mark kibort

I had a cop friend radar me at 70mph. speedo was off until I started using the 305x35x18s.

Now, its right on the money.

mk

mark kibort

should I post a picture of me with a measuring tape, the tire , a level, and todays paper??

Yes LIARs! I remember counting on their width numbers for the tire. that caused me headaches. toyo, yoko, hoosier, all REAL different for the same tire size. (based on tire rack sizing too! ) The Yoko A032R was .5" narrower! why was that ? because those numbers are ball park numbers and change often with molds and manufacturing tollerances.

you know me, i only deal with actuals!

GezZZZZZZ !

EDIT: Here is a pic with a level. 25.75" and this is a shaved worn tire. when new shaved, more like 26" ( which is half worn vs a new tire)

mk

SharkSkin

The point I was making is that an OB can definitely go faster than 130... On that particular trip I had sorted out the belly pan issue.

Mark, unless your tires are filled with concrete, or do not deform under the weight of the car for some other reason, that is not the correct way to measure rolling radius. You are measuring diameter and can only calculate radius from that, not rolling radius. You will derive an inflated speed @ RPM figure measuring that way.

mark kibort

Why dont you explain to us, why that is?

Radius and diameter are not even the way you "really " want to measure. What you are really calculating is Circumference. Now, do you think that changes with extra weight in the car for example. So, the real question to you would be:
Does the circumference change with weight on the tire, and contact patch deformation??

Anyway, I was only suggesting that the number will be much lower the specs and the spec is what I am referring to and this is an accurate way to verify that. So, I posted the tallest that the tire will be, besides having full tread,which certainly would add .25" to that number or a little more. So, in reality, and to your point, the tire would probably be less than 26" for a 305x35x18 and speeds would be represented by the numbers I provided.

Mk

SharkSkin

Yes, it changes with weight on the car unless the tires have zero flex. The circumference of a tire that you measure by wrapping a tape measure around the tread will be different than the rolling circumference measured with the weight of the car on the tire, by marking a line on the sidewall, rolling the car forward until the line is again in the same spot, and measuring the difference traveled. Try this with any vehicle. This is because the tire deforms under weight. The true rolling radius of a tire can easily be 1/2" less than the radius as measured using your method. Trying to get a measure of circumference without knowing the true rolling radius compounds the error.

If the difference between unloaded radius and rolling radius is 1/2", you'll double that when you derive diameter to calculate the circumference so your error would then be 3.14" on the circumference. On a 26" tire you are approaching a 4% error. This may be close enough to make the point you are trying to make but it's not accurate. This is a 5mph error at 130mph.

Without knowing what the spec is supposed to be it's hard to say. The Performance Chassis link provided by Dan gives diameter... if they mean for an inflated tire resting unloaded like in your pic, your numbers are comparable. Taking these numbers and plugging them into a formula that needs rolling radius will have error built in. Measuring rolling radius directly isn't necessarily easy, especially if camber is nonzero. It's usually easier to measure rolling circumference directly.

mark kibort

So yes, it all makes my point. The numbers are not correct from the manufactures and they are a lot less than what folks hear are claiming.

I will do the test with measured off the car tire circumference vs loaded rolling circumference and see what we get. I dont think it will be 4% but Ill use a tire that doesnt have a stiff side wall where loaded, there is very little deflection. Ill use the BMW tires 235x45x17s and what we get.

mk

SharkSkin

Looks like a typo above? Those are the same thing. To see the error I was talking about, use the level & tape measure method or wrap a tape around an unloaded tire, then check the rolling circumference. The terms rolling radius and rolling circumference imply "loaded".

Also I picked 1/2" as a reasonable number that resulted in easy math. The actual deflection will vary with multiple factors. For example, I run 29psi in my rears, some people run 40psi. This would make a measurable difference.

mark kibort

Yes, typo or tought typo. fixed it.

Now, you are guessing about a "measurable difference now, with 29 vs 40psi . But heck if you can prove that, I would be impressed.

