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Hey you guys, what is everyone running for tire pressures for the street?

I'm running 18 psi front and rear. At 20, the ride was too harsh. I haven't tried anything lower.

edit: I've got 205/50-15 tires.

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Chuck.

“Any suspension will work if you don’t let it.” - Colin Chapman

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I am running 20 front and rear on a 205/50/16 tire, relative to Chucks I am 2 pounds higher than his 18, but my car probably weighs a bit more than Chuck's (my assumption).
Either way that should ball park a starting point.

Al

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Super Seven 3.4

You are correct. I read where you are ~1650. I'm at 1400 without driver. Actually, my car is 1400, not me

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Chuck.

“Any suspension will work if you don’t let it.” - Colin Chapman

Visit my ongoing MGB Rustoration log: over HERE

Or my Wankel powered Locost log : over HERE

And don't forget my Cushman Truckster resto Locostusa.com/forums/viewtopic.php?f=36&t=17766

Thanks guys, I'm at 1275 lbs w/o driver and have been running about 22lbs. I might drop a bit now. It feels fine but I haven't really worked it hard yet. David

I'm at 1275 w/o me, and I've run between 18 and 20psi. A lot is going to depend on the tires - my Sumitomo's are s-o-f-t.

I run around 13psi on slicks.

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The Lethal Locost
The Lethal Locost 2 - Even More Lethalerer

WARNING -- liberal arts grad here. But, aren't we into one of those "springs run in series" high school physics things here?

Cause the tires do act as springs, as (of course) do the springs above them. If I lower the spring rate of the tires by letting out some air, don't I transfer the spring function to the tire from the springs, and probably some funky (bad) things would result?

On the other hand, if I put in softer springs, the spring function would remain with the springs, albeit a bit more 'cause the springs are springier?

Then again, the shocks have something to say about harsh rides too I hear....

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Geek49203 aka
Tim Wohlford
Louisville, KY
Hayes front, S10 +2 rear, Lalo body.

I run 35 pounds on my Goodyear FuelMax Assurance tires, my car's a live axle book build and I feel every pebble. Now if I were a real hypermiler (instead of leveling off smugly at 100 mpg) I'd run the "51 pounds maximum pressure" listed on the sidewall...and then I'd feel every dime.The absolute nutso hypermilers run rated-maximum-pressure-plus-50% and trust there's adequate safety margin engineered into the tire carcass; not only would I feel every dime, I'd be able to tell you if it was heads or tales*.

Not that any of this matters to anybody but Yours Truly. Never mind.

*Heads slope up gradually over the dead president's face, tales have a straight sided torch in the middle and the rise is more abrupt.

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Locost builder and adventurer, and founder (but no longer owner) of Kinetic Vehicles

The way I was taught to get baseline pressure involved sidewalk chalk and a long driveway. ..

You put a chalk mark across the tread and roll the car back and forth, then adjust the tire pressure until the chalk rubs off evenly.
Then you do it again, driving it around the block making "s" curves to see sidewall flex.

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"There are times when a broken tool is better than a sound one, or a twisted personality more useful than a whole one.
For instance, a whole beer bottle isn't half the weapon that half a beer bottle is ..." Randall Garrett

Thanks all you guys, I have 22lbs in them at the moment and they feel OK but I'll play around a bit now. Also I'm not sure why I asked this in this section but mods please move it to a more appropriate section if you can. Thanks David

Alternatively, corner weight the car (put a scale under each wheel and measure the weight at the tires). Then take that weight (W) divide by the max load of the tire (L) and multiply by the max pressure of the tire (P) to get your inflation pressure (I):

W/L x P = I

So if you had a car that was 1600lbs evenly distributed at 400lbs per wheel (W), a tire with a load index of 87 (1201lbs max load) (L), and a max pressure of 50psi (P) you would end up like this:

400/1201x50=16.65psi, I'd round that to 17.

Use that as a minimum, adjusting the pressure from there will affect handling, but going below that will be overloading/under inflating your tires.

Thanks for the math Firestorm. I've been doing it the hard /expensive way all these years, periodically adjusting pressure to figureout what is correct for even wear when the wheels and tires are swapped and the sticker in the door jamb no longer applies.

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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12

Yes thank you Firestorm, I haven't done a corner weight on it yet but will this winter and will see what I get............David

Yup, if you are running less than say 25 lbs, you have too stiff a spring and/or damper and you are relying on un'dampened air springs, ie; the tyre, to do your suspension work, not good.

For a comparison a Lotus Elise is 1600lbs (650 fr/950 rear) and ideally runs around 25/28 and they would be one of the lowest pressures for a production car in the world, I happen to know that Smart's are 29psi all round (they are also 1600lbs but would be softer sprung and higher sidewall than a Lotus).




Typical common shocks simply force oil through a set sized hole so hitting sharp edges can be too much attempted flow through the hole. Open up the hole and then the oil flows too fast at slow speeds.

I recall a top level Ford engineer laughing at me when I changed shocks on my 84 Turbo Colt (loved that car!). As he suspected, I really didn't know what shocks did for suspensions.

Now that I know, it's amazing how many cars I see on the road that need new shocks/struts. I am amazed at how many cars are going doing the road doing (roughly) 1-second oscillations. I recall seeing an old pickup on the West Virginia pickup that was going at least 90 mph, with the bed of the truck oscillating so hard that the tires occasionally came off of the ground!

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Geek49203 aka
Tim Wohlford
Louisville, KY
Hayes front, S10 +2 rear, Lalo body.