Here's a question that for no particular reason I ask...other than I want to know! I'm sure this board has plenty of suspension guru's...

Question: What is the reason for a smaller size front tire/wheel than the rear? Particularly on race cars? On my street 911, or even race 911's, it's usually real big meats in the back and much smaller in the front, even with fiberglass fenders that can house larger tires up front. On my race only 911, I run the same size slicks all the way around, as do many 914's.

Most track cars seem to run bigger tires in the back, front engine, mid-engine, or rear engine.

Why?

I look forward to the education!

Thanks,

Bill P.

P.S. We were sitting around last night wrenching on the race car and this came up....nobody had a decent answer...

ooops...typo...my race only car is a 914....

cause you have rear wheel drive.

well, on the 911 and 914 anyways. the more meat in the back, the better the chance of getting all that HP on the road instead of into smoke from spinning tires ...

Andy

2 general reasons you might want big tires on the rear: weight in back and/or lotsa power.

911's have their weight biased toward the rear, so they need to have more tire in the rear to keep the back end from coming around in the turns.

Any car with lots of power (and rear wheel drive) will want larger rear tires in the rear to be able to transmit the power to the ground without breaking the rear end loose.

A front engine/FWD car will not need bigger tires in the rear. A front engine/RWD lower power car will not need bigger tires in the rear.

Mid-engine cars are well balanced to start with, so generally, the same size tires all around will keep the car balanced UNLESS you have enough horsepower to warrant larger tires in the rear.

This is all very very general, and there are always lots of exceptions.

Demick
'74 2.0

1) You want your tires to have the largest contact patch possible.
2) Steering effort increases with the size of the front tires.
3) Read This

if you look at brad's tube car, it has the same size tires all around but a wider track in the front!
makes for very good handling in those tight corners.
as long as you have enough meat in the back to keep the tires from spinning ...

Andy

What I'm facing is the possibility of my car being reclassified within th PCA. Our local chapter (Golden Gate Region) is looking at redoing the classes based on a points system. I had built my car to the upper end of the current classes and now may be moved up next year based on the new points system...don't know yet.

My car will stick pretty well with the 15x7 Hoosier slicks all around (per my class rules), but the next class up would allow bigger meats. I may do it anyway. My car has a 2.7 litre 6 in it with all the bells and whistles, putting out about 230hp.

I can definitely tak advantage of more rubber in the rear as I have no problem spinning the back wheels with the tap of my foot at an Auto-x...at TT's I try to be a little smoother....

I may get a set of bigger rear tires to try and practice on to see if I like 'em...

I was just wondering if larger tires in back would upset the car....

thanks,

Bill P.

Just remember that it is basically a balancing act between front and rear traction. There are tradeoffs with any change you make. Add more rear traction, and you'll push more. You can compensate for the push with other methods besides tire size like tire pressure, sway bar, and shock adjustments. Of course, if you're already loose, then increasing rear traciton will help. In my opinion, it is more important to have a balanced car (in terms of traction) than absoulute grip at either end.

If the rules are changing, then you really should wait to find out what the new rules are before making changes. I don't know anything about the new rules besides the idea of making it a points based system, but I'd hate to make a change like that only to find out that the rule changes are different than what I expected.

Demick

Hi Demick,

Thanks for the input. I'm just doing my homework as we get more info on the possible rule changes. I have access to large rims and tires and am trying to figure out which tire sizes to buy for testing purposes. I'm standing pat for the GGR series I'm running, but lately I show up at other region's events and run different suspension settings to conduct car testing. I am now about to try different tire and wheel sizes.

Hopefully I won't be bumped....I'll have to really lighten the car drastically to remain competitive in the next class up....

Bill P.

Bill,

I think you mis understand the logic of classifying cars by points.

There will be no "next class higher" by the current definition, if you want to stay with the tires you have, the points you tally will be associated to the tires you have.

New, wider tires, might get you more points, reducing weight might also get you more points.
The key is to see if you could have wider tires than you already have, or lower weight than you already have, without tallying more points than you would with your exisiting weight and tires......
make sense?

It's too soon to know where the lines of demarkation would be for various changes to your car.

I can assure you there will be a great deal of mis-information about this topic, especially from those with cars who dont want to admit to all their current (legal, marginal or illegal) modifications - as is necessary for any points system.

I encourage you to consider the points system as a positive potential change, I bet you could make some interesting changes to your car that are forbidden for your class by the exisiting class rules - but would be entirely possible with a points based system.

Regards

Hayden PTBT

I like to believe the "track" width between the left and right tires makes a difference in traction. If you had 205's all around, but the rears are father apart than the fronts, you probably would have more rear traction. On my CRX, I have tried weird stuff like running 225's in the front, and 205's in the rear. It gave the car some ultimate steering response. But it also felt a bit sluggish when accelerating too.

Hi Hayden,

Thanks for helping me understand the points based system. I've been looking at other regions that use the system and I am personally okay with it. I actually don't look at it as a negative, but rather overdue considering the years that have gone by since the last review of the Golden Gate Region's class structures.

The reason I am testing so much in other regions is because I have a blast doing it! I like screwing around with the car and not only challenging myself with learning and improving my driving skills, but also my car setup knowledge. It's thrilling to me to make an adjustment, feel a difference, and ultimately improve my time with the adjustment.

I'm hoping I can run a wider wheel/tire combo without adding more points. Given the HP I am running, I think my car is faster with bigger rear tires from the limited testing I've done (I've borrowed tire/wheels from a friend). I also would like to lighten the car after the season if it makes sense in the scheme of things.

Basically, I'm just learning and preparing as quickly and in the best way I know how -- doing it. And although it's all for fun, why else do it, I do want to chase you for TTOD sometime in the future

The whole game plan is to be as fast as I can given the resources I have (both mechanical and mental). The mechanical part is coming along fine...it seems the mental part is the one lacking!

As this is the first year my wife and I have auto-x and TT'd, we've been sorting the car all year. We now are at the point where we have a baseling we can compare changes to. Hopefully the proposed changes can be implemented early in the off-season, allowing ample time for car preparation during the winter.

That's for all your help!

Bill P.

I meant "Thanks for all your help!"

What time is it anyway? what? midnight? I've got to get some sleep.....

Bill,

No Worries, I think it is important to share clear information about topics like this, especially to combat the inevitable selfishly motivated mis information that will begin to circulate.

If GGR members can craft a good points system and it is accepted via the appropriate mechanisms, I think it will be refreshing for all car owners.

With my own car being relatively unmodified, I am keen to see if there is anything we can remove that is adding points that I believe are disparative to the performance gain.

Points systems are cool for allowing you to choose your points bracket, never will you have to build your car up to the modification ceiling allowed for your class.
Now you can choose how to accumualte points in a wide variety of ways. I expect to have different build configs for my car that equal the same points, but have advantages for different styles of events - to me that would be part of the fun. It might be a simple as wheels and tires, or more extreme like alternate decklid / spoiler config.

You are doing a good job with your progress and having the opportunity to test is always going to generate information.

Regards,

Hayden PTBT