I dont believe the circumference changes with tire deflection with weight or changes in air pressure. But, that will be easy enough to prove.

mk

SharkSkin

It seems you may have taken that out of context. What I said was:

Other factors are sidewall stiffness, profile, weight on the tire, etc.

Measurable... depends somewhat on your methods I suppose. Significant? Probably not. But when differences are stacked, for example if you measure the same tire at 40psi unloaded vs loaded at 30psi, in the midst of a discussion where the numbers being tossed about are well within the 4% error in the example I gave I think it's relevant.

You can't take a measurement of an unloaded, normal tire and make a precise calculation of speed based on RPM with that measurement.

mark kibort

I guss you missed my drift. What I was alluding to that if the tire deforms, the circumference doesnt change, even though the radius from the ground to the axle does. I have to do the measurement that you suggested to see if there is a difference. BUT, in no way with all the variables stacked up , would you ever see a 4% difference.
Tires have a structure, they are not balloons. 40psi vs 29psi, which both are fully inflated, will appear to be the same as far as size. The difference is the spring rate of the structure at which the tire reacts to dynamic forces when driving, including rolling friction.

dprantl

Very interesting. I'm curious to see what the loaded rolling radius of my 275/35-18 tires is. I'm going to try and measure it today to see what the difference is.

Dan
'91 928GT S/C 475hp/460lb.ft

SharkSkin

No need to let facts get in the way, eh? Maybe you should contact this patent holder and this one, and tell them they have their facts wrong. Also this book author... with a little more googling you can find thousands of people you could educate on this.

From your statement above it seems you may have lost track of my assertion: You may see a 4% difference between wrapping a tape around an unloaded tire vs measuring the true rolling circumference on a normally inflated tire, absent ridiculously stiff sidewalls.

The difference due to tire pressure alone will certainly be less, but the difference is enough to use as the basis for tire pressure monitors, and it is enough that this is a well-known fact in automotive circles(well, most of them. )

Done.

/hijack - my intent was to point out a fact, not argue tire dynamics or split hairs over a perhaps 1% difference due to tire pressure.

mark kibort

No need to go on the offensive. I had to think about that one for a while, and you are right. I had to use an extreme example, like a flat tire and the effective rolling radius would be the shortest distance to the ground. So, you are right. However, the main point was that the pressures and varaiances in tire diameter vs manufacturer is going to be effectively less than 1% I would bet.

Ill measure my tahoe tires which have a lot of flex in the tire sidewall an see what the factor is.

Also, its no where near 4%, because with a 4% difference, if you have the tires up front, you get a RADICAL pulling to one side. Ive done this with a 255 40 and a 245 x45 by accident. the effect was huge for only a 3% difference. Ive also raced (practiced by accident ) with 50psi vs 30psi and didnt notice a thing, other than handling differences .

SharkSkin

I had hoped that my liberal use of smileys would make it clear that I was ribbing you, not trying to "go on the offensive". Apologies if that wasn't clear.

The "main point" was that rolling radius is different from "1/2 the diameter as measured on an unloaded tire" and that this will affect speed/RPM calcs. Tire pressure was introduced to point out one of the other factors that figures in... It was not my main point at all.

I don't see how your discussion about pulling is relevant to the main point I was trying to make. Do you mean you had 30psi on one side and 50psi on the other? How does this relate to 3-4% when I clearly stated that I felt the difference due to pressure alone would probably not be significant, and may or may not be measurable depending on your methods?

mark kibort

No problem.

The point about pulling is that you would have to see a 2-3% difference to get pulling on the front wheel or rear wheels (as I have done with rear tires too, but pulling only happens with WOT). So, I dont think that air pressures or side wall differences could create this much of a delta, but thats just a guess, dont hold me to it.

The main point here is that 305x35x18s are not 26.5 or 26.3" in diameter, even if they are pumped up to 40psi. so, the speeds indicated by RPM would be close to what I have posted. more than likely, they could be less due to the facts you have mentioned regarding active, loaded radius due to air pressure and sidewall stiffness.

Mk