For STF class there is no restrictions for offset on wheels or spacers.

 

Yeah your right I did get mixed up :P Area is the same as long as pressure and assuming the stiffness of the tread is constant. No need to dwell on that and I've read a couple threads about the effects of thin vs wide and since and set myself straight.

I can see why for learning narrower is better as you slide rather then break loose suddenly all together.

I also read a tirerack review someone made with the Kumos on the fit and found they only lasted 25k on their car. Although the handling was good just the treadlife was no better then the stock dunlops.

Today I started doing some chalk testing. I have been incrementally increasing the pressure by 2psi on the front 2 tires from 33lbs up to 39 so far and found that the rolling isn't that bad that I can't use them. Still at least 1 more round to go with the fronts then I will mess with the backs, but they should end up being a couple of psi lower. It seems like the ride has been stiffening up some as I have increased the pressure. Seems like you were right about the pressure making a difference.

This I think concludes this thread for the time being as I have now decided to stick with my current dd tires for the time being and try to improve my technique. At the next autocross I will definitely chalk the tires for at least one of my runs to check for any rolling issues.

I will throughout the season ask around about a used set 15x7 rpf1s.
Thanks for the all the advice.

 

Its easier if you define the forces between tire and ground as thrust forward, friction laterally, and weight on the tire the normal force. Thrust is of course expressed as the force from torque, friction is a ratio of friction to normal force. When those forces are factored the acceleration, cornering, and work are defined.
While many try to say that the pressure exerted by a tire is dependent on normal forces devided by the air pressure, thats arguable due to the carcass is not a thin-walled flexible balloon but a semi rigic structure that does not change uniformly with changes in pressure. Thats why the tire width can add surface contact area, and dependent on friction can change the surface contact pressure without changing air pressure, at all - and why up to a limit adding width is advantageous. Worse, friction is entirely dependent on the nature of the surface. If the solutution to friction, normal and thrust vectors were that easy to define we'd all be Sebastion Vettel - last year, not this year. Power counts, thrust.
have at it.
I have guys here who follow the TireRack champs and they unanimously say Direzza D11, last yearstar, and BFG rivals are the ones to run. if you can't run Hoosiers.
And no matter if wheel width and offset aren't governed, limit offset changes to those that keep track width wuithin one-half inch, keep the overall tire diameter within one-half inch of OEM, including if you can change wheel diameters to enhance sidewall height reduction for better accuracy in steering.

 

Your point is that the area gain wasn't nonexistent going wider. Also that very few things in physics/ dynamics are explained simply linear, and can only be explained with integration. I agree to that.

I was trying to keep it "G" rated and not label force vectors, not everyone understands which leads the thread off track (like it just has, lol).

The normal over pressure is a general rule of thumb. If you raise the limits to +/- infinite of course the contact area patch will be increased. But the limit is 15mm wider... The contact patch area gain is consider negligible at that point. Track-days and auto-xing orbit around "rules of thumbs". Micro-monitoring and interpolating real time data is not practical, like in F1... and honestly boring, unless you're an engineer. Are you a ME?

 

Over thinking all this. Your driving a Fit. Buy Direzzas and be done.

 

Stock class with Stock steelies? Just get 195/55/15 Yoko S.Drives. Take the hubcaps off, go drive the car.

Steelies are only 17lbs, it's not that big of a deal.

Honestly it's just autocross, not F1. No offense to the coneheads in here.


Edit:

Most tires also have a sidewall indicator you can use if you don't happen to have chalk. Not sure if the stock tires have them, but most do?

This is from my run last night pushing a Miata LOL

 

Typical Engineers. lol. We're too inquisitive.

 

You are right, but if you roll the tire over too far, when you add air to compensate you will need chalk to reflect the new tire roll. That triangle is the max roll that the tire should see. What were you running for psi?

 

The normal force changes with the load on the tire in corners compareed to straught ahead and with the speed andb angle of the tire to straight ahead.

 

LOL just giving you guys a hard time because of that, I work with engineers all day long I know how you are

That's true, but at least you'll know after your first run if you're way off base and can add air, I mean if you don't have chalk anyway. You could also scrub it off a little bit after a run.

For that pic I am running 36psi on 205/55/15 S.drive on 6.5" wheels, canyon run, fairly technical, several 2nd gear turns at grip limit.

 

Worse, not only ME but ChE and QE with more than 50 years in racing in NASCAR, SCCA and IMSA as driver, race engineer and team owner.
Your references to integration are not applicable. The mathsolution is not possible simply because the sructure and compounds used in racing are not anywhere near absolute. any calculation would be an attempt on variablres not quantifiable.
In truth a tire is not a infinitely variable struct=ure as most ideas of tire pressure directly related to tire contact surface presume. as most tire evaluations at the track will demonstrate, unless the tire pressure is changed by 5 psig or so you will not see changes in contact are and contact pressure. When yoiu change air pressure by a small amount the area doesn;'t change directly, only the contact pressure changes. You don't get that by inegration, onlyby measurement, most often lap times.

