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Discussion Starter #1
I'm curious as to what tires and psi everyone is running. I recently got a set of new tires and am starting to play around with tire pressure. With my setup right now I'm facing some mid corner exit understeer. To combat understeer, should the rear tire pressure be over or under inflated?

My current setup for my 2014 Mazda 3 Hatch:
- Federal RS-RR 235/40/18 F & R
- 35psi front and rear hot (too high?)

Alignment setting:
- 0 toe, -3F -2R
 

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*The Electrician*
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You need to learn how to chalk your tires so you can learn where the edge of your grip is between runs. Also you can't just set it and forget it, as temps go up so will your psi. This past weekend at autocross I had to bleed 2-3psi from both front tires and 1-2psi in the rear 2 between each run to keep them in the optimum window of grip. My fronts are set to 39psi and my rear to 35psi, Firestone Firehawk Indy 500's for tires. Sounds like you need a rear swaybar upgrade(if you don't already) to help combat that understeer because I never felt that I was plowing unless I made a mistake on entry which I correct for with more braking when such mistakes happen.
 

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Discussion Starter #5
You need to learn how to chalk your tires so you can learn where the edge of your grip is between runs. Also you can't just set it and forget it, as temps go up so will your psi. This past weekend at autocross I had to bleed 2-3psi from both front tires and 1-2psi in the rear 2 between each run to keep them in the optimum window of grip. My fronts are set to 39psi and my rear to 35psi, Firestone Firehawk Indy 500's for tires. Sounds like you need a rear swaybar upgrade(if you don't already) to help combat that understeer because I never felt that I was plowing unless I made a mistake on entry which I correct for with more braking when such mistakes happen.
That's currently my method for finding what tire pressure to go with for the front. My tires seem to like 33psi. I'm gonna experiment with low pressures in the back.

I currently have a progress rsb but am looking to get the stiffer JBR one or up my spring rate.
 

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*The Electrician*
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You need to learn how to chalk your tires so you can learn where the edge of your grip is between runs. Also you can't just set it and forget it, as temps go up so will your psi. This past weekend at autocross I had to bleed 2-3psi from both front tires and 1-2psi in the rear 2 between each run to keep them in the optimum window of grip. My fronts are set to 39psi and my rear to 35psi, Firestone Firehawk Indy 500's for tires. Sounds like you need a rear swaybar upgrade(if you don't already) to help combat that understeer because I never felt that I was plowing unless I made a mistake on entry which I correct for with more braking when such mistakes happen.
That's currently my method for finding what tire pressure to go with for the front. My tires seem to like 33psi. I'm gonna experiment with low pressures in the back.

I currently have a progress rsb but am looking to get the stiffer JBR one or up my spring rate.
33psi seems kinda low, sounds like you could be pushing harder. I don't think a stiffer swaybar is the answer, if your progress rear swaybar is set to full stiffness you should not be getting much understeer. Some better tires would definitely help too. Last season I had understeer like a pig, even with my progress rear swaybar. The answer was definitely better tires because Im getting great rotation now and very little plowing. The difference is day and night. Im not getting oversteer, just some very light plowing if I get my line wrong but otherwise its pretty neutral which is a very weird feeling for a FWD, but lol the new Civic Type-R's get oversteer so.....lol yeah. Also, we have 2 new civic Type-R's in our club, one has stuffed 285 wide tires up front, the other guy, 295 up front, my gawd wtf lol, superman front grip.
 

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New member.

Not new to autocrossing (25 years)... Instructor with the SCCA Starting Line School and the Evolution Performance Driving School

2018 Mazda 3 Sport Hatch (6MT)

SCCA HS

Koni Sports
Zero toe F & R

225/50/16 RE-71's on 16x6.5" +45mm Kia Optima LX wheels (scored a new set for just $160 in all! LOL!)

F: 34-36 cold, depending on the course
R: 34-37 cold, depending on the course

I'm not a big fan of big rear sway bars in GS and HS cars because we can't change spring rates to compensate for all that weight transfer being thrown forward.
 

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225/50/16 RE-71's on 16x6.5" +45mm Kia Optima LX wheels (scored a new set for just $160 in all! LOL!).......


I'm not a big fan of big rear sway bars in GS and HS cars because we can't change spring rates to compensate for all that weight transfer being thrown forward.
????
Sway bars cause a forward weight transfer?
A bigger rear sway bar makes a significant difference in how this car handles. It has a lot of under steer designed in for liability reasons. Changing the rear bar tightens up the rear end so it doesn't push as much, allowing the front tires to grip better in a turn. Have you tried one in your car?
If you are going all out on the track you should install a front bar also.

225s might be a bit wide for a 6.5" rim? Seems like those RE-71s would be slopping around a bit seeing as how the tread face is quite a bit wider than the rim width. They have a ton of grip when used correctly but it kind of defeats the reasoning of having purpose built tires when they just squish around and can't do what they were intended to. If performance is the goal keeping the tread face the same size or slightly narrower than the rim width is what you really need........
 

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????
Sway bars cause a forward weight transfer?
A bigger rear sway bar makes a significant difference in how this car handles. It has a lot of under steer designed in for liability reasons. Changing the rear bar tightens up the rear end so it doesn't push as much, allowing the front tires to grip better in a turn. Have you tried one in your car?
If you are going all out on the track you should install a front bar also.

