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Baja Forums -:- VW Volkswagen Bug, Baja, Bus, Sandrail and Thing -:- VW Volkswagen & Baja Bug General Discussion -:- Tire Pressure and Highway speeds
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Tire Pressure and Highway speeds
Bug, Baja, Thing and Buggies. Most every thing that will not fit any any other area. See list of other Forums for better topic placement...Volkswagen General Discussion
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fader426
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Post Post subject: Tire Pressure and Highway speeds
Posted: Fri Jun 20, 2008 05:39 AM
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Hey All-
Just curious what tires pressures should be run on the street with a Baja. I know it varies with type of tire and driving, I'm just trying to get a general idea.

Also, what highway/freeway speed is average?

'69 Baja, 1835cc, midrange cam, single Zenith 32NDIX, stock transaxle and gearing.

Any input is appreciated, just trying to get general info and opinions.

Thanks-


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WaBaja
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Post Post subject: Re: Tire Pressure and Highway speeds
Posted: Fri Jun 20, 2008 07:11 AM
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whatever the tire says on it?...
I did 50mph today in my baja bug! DIdnt want to speed too mucH!


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Post Post subject: Re: Tire Pressure and Highway speeds
Posted: Fri Jun 20, 2008 08:04 AM
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tire size rim size and weight of vehicle will all change the handeling of a vehicle given a certain pressure. on a bug i would assume you would run less pressure than most cars so i would guess 30?... best way to be sure is get a tire marker (oil pen) and put lines across the tire in the width direction drive said vehicle about a block and see where the marks are left...

marks on out side but not center means your overinflated

marks on the inside but not the out side means your under inflated

marks on one side rather than the other is a alignment issue normally camber but could be toe.

try that and see what ideal pressure for your set up is then drive it if the car feels like its "ice skating" you probally have to much pressure

if it feels like every line in the road pulls the car then you probally dont have enough air in the tire

also keep in mind that air pressure affects rolling resistance therefore less resistance means better gas mileage!
~chad


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57baja
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Post Post subject: Re: Tire Pressure and Highway speeds
Posted: Fri Jun 20, 2008 09:41 AM
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We did a thread on this not too long ago. I shoulda kept it alive, since I'm still playing with street pressures, myself.

Scientifically, there are many variables which immediately come to mind that one must consider. We all know that tires wear more toward the outside when they are under-inflated, and excessively on the inside when they are over-inflated. However, we must consider other things when determining proper tire pressure for street use.

One thing that we are forced to think about, especially when gas is 5 bucks a gallon, is mileage. If tire pressures are increased, there is less drag, or friction, that occurs between the tire and the road because less of the tire is in contact with the road. Therefore, our mileage increases. Think of it in these terms: Try pushing a wooden wagon wheel down your driveway. Then, try pushing a round bag full of sand. Of course, this is a bit of an exaggeration, but you get the point. There is far less drag on the wooden wheel than the sand bag.

Another thing to consider is that the higher the pressure, the higher the rebound force, or bounce, you'll get from a tire. Ultimately, for safety and handling reasons, you want a tire to stay in contact with the ground at all times, especially on the road. This is where your shock selection really comes into play. Good shocks keep your tires on the ground, but unfortunately will not compensate for tire pressures that are too high. Think of this tendency in these terms: Bounce a basketball that is really full of air, and dribble one that is low. That high=pressure ball really bounces a lot, huh?

Then, we must, of course, consider traction. As a general rule, the more tire that is in contact with the pavement, the more we will have traction. After all, this is one of the reasons we air down when we go off-road, right? There is a point of diminishing return on this, though. If we have 2500 lbs of pure VW baja riding on 25 square inches of rubber (inflation low) we have about 100 lbs of pressure riding on each square inch. If we have 2500 lbs of that same baja riding on 10 square inches of rubber (inflation high), we now have 250 lbs of traction per square inch. You may have more or less traction depending on terrain, heat, tire tread design, wetness/slipperyness of the road, speed etc. with more or less surface area in contact with the road. Sometimes more force on less surface area will actually do a better job gripping the road than less force on more surface area.


