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Hey guys- Sorry if we've covered this before, but I don't recall if we did... The cars we build tend to be on the very light side... I'm wondering what tire pressure you've found that works for you on your 'light' car?

My build is unusually small and light (a fiberglass microcar); I'm right at 1000 lbs wet with 1/2 tank of fuel and no driver.. With me in it it's right at 1200 lbs total. She's at about 110 hp, so she'll handle and accelerate when prompted..

I'm experimenting with what tire pressure to run for my street tires, and I keep going lower and lower.. I think right now I'm down to 25 lbs (cold) both front and rear (my street tires are tiny; Firestone F560 radials @ 145 R 13). I have background in off-roading, where we REALLY lower the tire pressure to fit conditions (3-5 psi on our quads in the sand!); street tires of course need more, but where is the Goldilocks 'just right' spot?

To further complicate my question, I also have a set of track tires (100tw Toyo R888R @ 185-60-13), and as I'm just getting those into service, I'm needing to experiment with what PSI will work for those too (on a course/ track). I suspect that I'm so light that I'll have a hard time getting any heat into these track/sticky tires, so the temptation will be to run lower PSI on those I imagine..

Thoughts? Experience with just how low to go for such a light car?

Thanks-

--ccrunner

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1972 Honda N600 Restomod "ccrunner's N600 VFR800 repower"

1963 Volvo P1800 Restomod
http://locostusa.com/forums/viewtopic.php?f=36&t=16309

1959 Berkeley SE492 Restomod...
viewtopic.php?f=36&t=19397

1959 Berkeley (#2) SE328 (170 HP-DCT bike motor in a wee car..) "Slow Build Berkeley (ccrunner)"

On my 1200 lb car I was running 19f/18r on Re71s if that helps at all. Any lower and the center strip would be too cold.. Though I also had trouble getting too much heat into them without a codriver.

Kind of depends on the size of the tire too.

I run 18psi in my Locost for the street (185/60R13 on a 13x7), and about 13psi with 20x9.5x13 slicks on a 13x10 rim.

My (formerly) daily driver squarebody C10 runs 35psi in 245/60R15 (15x8) fronts and 24psi in 295/50R15 (15x10) rears.

An IR thermometer will certainly get you in the ballpark of what the car actually wants. You want even temps across the tread.

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The Lethal Locost
The Lethal Locost 2 - Even More Lethalerer

I've always been happy with 18psi, front and rear if I'm running twistys. 25psi if I'm doing strictly highway driving. Russ

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You meet the nicest people in a Locost Se7en.

1200# dry. Running Toyo r1r and Avon zzs.. 17-18psi has always worked well. Maybe 16psi autocross (any less decreased my performance)
As always mileage may vary in your hands but the reference points posted will probably get you to a close starting place.

Everybody has their own degree of success.

The grip the tire has on the rim lip is mostly determined by the rim od, tire profile, sidewall stiffness, and pressure. A pot hole is more likely to result in rim damage and unseating the bead/blow out. I would not run less than 20 psi with any auto tire and I also carry a mini-compressor with plug strips in every car. The lower you run it, the more often it will need to be checked to ensure the pressure is maintained.

With your specs, I would start out at 25-28 psi all around and monitor the wear pattern.

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McSorley 442:122x46x14
Collins 241:127x46x12

I second using pressures in the high teens. That said, the right way is tire wear and temperature, ensuring it's even across the face of the tread. Experiment with pressures and see what works best, and what's best at one end may be different than the other.

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Midlana book: Build this mid-engine Locost!, http://midlana.com/stuff/book/
Kimini book: Designing mid-engine cars using FWD drivetrains, http://www.kimini.com/book_info/

Thank you guys for your feedback.. You've validated my thought that at 25 PSI (cold- on the street tires), there's still room to lower the PSI..

I was reluctant to drop them any further simply out of my old-school habit of running car tires up in the 30-x PSI range.. We're putting these tires on vehicles much lighter than they were designed for, so I'm glad I asked about this.. Surely I have some trial and error to learn where things should be on this particular car, but at least I know I'm not crazy to be considering dropping the pressure even further than the mid 20s..

thanks again

--ccrunner

_________________
1972 Honda N600 Restomod "ccrunner's N600 VFR800 repower"

1963 Volvo P1800 Restomod
http://locostusa.com/forums/viewtopic.php?f=36&t=16309

1959 Berkeley SE492 Restomod...
viewtopic.php?f=36&t=19397

1959 Berkeley (#2) SE328 (170 HP-DCT bike motor in a wee car..) "Slow Build Berkeley (ccrunner)"

Following as my new-to-me Midget will be getting new rubber as soon as we get it home, to replace the 10-12 year old tires.
Good to have some basis on starting pressures, though the Midge is 6-700lbs heavier than the Berk.

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1975 MG Midget
http://www.locostusa.com/forums/viewtopic.php?f=36&t=24382
13B Powered Rotus Geauxkart
http://www.locostusa.com/forums/viewtopic.php?f=35&t=20899

Congrats on the Midget! Will you post a build log on this site for it? A different site? At very least give us a picture

--ccrunner

_________________
1972 Honda N600 Restomod "ccrunner's N600 VFR800 repower"

1963 Volvo P1800 Restomod
http://locostusa.com/forums/viewtopic.php?f=36&t=16309

1959 Berkeley SE492 Restomod...
viewtopic.php?f=36&t=19397

1959 Berkeley (#2) SE328 (170 HP-DCT bike motor in a wee car..) "Slow Build Berkeley (ccrunner)"

I'll probably start a build on here to track the progress on it.
Hopefully it doesn't need much to be road-safe.

_________________
1975 MG Midget
http://www.locostusa.com/forums/viewtopic.php?f=36&t=24382
13B Powered Rotus Geauxkart
http://www.locostusa.com/forums/viewtopic.php?f=35&t=20899

Of 15" rims my starting pressure is 16/18 and go from there. At the track I will see how the temps are running and go a little lower if I don't think they are getting hot enough. Tires just come alive when you get this into there optimum working temps. Side wall stiffness can make a big difference, the original R888's worked great, but the R888S had a much stiffer side wall that I could never get up to temp. My favorite is the Toyo RA1, give great grip, are predictable, there best grip is just before the cords show so you can get full use out of them.

Graham