225s might be a bit wide for a 6.5" rim? Seems like those RE-71s would be slopping around a bit seeing as how the tread face is quite a bit wider than the rim width. They have a ton of grip when used correctly but it kind of defeats the reasoning of having purpose built tires when they just squish around and can't do what they were intended to. If performance is the goal keeping the tread face the same size or slightly narrower than the rim width is what you really need........
When entering into a corner, weight transfers forward and diagonally. Say it's a right hand turn. As you turn right, weight unloads off the RR and loads up the LF. Vice versa for left hand turns. This is why people who use coilovers for the proper reasons (motorsports vs stance) will cornerweight their cars with the "ideal goal" being a 50% crossweight between LF and RR relative to the RF and LR. This will give consistent handling characteristics regardless of left or right turns... In SCCA Street classes, especially HS, we have no such luxury as we can only change one swaybar and dampers are limited to two (2) adjustments: compression and rebound (really high-end stuff can allow for individual adjustment for low and high speed compression and rebound). For reference an off-the-shelf Koni Sport allows for rebound only.

So we have to resort to other measures to make the car work.

But for me, a big rear swaybar isn't the silver bullet.

Big rear swaybars help with turn-in because they LOOSEN that particular axle. Tightening up the car (as you stated) would actually increase understeer. But the more rapid rate at which it will turn in can also overload the outside front corner dealing with all that weight transferring onto it.

When I've run in classes that allow me to run pretty much whatever I want, I'll have shock valving, spring rates, bushing types, and swaybars that capitalize everything.

As for the tires, the 225/50/16 RE-71R has a rim width rage starting at 6.5", so the 225 is fine there. Check TireRack.com for proof. Squishy? Not at all.

My 2.0L 155hp does very well in HS considering it's grossly outgunned as the 8th Gen Civic Si (200hp and a helical LSD) and the Hyundai [Kia Elantra] GT [Forte5] (both have 201hp and tons of torque for the class)... The driver is largely responsible for that. ;)
 

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Maybe you should go back to square one and check a few things.......


A larger rear bar increases the virtual combined spring rate, so it "tightens" the suspension. In a turn, the larger bar limits weight transfer to the outside tire and so limits grip, reducing the tendency to push, or understeer. This does make the rear end feel looser. Corner weighting is entirely different and has no real bearing on the sway bars and vice versa, other than you can cheat a bit and preload a bar to favor turning in one direction vs the other.
If the front outside tire is being overloaded you can tweak the suspension further to make the rear end a bit looser, or you can add a larger front bar if allowed. There is a line you don't want to cross to avoid snap over steer.....

I know very well how tires fit, what they fit and why. I also know what tires to use to get the best grip from a particular rim. Those numbers published on TireRack are industry standards for safe use, nothing more. There is no "proof" of anything there. If you were to actually read the specs and understand them, you'd see than your tire, a 225/50/16, has a section width of 9.2". Now subtract .2" from that because you are going down .5" from the measured 7" used for that tire when calculating industry standards, and you end up with a tire that is 9" wide and has a 7.6" tread face on a 6.5" wide rim. That tire is going to roll off the tread face onto the shoulder in a hard corner and you will lose grip because the sidewall will not provide sufficient support.
Here is a pic I found that shows the problem of using a rim that is too narrow for the tire. I'm sure you can figure out whats happening here....and the result



Its also going to have kind of sluggish turn in also compared to a proper width tire. The rim will turn, the sidewalls will flex, only then will the whole tire will catch up. Squissshhhhhh....
Most guys with smaller cars use 185s or 195 on a 6.5" rim for best results. 225s are for street cruisers who don't want to feel the bumps.......
 

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I agree, swaybars have no real semblance on cornerweighting a car because the bars should be disconnected (even if it's just one endlink) so preload doesn't play into things.

One of the other reasons Street (old Stock) class autocrossers run a wider tire on a narrower wheel than one would like to see is because of the OEM camber limitations cars such as the Mazda 3 feature. The wide section width relative to the tread width helps in these instances. Ideal? Not at all. But, "Them's the rules" as the old saying goes.
 

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I'm curious as to what tires and psi everyone is running. I recently got a set of new tires and am starting to play around with tire pressure. With my setup right now I'm facing some mid corner exit understeer. To combat understeer, should the rear tire pressure be over or under inflated?

My current setup for my 2014 Mazda 3 Hatch:
  • Federal RS-RR 235/40/18 F & R
  • 35psi front and rear hot (too high?)
Alignment setting:
- 0 toe, -3F -2R
This is cornering on street or track? In either case you are likely setting the pressure too high. Definitely if you are talking track or autocross.

If you are street driving it is going to be hard to find an ideal tire pressure because when you cruise to work your PSI won't increase much. But when you do spirited driving it will increase. So 35psi is fine for commuting but probably goes to 40+ with spirited driving and definitely over 40 if autocross or track. The only way to know is check your pressure. Buy a quality pressure gauge (I like Longacre brand). Pull over when your tires are hot and check the actual pressure.

You want to aim for a cold pressure that when heated up is the pressure you want hot, which can only be done by checking hot pressures and using that information to adjust cold pressure.

On the street I just put 33psi cold and don't go crazy because I also have street tires.
Your tires are more performance oriented and you have good camber so your tires will heat up even more.

TLDR: get a tire pressure gauge and check your pressures hot, adjust your cold pressures to a spot where when the tires heat up you get the psi you like. 35psi cold is too much for your driving it seems.
 
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