edub

In the 718 GT4 Clubsport technical manual there is a paragraph on acceptable tire sizes so as to not disrupt PSM operation.

"As the PSM is only tailored to specific tyre circumferences on the front and rear axle,
a learning algorithm has been implemented that detects and compensates for the deviations
in wheel circumference on one axle or on individual wheels under certain conditions.
This is done to guarantee the best possible performance. The learning thresholds of this
algorithm are a deviation of maximum +/- 6% from the tyre circumference entered in the ICD
(see Determining wheel circumferences for ICD on page 411).
The tyre circumferences of Michelin (Slick/Wet) wheels are implemented in the ICD on
delivery (see Tyres in as-delivered condition on page 358).
• Front axle: 25/64–18 (rolling circumference Ustat = 1990 mm)
• Rear axle: 27/68–18 (rolling circumference Ustat = 2200 mm)
Before using tyres with a new tyre specification, measure the tyre circumference very
precisely and store it in the ICD as "DRY" (slick tyre) or "WET" (rain tyre)."
* ICD is "Intelligent Color Display" a native component of Clubsport cars


Is there information on what tire size deviation is acceptable to a standard car? If you exceed that deviation what is the impact?

was thinking about running a size which apparently is 5.6% (265/35/18) smaller than the stock 245/30/20. And if I run those 18" to the cords, the diameter would be 6.4% smaller than stock 20" tire.

Thanks!

enduro

I've always operated with the assumption that the car has no idea the actual tire raidus, just the relative radii (unless it consumes a GPS channel), therefore for TCS and ABS purposes, I pay attention to the difference/ratio of the front to the rear (vs what OEM ratio would be). Would be interested to learn otherwise. Additionally, I use the revs per mile published by tire companies to back out the effective radius, not the nominal size.

edub

I suspect the car can identify the actual tire radius per wheel - the ABS sensors indicate wheel speed relative to engine crank speed, output shaft and selected gear. We know the rev match function is robust enough to operate properly despite gearing changes, and you can see reported yaw and wheel slip are measured in the porsche track app.

That said, I can't imagine why the PSM system would care if you run smaller wheels or not, given it can measure everything independently.

enduro

None of those parameters can be used to calculate tire radius, just relative radaii :-)

edub

Really? I was thinking this, what did I miss:

Known:
Gearing
Engine and output shaft speeds
Wheel speed
Baseline speeds of gearing, engine and wheels on stock tires.

Then you just have to do the maths between the baseline and whatever you see your wheel speeds at.

I'm not saying it's being done because I don't know that, but the data is all present to make a conclusion.

Am I wrong?

enduro

My understanding is wheel speed if, given in mph or kph, as measured by the abs sensors, is just an angular speed * an assumed radius. They really only know angular speed. Only way I can think of that the car can know speed without assuming radius would require using a GPS feed or some kind of radar. Which I do volunteer might be the case for some or many modern cars, but I'm assuming not for my GT4 as my speedo has no idea my tires are smaller than OEM radius.

enduro

Now clubsport may have integrated this telemetry so maybe it does know. For my GT4, just checked my wheel speeds (OBDII channel) vs my AIM GPS from a lap. Actually fun watching how much of an effect slip angle has, but FWIW, it thinks my front wheel speeds are around GPS speed when things are settled in a straight, but rear wheel speeds are on average quite a bit faster than GPS speeds (but get similar under braking) implying it's guessing the wrong radius (guess is larger than actual) for my rear tires.

beez

The generally accepted variance in tire diameter (or circumference) on Porsche street cars, including GT street cars, is ±3% from the original OEM tires as delivered from the factory. This is also the variance front-to-back. I've found if you stay within this variance with the tires themselves, you'll usually be within that window front-to-back. The ABS system and wheel sensors are what measure this variance and the system is programmed to accept a ±3% difference. GPS is not involved.

Exceeding this window can bring problems with early or no ABS intervention, unexpected ABS and/or PSM intervention, and even "ice pedal" where you press the brake pedal, and there's no braking. I use tiresize.com to compare prospective tire sizes to the original OEM measurements.

Porsche factory race cars and the clubsport versions of the Caymans use the "motorsport" ABS system made by Bosch, which can be programmed for different tire sizes, and different levels of intervention.

edub

I would love to see a source citation if you know of one. I have also seen the 3% spoken about but no source mentioned.

https://www.wheel-size.com/calc/?whe...cl=46mm&sr=0mm is pretty cool as well and adds a bit more visualization than tiresize.com

Thanks

enduro

@beez out of spec front to rear radius ratio makes sense, but if GPS is not involved, how does it know the tires' radius? ABS sensor is just a hall effect counter. All that'll know is angular speed, acceleration, etc.

Only thing I can think of is that for a given steering angle, lateral acceleration may be different than expected or for a given angular speed for the wheel sensors, and longitudinal acceleration may be different than expected for a given angular acceleration; or maybe some other derivatives|gradients. I expect the TCS system will have accelerometers.

I think I just answered my own question, but if someone has detailed knowledge of the systems and algorithms, genuinely would like to understand why and how absolute radius affects its operation such that a 4% delta from spec can cause problems, assuming front to rear radius ratio is in spec.

'Ice Pedal' is one very insidious thing, seen the effects at the track (not on my car) and they aren't pretty.

slivel

I haven't seen a source either but out of curiosity tried to do an analysis based on tire diameters. According to Tire Rack, specs for a Michelin PS4S in a 245/35/20 the diameter is 26.8 inches and sports 9/32 of tread when new. If you run the tire to the wear bars (usually 3/32), then you will have decreased the diameter to about 26.425 inches or about 98.6% of new. So the 3% number sounds reasonable.

enduro

FWIW, a quick spot check of 234 45 20 tires on tire rack shows a >4.5% range in effective radius.

tire rev/mile rad / min rad (effective)
Pilot Sport 4S - Size: 245/35ZR20 k1 f 777 1.005
P Zero Run Flat (PZ4) - Size: 245/35R20 775 1.008
Eagle F1 Asymmetric 3 - Size: 245/35ZR20-na0 p 781 1.000
Ventus V12 evo2 - Size: 245/35ZR20 754 1.036
Ecsta PS91 - Size: 245/35ZR20 746 1.047
ContiSportContact 3 - Size: 245/35ZR20 781 1.000
ADVAN Sport A/S+ - Size: 245/35R20 776 1.006
Pilot Sport Cup 2 - Size: 245/35ZR20 777 1.005
Sport Maxx Race 2 - Size: 245/35ZR20 781 1.000
Pilot Sport Cup 2 R - Size: 245/35ZR20 777 1.005

jwr9152

Evidently it can be added with PIWIS. Looks like clubsport mode can be enabled and you can change the circumfrance of your tires.

https://rennlist.com/forums/718-gts-...l#post18485119

edub

Wow thats awesome, thanks for the heads up