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At the last divisional SCCA race my car was corner weighed. 69 Ghia.

LF 330 lbs
RF 345
LR 540
RR 524
Total 1739. Minumum allowed in EP 1680.

That works out to about 63% of the weight to the rear.

I'm running the same size tire on all four(195/60 14). My working theory is that I should change to tires that are wider in the rear than the front. I would like the lbs per square inch of tire patch to be the same front to rear. So a 13% wider rear tire would be better.

Any thoughts?

Theo

Theo,

I ran my Ghia in EP in 2002, and I think in that year the maximum wheel rim width was 7" and the max track was 60".

Talking to some folks in EP then, they told me that was the rim width spec for the similar SCCA road racing class, and they made the 7" the same in Solo 2 so the road racers could bring their cars into EP and have a place to play.

Since the 7" rim width limit for road racing discouraged very wide tires, the tire manufacturers came up with the cantilever tire concept for road racers. They made super stiff sidewalls on the racing slicks where the tires were cast for 7" wide rims but the tire section width was on up like 9" wide and more.

The Ghia's stock track was in the mid 50" range if I remember right... maybe 55" or 56", so to max out the track, I ordered custom made Diamond Racing Wheels 13" in dia. and 7" wide but hugely offset (caution: bad for bearings!) so my track was just a hair under the 60" limit front and rear.

Onto those 13" x 7" wheels, I put Goodyear cantilever road racing slicks that were 20" (tire dia and not wheel dia) x 8" section width and R430 compound (actually probably too hard for autoxing). These are not optimal for my 7" rims. I've been told that a better set up is more like Hoosier 20" x 9" cantilever in R25 (might melt on really hot days!) or R35 compound. Some rear engine drivers put R25 on the front to heat up faster and R35 on the rear to withstand the heat. Some of the really good drivers in DMod don't like the cantilever tires at all and go to 8" wide wheels front/10" and more wide wheels rear to support the wider slicks.

In terms of your question about putting wider tires on the rear, what type of handling characteristics are you having?

When I started out in 2001, I was running street tires... Yokohama AVS 195/60 x 15 on the Ghia, and the handling was so poor on the Ghia that I literally couldn't get the thing to turn into sharp bends.

While the Goodyear slicks helped with grip, it was really adding better shocks (Konis) and swaybars (Whiteline) that began to give me the control I wanted and the increase in oversteer that helped the car rotate.

I guess what I'm asking is whether your car is pushing or is loose before I'd make a call on a wider section width in the rear.

There are a bunch of those charts on the Internet that show how to increase oversteer/rotation or increase understeer for variables like wheel width, camber, spoilers, swaybars, shocks, etc.

I've got one of them I'm looking at and will try to scan the whole thing but here is what it says for some of these variables above:

To increase oversteer and reduce understeer:
Front tire section width: Increase section width
Rear tire section width: Reduce section width

(To reduce oversteer and increase understeer, do the opposite on section width.)

To increase oversteer and reduce understeer:
Front wheel width: Wider
Rear wheel width: Narrower

If your car is handling perfect right now, and you're getting some nice drift/slip angle, be careful about widening your rear tires, as it could make it push some.

Here is a link to one of these charts:

http://www.rsracing.com/tech-tire.htm

These are some experiences I've had, and take with a grain of salt. I haven't driven the car in a year due to blown tranny, divorce, car in storage, etc. and am now finally back to working on welding in new pans and am putting in a Kafer Cup brace along with the new tranny and 1915.

I have heard that in the next year or so we will see loosening of EP rules (and Prepared rules in general) to be more of a stepping stone from Street Prepared. That means fuel injection and wider wheels and tires will be allowed so that guys running Street Prepared who have those now can more cost effectively slide into Prepared to go faster and do more mods.

There is talk on the DMod list I'm on that maybe in 2006 DMod tub cars will be put into A Prepared with a sliding weight/displacement formula and DMod will become dedicated to tube frame cars like Locosts and rebodied sports racers. Man, there are a lot of rules in SCCA!

Jeff
'71 Ghia DMod
(we can rebuild it... we have the technology)

Jeff,

Thanks for the informative post. Wow. So much to learn.

I'm a little surprised at the understeer people report with Ghia's. My Ghia has always oversteered big time. I've been terrified to push hard on the street since a snap spin on the street years ago. My Ghia has always handled great but once you broke the rear loose you were on for a ride.

I have a theory that the proper road race setup for a Bug or Ghia is to have the front end about 1 inch taller than the rear when measured at the pan. If the pan is lower in the front it will push. This was something I learned setting up Baja Bugs and running off road. Any thoughts?

In racing trim the Ghia is very neutral with a slight tendency to oversteer. It handles awesome... I ran a Subaru WRX last year (2nd place in season) and the Ghia is faster. I can drift the corners and the rotation is great. Perhaps I should be happy and drive.

I did put in a kaufer style brace a week ago but have not race yet. It does feel a lot different with the brace in.

Again, thanks for the post. This forum is great.

Theo

Theo, what exactly is the difference in feeling? I am building my '71 super and have a csp brace to install but would like to have an idea of what it will do. I have not driven my car in almost three years and have not even had a chance to drive another bug since mine tok a brake. I can't remember what my car even feels like. I already have sway bars on the front and rear and will at the Kafer brace soon.

Would you give some before/after info?

Thanks, Bert

Burt,

My 69 Ghia was converted from an auto. I don't know if this makes a difference but the frame horns flex a lot without the brace. They flexed so much I have inspected for cracks more than once.

Without the brace I could feel the trans move under acceleration and hard cornering. It feels like the rear of the car has slop in it. It handles great but is not precise. I could not drop the clutch without dangerous wheel hop.

With the brace the rear feels solid. I can feel the bumps more and there is no (or very little) slop. I can drop the clutch without much wheel hop. It does have some. When cornering hard the rear feels 'blocky' It's like the rear end wants to go strait and it has some jerkiness when turning. I have not had the rear aligned so this may contribute to the feeling. I have not raced since installing the brace but I worry I may have lost some rear traction by stiffening the rear.

Theo

EASY RIDER wrote:Theo, what exactly is the difference in feeling? I am building my '71 super and have a csp brace to install but would like to have an idea of what it will do. I have not driven my car in almost three years and have not even had a chance to drive another bug since mine tok a brake. I can't remember what my car even feels like. I already have sway bars on the front and rear and will at the Kafer brace soon.

Would you give some before/after info?

Thanks, Bert

just another voice on theo's original tire size question.

i think your right on the money theo.

for peak roadholding wider rear tires (and rims) is the way to go for a tail heavy car. if you keep tires all the same, your rear tires have more load in the corner and run larger slip angles, and will limit you. look to the 911 crowd for a cue: 10-18% wider in the rear. i run 205/55 on 6" fronts, and 225/50 7" rears.