275/295 Toyo RR Diameter Question

[DRLane]

Anyone see any issue from a vehicle dynamics perspective with the rear having a 1/2" smaller diameter than the front tires?

275/35/18 - 25.5"
295/30/18 - 25"

My thought was at the time to replace swapping to 305 at the rear and keeping the 275 on the front. Then the variance would be ~1" smaller front to the rear.

Or, suck it up and buy new 10.5 fronts and run 295 square.

[tsk94]

I've run this exact setup with 0 issues. It won't be problem.

[DRLane]

Thx! I'm 12 heat cycles in so too late now but was especially curious for the future.

[pbonsalb]

I am running 265/35/18 front and 285/30/18 rear on the street. Theoretically it could affect ABS or traction control, which are likely triggered by front/rear speed differences. The bigger the gap from the stock difference, the more likely.

[fsmtnbiker]

I suspect this will degrade ABS function. I already feel its not amazing with a square setup, and I feel the stability control kicking in all the time on the street at higher speeds. Dynamically, probably fine, but as far as the electronics go this car is really bad at sorting out different tire diameters vs the OEM sizes.

[DRLane]

Thanks gang, only running this RR setup on the track with stability control off. Haven't noticed any real braking degradation as I'm not trying to engage ABS at threshold.

[ThunderMoose]

Originally Posted by DRLane

Thanks gang, only running this RR setup on the track with stability control off. Haven't noticed any real braking degradation as I'm not trying to engage ABS at threshold.

I may have asked you this before but do you have datalogger? You'd be surprised how much ABS kicks in under hard braking.

[DRLane]

Originally Posted by ThunderMoose

I may have asked you this before but do you have datalogger? You'd be surprised how much ABS kicks in under hard braking.

I do, Solo2 DL. What should I be looking for?

[pbonsalb]

Originally Posted by fsmtnbiker

I suspect this will degrade ABS function. I already feel its not amazing with a square setup, and I feel the stability control kicking in all the time on the street at higher speeds. Dynamically, probably fine, but as far as the electronics go this car is really bad at sorting out different tire diameters vs the OEM sizes.

Are you using stock or Euro spec traction control? I have Euro spec. I am not a coding expert, but wonder whether there are parameters that can be changed to address front/rear changes. With the Racelogic traction control on my E36, I specify the front/rear diameters, but that is a programmable aftermarket unit.

[ThunderMoose]

Originally Posted by DRLane

I do, Solo2 DL. What should I be looking for?

You should have a channel that indicates when ABS is triggered. Its binary. It's ASC REG on my E46 M3.

[DRLane]

Originally Posted by ThunderMoose

You should have a channel that indicates when ABS is triggered. Its binary. It's ASC REG on my E46 M3.

Doesn't show anything - The brake switch is clearly working but ABS trigger is 0. Solo is connected to the CAN.

[Richbot]

Chris - I think I know and have experienced the effect you're seeing - traction systems/stab control engaging when they see a rear wheel speed faster than front by more than x revs for any reason, I see it too on my car on a square setup going around higher speed corners a mid-corner bump can be enuogh for it to do a light intervention. But I think with a smaller rear diameter the inverse is going to be true, it's never ever going to see what it thinks of as "lockup" and ABD might start feeding more pressure to the rear too on a square setup, and pushing some brake to the rear might actually be helpful. Would be cool to log those brake pressures.

[rhyary]

Originally Posted by DRLane

Doesn't show anything - The brake switch is clearly working but ABS trigger is 0. Solo is connected to the CAN.

My ABS also shows 0 even when i can tell it is triggered at the end of the straight mostly on day 2 or 3 front tires.

So the AiM parameter is there, but it is it receiving data

[fsmtnbiker]

Originally Posted by Richbot

Chris - I think I know and have experienced the effect you're seeing - traction systems/stab control engaging when they see a rear wheel speed faster than front by more than x revs for any reason, I see it too on my car on a square setup going around higher speed corners a mid-corner bump can be enuogh for it to do a light intervention. But I think with a smaller rear diameter the inverse is going to be true, it's never ever going to see what it thinks of as "lockup" and ABD might start feeding more pressure to the rear too on a square setup, and pushing some brake to the rear might actually be helpful. Would be cool to log those brake pressures.

Unfortunately I think the problem will be two-fold:

TCS will be engaging constantly at higher speeds, because it sees higher rear wheel speeds than front. This is easy to fix, turn off the TCS.

ABS will see the front tires spinning slower than the rears under braking and dump front brake pressure (and add rear up to the limit of it's biasing capability). Overall I don't think it would be a net positive because the car will not have enough front brake pressure and the ABS intervention will last much longer than it should as the front wheel speeds never 'recover'.


I'm tempted to try running a slight stagger to get to the OEM diameter split to see if it allows better trail braking behavior. I find if I get into ABS at any time while I'm trail braking, this car is a real challenge to get to turn in smoothly. It just gets less predictable. One of these sets of tires maybe I'll try it. 295F/315R seems like the easiest possible fitment, but the 315s on a proper size wheel don't really actually fit and I don't want to bend sheet metal yet.

