
LF 563 RF 547
LR 529 RR 465
That's with a relocated battery in the right rear. And no hood.
That right rear will throw you off a little.

Are you talking about cornering on winding roads or on an oval? With the weights you have I think you should be cornering better on left turns than right ones and MUCH better when you aren't braking (ie. going uphill).

I find it odd that flipping the ball joints gave you a better cornering setup. Only because I accidentally flipped the ball joints one time and it set the car up to dance in a straight line. Cornering wasn't even close to being better.
 

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Opeler
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382 Posts
Discussion Starter #18
Corner weights

What are you corner weights?
I just finished weighing the car tonight. I used two digital 400 lb capacity bathroom scales at each corner with a piece of 2x10 spanning each pair of scales. The weights may not be 100% accurate but I think they should be pretty close.
LF 713, RF 606
LR 538, RR 503

56/44 front/rear weight %
*weights were measured with 190 lb ballast in driver's seat and 40 lb ballast on drivers floorboard(to simulate weight of drivers legs on floor) and 1/4 tank of fuel.
 

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Just Some Dude in Jersey
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14,995 Posts
Holy Cow!(No pun intended! :veryhappy) You need to lighten that driver's side!

Option 1: Sit side saddle and put your feet in the passenger footwell.

Option 2: No more beer for you!

Option 3: Chop off legs, place on spare tire ledge.

Option 4: Remove left front wheel, REALLY lean into those right hand turns.

Option 5: Remove brake pedal/MC/Booster/etc. Slam into car ahead of you, thereby using HIS brakes.

:lmao:

:veryhappy
 

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Über Genius
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9,314 Posts
Holy Cow!(No pun intended! :veryhappy) You need to lighten that driver's side!

Option 1: Sit side saddle and put your feet in the passenger footwell.

Option 2: No more beer for you!

Option 3: Chop off legs, place on spare tire ledge.

Option 4: Remove left front wheel, REALLY lean into those right hand turns.

Option 5: Remove brake pedal/MC/Booster/etc. Slam into car ahead of you, thereby using HIS brakes.

:lmao:

:veryhappy
Nah, much easier.

Just make left turns only.

Remember, two wrongs don't make a right. But three lefts do!
 
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