To go back to tire recommendations, check the TireRack nationals and see what the winners wear. Right now, based on last years and this years SCCA solo results the Goodyear rival and Dunlopr Direzzas both star D11 and D11's are tires of choice. If your ruls allow any tire then of courseHoosiers are the choice. If you have access refer to Grassroots Motorsports for good tire evaluations in autocrossing. Also check TireRack test results for lap times; forget the reviews, stopwatches are the rule of law.
Yes, that mans tires mounted for a./c and another set for street. If you can have offsets other than OEM add either less offset wheels or spacers but not over 1/4", 6mmm. And if you really want to get seroious use the lightest wheel you can find to fot your tire and offset.
good points made about using chalk to know when the tire wear contact 'rolls'ver the edge but if you stichk with pressures over 40 psig you'll be
OK. Pressures as much as 50 psi cold are common in a/xing due to the short time 'on track'.

 

You're correct, the contact area under loading and unloading conditions.

The topic is about increases contact patch over the whole range of operation (loaded and unloaded, in a turn and on a straight). Which are the three "rules of thumb" of autoxing: Less PSI, more vehicle weight or taller tires. Autoxing is as basic as racing gets.

 

That's a hell of a resume.

 

mahout has a good point and one that i've always known and why I sometimes shun the technical jargon when it comes to tires and pressure.

It's very difficult to plug the numbers in, as he said, because tire compound, sidewall structure, belt structure, air temperature, track temperature, etc etc make it almost impossible to do it "the easy way"

Just get out there and drive it. If you don't like it, play with it until you do like it. I haven't done much to the Fit, but on all my other cars it was a morphing process just fixing it how I liked it, and when I was done, it was perfect. For me anyway.

 

Good question; here's my answer to bench race:
Vehicle weight is better off less if possible, not so much for tires but for minimizing opposion to acceleration and weight transfer; weight transfer is a topic to investigagte. Can you minimize weight transfer>? Can you add OR REDUCE antisay bars. On a Fit it is very advantageous to disconnect the front antisway bar to 1) improve turn in and 2) let the camber increase friom OEM. The Fit front camber decresaes with compression.
2. The issue on psig is a matter of finding out what works for your car.
It generally takes an afternoon on an empty parking lot on marked corners and stopwatches. Generally speaking 40 psig front is a good place to start with perhaps 36 rear3. tire selection is determined by again those stopwatches. Using tirerack and GRM to narrow your choices for both tires and wheels is a good start followed by checking the tirerack solo championships is a damn good statr. Lighter wheels count more in racing than a/x though penalizing with heavy tires is not useful. Most steel wheels weigh about 20 lb best and best alloys are about 10l bfor Fits.
One aspect not often considered in tire selection is the weight of the tire. A 18 lb tire is advantageous over a 20 lb simply because as the weight increases its harder for the engine to turn over more 8 more pounds of weight some 12 inches (1 foot) in radius means a loss of 8 pounds of torgue and the Fit isn't graced with an abundence of torque to start with. That doesn't mean you take lighter over lap times but if two competing tires have different weights choose the lighter. Those lap times from tirerack or GRM are not likely to have compared the exact isuize you want for your FIT but compounds/lap times are a good base line. And when you go wider - and likely heavier - go a tad less diameter to reduce the increase in loss of torque by reducing that radius. Even a quarter-inchg makes a difference but pretty much an inch won't help. Use tirreacks specs on tires to ge weight and diameter info both on wheels and tires. If you get a choice to select both wheels and tires choose the lesser weight combinbations on the acknowledged better tires..
And thats our 3 rules of autocross tires I shoulld also add that 1/4" spacers of wheels with 6 mm less offsetr will help camber but not as much as disconnecting the front bar.
All of this must be factored against your local rules. From my experience that variation is huge unless its SCCA.
Now know why its best to start racing in a/x, its less but by no means lesser complicated and is a doorway to understanding the implications of technical importance in any endeavor, racing perhaps more than others. At least in my career I found most mechanical and chemicl engineering a bit more uncertain that racing, especially when the budget is a whole lot tighter.
Good luck.

 

Mmm I find one statement at odds with the other.

Removing the front bar just lessens the effective spring rate in the front during cornering, allowing the rear to rotate more, but I don't see how adding roll will help the camber curve at all on a macstrut car?

 

The Fit has a macpherson strut. A macpherson strut increases camber when loaded (up to a point). Can you explain what you mean?

 

The softer the front suspension the more the car will want to turn in. That is correct. But makes for a unbalanced drive removing the sway bar. Id be interested to see if any national winners in the STF ran with out a front bar and on what cars. There are other ways to introduce turn in; toe out in front, weight distribution rearward, tire pressure higher in front, tire width combos (front to rear)... Sway bar removal is the budget friendly way.

Sway bars in a turn attempt to keep all four in contact with the Ground. When you remove the front sway; under a hard say right turn the front left tire contact increase, while the right front tire decreases, making the front left tire a "pivot point".

 

i heard the new bridgestone re11a is probably the best choice for autocrossing. has anyone tried it here and can share some experience on it?

 

Yeah turn in and pivot points and all that are great, but at the loss of total grip unless you've got some serious tires. You're battling loss of grip from a crappy camber curve on both front tires as well as losing effectiveness of one of your steering tires. There's way more effective ways to get better turn in than unbolting the front bar. That's an on-track end run last resort that's simple to do in a pinch when you've got no other choice, not an end-all answer IMO.

I especially don't see the point in STF where you can run coilovers and play with spring rates, dampers, and tire pressures rather than unbolting bars. It's not stock class. In addition, you can't use r-comps, just club rubber, so you have to battle that loss of grip I was talking about previously.

This is all hypothetical of course