PSI Recommendations: Yokohama S-Drive 17"

NukleoN · 09-18-2009, 07:18 PM

Howdy,

I have the Yokohama S-Drive 17" tires, 195/40 front and 215/35 rear. The MAX PSI on the sidewall reads 50, which I know is just the max. However, I am wondering what people would set their PSI at?

I am currently 35 PSI front and 36 PSI rear (I always give the fronts slightly less on motorcycles but it might not matter for the smart car).

What is everyone running and how do you like it? I want to make sure I am not running the pressure too low.

Thanks.

SmartChip · 09-18-2009, 07:49 PM

Nuk, I'm running 17" Yoko's,195's front and 225's rear. Tire pressure is 36 lbs. front and 42 lbs. rear. If you roll over a bump, you want the tires to feel reasonably hard. If they feel mushy you need more pressure. Hope this helps. Cheerz, SmartChip

Old smart · 09-19-2009, 07:20 AM

The passion’s 155 fronts are 29 psi and the 175 rears are 36 psi. The pulse’s 175 fronts are 26 psi and the rear 195s are 32 psi. Information from the 450 BRABUS tire pressure label (on fuel filler door) has its 175mm fronts again at 26 psi and the 205mm rears at 33 psi. (Note, these are smart/BRABUS figures, not something picked out of the air). The curb weights of all three vehicles are within 99 lbs of each other, the 450 BRABUS being the heaviest.

As is evident, as tire tread width increases (and supported weight remains constant) there is a proportional decrease in the specified tire pressures – a factor of 0.9 x pressure in the case of the passion/pulse (e.g. 155mm/175mm = 0.9; 0.9 x 29 psi = 26 psi).

So, proceeding likewise for your 195 fronts, 155/195 = 0.8; 0.8 x 29 psi = 23 psi, and for the 205 rears, 175/205 = 0.85; 0.85 x 36 psi = 31 psi.

That is plenty of pressure for a car as light as a smart. The idea is to distribute the pressure of the tire evenly across the contact patch, from sidewall-to-sidewall. Too much pressure, the centers wear too fast; too little, the shoulders wear too fast; in either case, roadholding is diminished. In reality, camber affects the inner and outer sidewalls differently, and you work with an average of the two compared to the center. That is best done with a pyrometer.

When I campaigned a 1200 lb Caterham Seven “clubman” in SCCA, EMRA and NASA the 185s were at 18-20 psi, depending on the track and weather conditions. The contact pressures (evidenced by tread heat) were regulated using a pyrometer. A tire in street use need not be so fiddly.

**jwight** · 09-19-2009, 07:32 AM

Just pump 'em up to the max pressure shown on the sidewall!

Actually, don't do that (just in case somebody missed the sarcasm)

NukleoN · 09-19-2009, 08:14 AM

LOL jwight.

Great answers, thank you.

Had to edit my rear size, it's 215/35 not 205/35. I used the formula you came up with and came up with 29 PSI rear (23 PSI front is the same).

From what I understand, tire pressures are run lower on the track for more grip, but I guess if you're using a pyrometer you're always getting optimum grip as track and air temps change. Sounds like this formula gets me in the ballpark for the street. Thanks!

Old smart · 09-19-2009, 08:33 AM

Quote:

Originally Posted by NukleoN

LOL jwight.

Great answers, thank you.

Had to edit my rear size, it's 215/35 not 205/35. I used the formula you came up with and came up with 29 PSI rear.

From what I understand, tire pressures are run lower on the track for more grip, but I guess if you're using a pyrometer you're always getting optimum grip as track and air temps change. Sounds like this formula gets me in the ballpark for the street. Thanks!

Pressures may often be lower in competition, but remember that those tire tread widths are usually quite wide - the same theory holds.

As it happens, smart sets the 215s on the 452 BRABUS roadster at 29 psi. I don't know how the 452's weight and weight distribution compares with the 451.

NukleoN · 09-23-2009, 02:43 PM

Ok this is weird. I set my front tires to 24 PSI and rears to 29 PSI, but my tire pressure monitor keeps lighting up. Is this typical or is my car trying to warn me about a variation from expected stock PSI pressures? Maybe there's a way to 'reset' it?

Old smart · 09-23-2009, 03:12 PM

Quote:

Originally Posted by NukleoN

... Maybe there's a way to 'reset' it?

Yes, reset it per the OpMan. Don't just leave the setting the same as "the before;" reset it to "the after."

NukleoN · 09-23-2009, 03:32 PM

Quote:

Originally Posted by Old smart

Yes, reset it per the OpMan. Don't just leave the setting the same as "the before;" reset it to "the after."

Ok I just realized you meant 'Operaters Manual' by OpMan. I thought you were talking about some member on the forum. Never heard it abbreviated like that before.

I'll check it and reset it though. Thanks.

NukleoN · 09-29-2009, 04:05 PM

I am still looking for opinions regarding optimal tire-pressure for the OP specs. I am using 24 (front) and 29 (rear) PSI but this seems really low to me. Are there other methods for calculating correct PSI with a tire which has a MAX PSI of 50? How do I know if 23/29 PSI is too low for me? Resetting my TPMS doesn't 'stick' and it lights up everytime I start the car (yes, I reset it when the tires were cold).