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I have been running 18/28.  Maybe the front is too low.

Perhaps someone can describe the procedure where one puts chalk on the treads to check for full contact at various pressures.

I have a thread depth gauge. After 3 seasons, wear is consistent across both front tires. (18psi) the rears are a moot point due to my caster.

An alternative way to determine your proper TP is by checking them when hot. Your proper inflation is when your hot psi is =10% of your cold psi. Any more or less than 10% and your tires are either under or over inflated.

Last edited by dlearl476


@Michael McKelvey  Easy process:  get a piece of driveway chalk and draw a 1” wide line all the way across your tire tread, sidewalk to sidewall, on your tires.   Go out for a typical drive ( “don’t drive like my brother” ).   When you get home ( or home equivalent ), inspect your tires to see how much of the chalk is now gone.  
You should, ideally, see about 80% of the chalk width versus tread width gone, all across the tread width.   Less than that says your tire pressure is too high ( the tire is too hard ).  Wider than that, the pressure is too low, especially if the chalk towards the sidewalls is gone but the chalk in the center is still there, showing that the tread is “pooching” up in the center.  

@edsnova posted:

So...18 psi cold and 1.8 psi hot? Got it.

Dang, Ed. I never thought of it that way! You know how much math that would have saved me over the years?  

TP is much more critical on MCs than it is on cars. Every time I put a new pair on any of my MC's, I did extensive testing to determine the proper TP.  My Ducati was a completely different bike between 36/42 and Ohlins recommended 32/38 on Michelin Sports.

All my tire gauges read both.

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Richard, I've also been wondering about the Willhoit suggestions you quote.

29 in the rear isn't too far off from what gets kicked around here. (I think I've been running around 28 for a while now.)

But 26 for the front is maybe 4-5 lbs higher than where the forum consensus usually ends up (ha-ha, he said 'consensus').

I think most of us start out around there but usually end up lowering it to avoid the merciless pounding that usually results. My car definitely has a nicer ride at 21 than at 26, although it probably handles  better at the higher setting.

So, maybe our (lowered) VW front ends just ride harsher than an original 356 and those cars are more comfortable at the higher setting? I don't know the answer to that having never driven an original car.

Another thing that makes us favor lower settings is that VW originally recommended something like 18-20 up front for the Beetle. What is often forgotten, though, is that that was for bias ply tires. Bias plies have much stiffer sidewalls than radials, so at higher pressures tend to ride like rocks, especially if lightly loaded (like on the front end of a Beetle). Also those skinny, high aspect ratio tires were actually meant to be a significant part of the suspension - they supplied a good bit of the compliance. So, another reason for lower recommendations.

I don't know exactly when Porsche made radials standard. Michelin developed the 'X' in 1949 and they were pretty widely available by the mid '50s, but my hunch is they had to be optioned until probably the early 60s.

So was the 356 front end sprung differently than what most of us end up with a lowered VW beam? And does that make higher tire pressure up front more bearable on those cars?

Dunno.

Anyone got answers?

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18 is TOO LOW for Danny!

Most of us don't have a VS Spyder. Most of us don't drive like Danny. Hell, most of us probably can't drive like Danny.

I think I set my Spyder's fronts to like 22. That seems pretty good. I'm running Vred Sprint Classics with tall sidewalls on that car. I plan to drive that car in a Danny-like way some before settling on final tire pressure recommendations. It might well go up a pound or two.

My TDR has Hanhooks or something, also 165/80s, and that car likes 16 lbs on the front.

That is because it is significantly lighter up front than a Speedster, much more so than a Spyder. With 22 lbs of front it's OK. A little stiff. 22 is about what that car wants for auto-x duty.

When I got it it had 30 up front and it was not only harsh riding, there was no front traction. I can report that 16-18 does not roll the tires off the rim, and does not squeal around corners.

Why would this be, you ask?

Because tires are made to be inflated against the forces (i.e. weight) opposing them.