I'm rambling, huh? Sorry. I think about these types of things in great detail. Bear with me.

OK, let's think about another factor: What about the amount that a tire actually flexes, particularly front tires, when they turn. When you turn at high speeds (even low speeds), such as the freeway, your tire flexes, or rolls over your rim to an extent. When it rolls over, it exposes the sides of the tread to tremendous forces and transmits this force into your steering, and affects the handling of the car. If you have really aggressive front tires, they can get really jumpy on you, as you will feel the sides of the treads ripping at the road if the inflation is low. That's why they tend to wear out quicker in the sides. However, if it is too high, there is not enough surface area gripping the road to keep you in control.

Car weight: Think about this for a second. Our beloved bajas weigh much more in the rear than they do in the front, right? Therefore we should be concerned with the traction that we have in the rear versus the traction on the front. In a perfect world, we should aim for better traction in the rear. What about when we are braking, though, at high speeds? Hmmm... Inertia puts more pressure on the front now. That's why cars generally wear brakes out twice as fast on the front than the rear. Maybe we want more traction on the front. Think about this, though: If we are braking hard and our front wheels lock up (lose the ability to grip the road), now we lose control of the direction that we are steering the car in. Therefore, we must keep in mind, amongst other things, our brake bias, front to rear. We need to keep the brakes from skidding the tires.

Conversely, when we are accelerating around corners, particularly in wet, icy, or slippery conditions' we need to make sure that our bugs don't go spinning around ass-end first (I did that in mine once in the rain on the freeway.... it wasn't fun). Fortunately we are at a distinct advantage over other vehicles since the engine is in the rear.

I know I'm really rambling now, but I must complete my thought. Bear with me.

Most of us run different tire sizes in front and in the rear. Tire diameter, as well as tread width play a crucial role in this. Think about another thing: It's going to be much easier to stop a 28" tire than it is a 33" tire. If your tires are different up front than they are in the rear, this will be compounded as well. All factory cars that I know of have the same size tires on all four wheels. In our case, it isn't so. Therefore, brake bias is going to play a crucial role here, and it will actually work against us in many respects, unless designed around this. Of course, to compound things, our front tires are usually narrower than our rears, too. That means you will have less tread in contact with the ground, all other things being equal.

These are just a few things to consider when you think about what pressure to use. It's really just a drop in the bucket, though. There's something to be said for ride and comfort. Each and every car is different, as well as each and every driver. Weather and road conditions play an important role, too!

For me, like I said before, I'm still experimenting. I just changed mine the other day to 17psi up front and 25psi in the rear, and I'm going to see what it feels like. It's really a trade off, when you boil everything down. You're going to make some sacrifices: Mileage vs. wear, comfort vs. safety, cornering vs. braking, etc. Make an informed decision, and do some experimenting.

Sorry for the rant, but I hope it helps someone out there.


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Post Post subject: Re: Tire Pressure and Highway speeds
Posted: Fri Jun 20, 2008 10:13 AM
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Dudes!!!! I GREATLY appreciate the time spent on explaining the SCIENCE of tires!!! BUT, I don't need (someone else will, for sure) to know the science about tires. I build and road race Italian motorcycles, so how tires work I am VERY familiar with. (and trust me, you don't even want to know how small a 100+ hp motorcycle tire's contact patch is, when leaned into a corner)

I really just wanted to skip the tech stuff, and find out real-world what you guys are running, and also what highway speeds.

Myself, right now I have 25psi in all 4, and haven't gone past 65 mph yet. I have very little rolling resistance, and off the line hauls butt. The steering under 40 mph seems slow and feels like the tires are flat, past 55 mph gets a little twitchy, but not unstable. In fact, with my stock IRS, ball-joint suspension and single Bilstein shocks, it rides REALLY good past 45 mph. That said, past 50 mph the steering is smooth and responsive, but revving the piss out of the engine.

57, I did find your tire thread. It's good.
90xj, I like your pen-marking method. Good tip.
Wa, my motorcycle experience says NEVER fill it to what the tire says, you'll land on your head!

Thanks for your responses.


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