[Richbot]

I see what you're saying - there's probably also only so much hydraulic pressure to work with for a given brake application, so as ABD does its thang it's asking you, implicitly, (because the fronts still have to do the fixed-by-physics work of slowing the car) to keep stepping down harder on the brake right when you'd be wanting to bleed off pressure as the car pivots in

Interesting

And sortof shades in the reason why so many people might report the car being easier to drive on a staggered setup, beyond mere built-in tire stagger understeer, which I am still not convinced has to be a real thing with the right suspension tune on the car

I bet the ABS-on trigger is only above certain thresholds also, a quick flutter from one channel the ABS might never trigger the signal the AIM is logging

[rhyary]

Originally Posted by Richbot

I see what you're saying - there's probably also only so much hydraulic pressure to work with for a given brake application, so as ABD does its thang it's asking you, implicitly, (because the fronts still have to do the fixed-by-physics work of slowing the car) to keep stepping down harder on the brake right when you'd be wanting to bleed off pressure as the car pivots in

Interesting

And sortof shades in the reason why so many people might report the car being easier to drive on a staggered setup, beyond mere built-in tire stagger understeer, which I am still not convinced has to be a real thing with the right suspension tune on the car

I bet the ABS-on trigger is only above certain thresholds also, a quick flutter from one channel the ABS might never trigger the signal the AIM is logging

a front 275/30/19 rear 275/35/19 had a very nice feel to it. I am typically on 295/30/19 rear and I wonder if the larger diameter of the 275/35 was contributing to what I was feeling.

In the context of diameter discussion.

[dparm]

I don't buy the "changing tire sizes screws up the ABS". It's not like you've gone from 15s to 20s, we're talking very minor differences in overall height. The tire's diameter changes with speed, temperature, etc. anyway. The ABS computer is smart enough to compensate.

[pbonsalb]

I am not sure it is. I think the ABS computer measures wheel speeds and tells the pump how to modulate fluid pressure to prevent too much if a difference. My assumption is that it is programmed to stock tire diameters.

However, it may have a lot of flexibility. I don’t know. Could be that 5-10% speed differential is what it takes to represent impending lockup. But you would be using up that margin and thus be closer to activation by deviating too far from factory spec.

[dparm]

Originally Posted by pbonsalb

I am not sure it is. I think the ABS computer measures wheel speeds and tells the pump how to modulate fluid pressure to prevent too much if a difference. My assumption is that it is programmed to stock tire diameters.

However, it may have a lot of flexibility. I don’t know. Could be that 5-10% speed differential is what it takes to represent impending lockup. But you would be using up that margin and thus be closer to activation by deviating too far from factory spec.

There are several different tire sizes on the car from the factory (18", 19", and the 19" GTS). Is that programmed into the ABS computer anywhere?

[dparm]

The OEM 19" rear is 668mm tall (26.3"). A 295/30R18 is 634mm tall (25").

The OEM 19" front is 654mm tall (25.7"). A 275/35R18 tire is 650mm tall (25.6").


The front is effectively the same size so you're fine there. ABS on the front axle is more critical as this is where all of the weight shifts under braking AND it has to do the steering which is what ABS will help with under braking.

1.3" difference out back is not too extreme, IMO, and is what I see on a variety of track setups. There is front brake bias to begin with and those tires' contact patches will shrink substantially under hard braking. Just look at pictures of cars under hard braking and look at the amount of sidewall deflection. Heck, just look at the car when it's sitting still...the sidewall is going to compress from the car's weight anyway, it's not the full 1.3" height difference. I bet it's more like half of that. PLUS, everyone drops their cold pressures down.

The final point I'll make is that an RR is a very sticky tire and I wouldn't expect a ton of ABS engagement due to those being overwhelmed, but YMMV depending on temperatures, the track you're at, and the brake setup.

I would be willing to sacrifice a tad of ABS engagement in exchange for having more rear tire to help me get back to WOT earlier. You guys need to give the ABS programmers more credit. That system has to be able to cope with a variety of situations like over and underinflated tires, changing road conditions, old brake fluid, dirty lines, etc. This is not some rudimentary early 1990s ABS system we're talking about here.

[Richbot]

The car is designed around a square winter setup, but I do wonder if part of the "margin" being gone is by-design when they recommend that square setup, if you want to really minimize wheelspin/oopsies in the slick stuff you have the rears that much closer to the intervention threshold by default when you're on the OEM square winters

[dparm]

Originally Posted by Richbot

The car is designed around a square winter setup, but I do wonder if part of the "margin" being gone is by-design when they recommend that square setup, if you want to really minimize wheelspin/oopsies in the slick stuff you have the rears that much closer to the intervention threshold by default when you're on the OEM square winters

Narrower tires will dig into the snow better. I suspect that's one of the biggest reasons. The extra width out back isn't needed in inclement weather.