This was true with bias tires also. Whatever sidewall you have—thick or thin, bias or radial, white or black—you want just a little more air on the inside of it than necessary to handle the psi forces it will be subjected to.

This is why the sidewall says "MAX LOAD" and a number. Look at that number on your tires. With a Speedster, it's going to be a number that's more than half the weight of your whole car (it's 1169 lbs on my Vreds; the whole car weighs 350 lbs more than that). And that's weird, huh.

On your daily driver or monster truck, the MAX number on your tire will be well less than half the weight of the vehicle.

But with our light clown cars, we're never going to get anywhere near the max load of any tire we run. Typically on street car tires, that max load is calculated at 35 psi.

So to make the tire work "right" on a clown car that weighs half or less than the types of cars it was designed to fit, we have to reduce pressure inside the tire. Yeah, sometimes to half that 35 lb "MAX LOAD" pressure rating.

@dlearl476 posted:

I have a thread depth gauge. After 3 seasons, wear is consistent across both front tires. (18psi) the rears are a moot point due to my caster.

An alternative way to determine your proper TP is by checking them when hot. Your proper inflation is when your hot psi is =10% of your cold psi. Any more or less than 10% and your tires are either under or over inflated.

It's tread, not thread. And it's the camber in the rear, not caster.

And I think you mean 110% hot, right?

20 psi cold, 22 hot?

"Anyone got answers?" - @Sacto Mitch

As I'm driving along (throwing out the vibes), I'm guessing that my front TP are probably OK (whatever they are) and my rear TP are probably OK as well. In my mind, the answer is probably floating around in the same void as oil viscosity, trans gearing, total spark advance, fuel jet size and...

As I'm enjoying the day driving my Speedy, the pressing thoughts on my mind are: I bet I look REALLY dorky in this wetsuit: I'm sure glad that BIG dude I dropped-in on didn't spear me with his surfboard: I wonder if that fish taco place will be open later.

IMG_20141004_112738

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@Sacto Mitch posted:

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Tire pressure, taco place!

Tire pressure, taco place!

Tire pressure, taco place!

Chanting this rapidly three times while beating your garage wall in rhythm with the ceremonial, sequined, selfie stick will conjure the great El Guapo!  He will appear in a wetsuit sparkling with drops of Pacific seawater, Ray-Bans, and a knowing smile. The pressure in your tires will drop, the sidewalls will relax, and so will you!

Denizens of the great white north, chant with me!!!

"How is proper pressure related to tire size? For instance, if one changes from 165/80 to 185/65, how, if at all, would the proper pressure change?"

The short answer is, yes, proper tire pressure on the same car will vary slightly with different size tires.  I refer you back to my post, above, on marking your tires with chalk to see if yours are at the proper inflation pressure for your car and driving habits.  The chalk lets you see in a minute or two whether your selected tire pressure is right for your car.   If you need a visual to go along with that, then here you go:

tire-pressure-impact-on-tread-wear

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Last edited by Gordon Nichols

Look at that max load number on the sidewall, Mike. If your wider, lower-profile tires are rated to carry more weight, you might do well to drop the pressure a bit as compared to the tall skinnies (or whatever you're replacing).

Really, you should be able to feel it when it's too much: it'll ride like a buckboard AND handling will be worse.

If you're at the track (or Danny) you want to increase that buckboard effect only until you crest the point of diminishing returns on the skidpad. If you're like the rest of us you'll want to decrease the buckboard effect just until the car starts to squeal when you're accelerating around a traffic circle (as one does), then add about two-three pounds.

The chalk-across-the-treads method is more for Danny driving but it will tell you for sure if you're too low.

Front to back pressure should be close to the front-to-back weight distribution, which I think in speedsters is about 40-60.

And by the way... the only guys left on this board who are likely to wear out a set of Speedster tires are Jack Crosby and Jim Ignacio. So, really...just don't ride the rims, boys. It makes the rest of us look bad.

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