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PostPosted: January 23, 2021, 2:11 am 
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Joined: October 10, 2010, 10:26 am
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Location: Santa Maria, CA
Looking for your experience here guys.. I've built a very light BEC that I believe weighs about 850-900lbs wet with 1/2 tank of fuel... (with the driver and full of fuel it will be about 1050-1100 lbs all in). She's got about 105-110 hp.. (It's a 1959 Berkeley/ RWD).

I'm just getting it on the road, and I don't know what tire pressure to run? (this is the lightest car I've built).. At first blush I'm trying to get a little nicer ride (as is @ 32psi it's HARSH!), but I haven't even dialed the suspension in yet- Finally getting it on the road has got me to wondering what tire pressure range I should be running. These are my 'starter tires' to get it on the road- they're 155/70r13... treadwear 420.. traction A, temp A.. regular ol' DOT street tires. I'll be swapping out to different tires later, but I'm stuck with these for now.

So your thoughts on a ballpark on what to run.. I know it will be some trial and error, but this car is freakishly light, and if I leave the tires at 32lbs it's going to beat me to death :lol:

--ccrunner

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PostPosted: January 23, 2021, 2:23 am 
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Joined: January 18, 2015, 2:34 am
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My Birkin weighs 1,337 lbs. with a half tank. I run 17 F/19 R. This is with Hankook RS-3s.


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PostPosted: January 23, 2021, 3:01 am 
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Joined: March 19, 2011, 10:22 am
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Location: Holden, Alberta, Canada
I have no expertise figuring this out, but, when I was apprenticing as a mechanic back in the late 70's I used to do a lot of tire installs. When installing tires on small cars my boss would tell me "24PSI in the tires, no more!" The customers never came back to complain about handling or tire wear.
In the 70's those small cars were nowhere near as small as the Berkeley, I'm talking Pinto, Astre, Bobcat, Vega, and a Datsun, and maybe an Austin Mini.

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PostPosted: January 23, 2021, 8:32 am 
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Joined: September 22, 2005, 8:12 am
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Location: 4AGE in S.E. Michigan
I'm at 1267 lbs and run pressures at 18 Frt and 19 Rr. on 55 series tires.

1; You can try the carbon paper trick. Do they even make carbon paper now days? 2; or shoe polish on the tire.

1] Roll the wheel over the carbon paper set on top of white paper, and see what rubber contact looks like. Adjust pressure until you get full contact.

2] Put a shoe polish mark across the tire(s) and drive a short distance. Check how the polish is wearing away. Adjust pressure until you have complete contact.
Either method should get you within +/_ one ,possibly 2 pounds. Final pressure by road/track testing.
Davew


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PostPosted: January 23, 2021, 8:41 am 
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Joined: August 28, 2010, 7:53 am
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My Legends car is 1290lbs with driver fuel etc and i run 12-13 psi on slicks. I am sure a physics professor could figure out the weights and surface area, then calculate pressure, but I would just try 16 lbs and see how it looks/feels.
Remember, the air is holding the car up (it is an air spring) The tire contains the air.
A tire pyrometer can be useful on slicks and bias ply tires, but it doesn't really help on radial tires.


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PostPosted: January 23, 2021, 11:57 am 
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Hi ccrunner, my 1959 Fiat Abarth 850 weighs 1275lbs and wears 145/75 x13 radial tires that are stretched out because the wheels are an inch wider than the tires. The car is very happy with 22psi front and 24 psi rear on the road. The car is rear engined so weight bias to the rear and it works its outside rear tire very hard when cornering. For a slalom pressures are dropped by 3-5psi depending on the day.

I should also say that those numbers are off of my pressure gauge, I use the same one all of the time never any other gauge, and I've never compared it to other gauges so the numbers could be +/- a bit "in the real world".

Good luck! Bill

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PostPosted: January 23, 2021, 12:33 pm 
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MG said 22psi in the front and 24psi in the rear for a Midget on 145 width radials and that car would be ~50% heavier than yours. My Locost seemed happy at 18 psi all around on 195/50R15 tires. High teens sounds like a good starting point.

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PostPosted: January 23, 2021, 1:13 pm 
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Location: Cornholio OR "Where the magic happens"
Something is rattling around in my head about watching for a 3 psi rise from cold to hot.

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PostPosted: January 23, 2021, 2:24 pm 
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Just for another point of data, my Locost was:

50/50 weight balanced.
1400 lbs.
1st gen Azenis RT615 205/50R15 tires.
DOT rated tires.
Treadwear rating=200

I ran 99.9% on the street with 18psi all around. After 6,000 miles, the tires were not showing any favoritism on wear. All were worn equally . :mrgreen:

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PostPosted: January 24, 2021, 7:51 pm 
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Joined: October 10, 2010, 10:26 am
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Location: Santa Maria, CA
Thanks for the quick and insightful feedback guys.. I put them all at 18 psi and went for a short drive... there was a noticable change in the ride and handling- all for the better :D

I'll play around with higher/lower pressure as I start to get miles on it... Also I'll do the shoe polish test so I'm not blind guessing..

As always, thanks for the help everybody :cheers:

--ccrunner

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My '72 Honda N600 build log (bike engine in a microcar)...
http://www.locostusa.com/forums/viewtop ... 36&t=14452

My '63 Volvo 1800 with a turbo inline 4 build log (LNF Ecotec compliments a Svelte Swede)
http://locostusa.com/forums/viewtopic.php?f=36&t=16309

My '59 Berkeley SE492 build log (bike engine in a microcar)... "A Berkeley With Bite!"


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PostPosted: January 25, 2021, 9:33 am 
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Joined: January 7, 2011, 10:47 am
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Location: Bethlehem PA
During a test n tune at a local autocross 17-18 psi yielded the best time on the skidpad, that is what I have been running ever since.

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PostPosted: January 25, 2021, 11:24 am 
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That's about the weight of my old A Mod. I ran 9 psi on 10" wide tires.

A little quick math based on tire size I'd guess 20 psi or less

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PostPosted: January 25, 2021, 12:08 pm 
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Well, this is a factor I've yet to consider.
Glad to have an extra data point when I eventually pick up & get going with one.
Definitely wouldn't have guessed on running the pressures so low, compared to a typical passenger car. I think my 15 Mercedes was close to 40PSI all around.


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PostPosted: January 26, 2021, 10:06 am 
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JAMADOR wrote:
Well, this is a factor I've yet to consider.
Glad to have an extra data point when I eventually pick up & get going with one.
Definitely wouldn't have guessed on running the pressures so low, compared to a typical passenger car. I think my 15 Mercedes was close to 40PSI all around.


My BMW 318ti at 2800 lbs was happiest at 52 psi on BFG Sport Comps. That was settled on a skid pad and at SCCA Starting Line School.

Dropped down to 42-45 for daily comfort cruising.

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Too much week, not enough weekend.

OOPS I did it again
http://www.locostusa.com/forums/viewtopic.php?f=35&t=17496

Blood Sweat and Beers
http://www.locostusa.com/forums/viewtopic.php?f=35&t=15216


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PostPosted: January 26, 2021, 1:43 pm 
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I remember my RX-8 needing 45ish(or more?) before I stopped rolling onto the sidewalls of a set of Falken FK510's in auto-x. Wish I kept better records of that session.

I keep it a 36 for daily driving.

Best thing to do is, when the time comes, chalk the tires and see where it looks the best.


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