Pressure splits

XXX#40

Well-known member
I should know better (my time is too valuable for some debate)....BUT :)

How much does the psi change the profile? What brand of tire are we talking about?
When cutting a tire using a template, I see very little (ie .001" = one thousandths of an inch = way smaller than the width of a human hair) when adjusting air pressure by 2 psi. It isn't until you reach around 15 psi in a tire that you effectively change the profile (ie balloons out the center of the tire.) Recognizing that dirt racers will never see that high of PSI, why is it that you think that a real, although minute, profile change (if it is even measureable at all) would affect the handling? Furthermore, when running very low psi (ie 4-6 psi) in kart tires with the sidewalls pulled out as much as we do, and 350-400 pounds pushing them down onto scale pads changes the profile dramatically more - then consider how they deflect under loading on the track (ie acceleration, cornering, lateral g-forces, etc.) I think we would be really splitting hairs (literally) on a pound or two changing the profile being the reason that the kart handles differently.

No doubt that air pressure splits affect handling, but I feel that the tire profile (at the pressures that we race at) has very little to do with that change in handling.

I think of psi affecting the spring rate of the tire moreso than the contact patch. That IS something measurable.

Personally, I think too many racers race on scales -- Gone are the days of a guy being able to set up a kart by the seat of his pants. Instead we see guys plugging in "factory" set-ups and then wonder why they can't hit their behinds running up "norf" on our black dirt oval tracks and floppy sidewall tires.


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Brian Carlson
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Vector Cutz
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same could be said about guys at the track on the scales, if your on the scales at the track you are behind the 8 ball, we do just as Brian mentions, we have a favorite setup that we call the baseline, adjustments from that point are seat of the pants and laptime adjustments.
Really we are very seldom on the scales anymore, when we first started we spent hours just making
And Brian it may just be a 1 lb split, but if nothing else is change but just the split changed you can fell it by the seat of the pants.
We run no split, and have run negative split.
I say try both ways
I'm not sure what your argument would be?
The original post was resetting to same scale number, would there be a handling change?

If a 1 psi change to the left side creates a negligible change to scale weights, then the lf and lr changes cancel each other, for the most part.

Contact patch, both statically, and dynamically, is what makes the handling difference.
I agree, same numbers with a 1 lb split versus n spilt, the kart will handle differently, a change that can be felt while on track
 

95 shaw

Premium User
I'll add another thought.

If you had a 1 square inch scale pad, and placed it under the center of the contact patch, with the surounding area at the same level, what would it read?

My money would be on the same as the pressure in the tire.
 

racing promotor

Well-known member
So I'm understanding your point, your saying If a kart is scaled In the pits of a track to whatever set - up numbers and cambers with 6 lb of air in all 4 tires, then raced with whatever tires and Air, then you bring it in set it on those same scales with a 1 lb split 6 & 5, BUT you took the time to reset & match set - up numbers 100%, and reset cambers and toe to match 100% and returned to the track with the same tires and pressures as first run your going to notice a handling difference that would equal same affect as adding a 1 lb additional split after leaving the scales and NOT resetting anything ?
 

95 shaw

Premium User
Contact patches are still going to be different.

Size and shape.

Will handling change?
Thats a crapshoot depending on the charateristics of the tire being used, and where you are relative to optimum for track conditions to those charscteristics.
 
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CarlsonMotorsports

Premium User
Some will argue the tire sidewall construction makes a difference.
Dynamically, yes. Statically, no.


A balloon at 5 psi with 100 pounds on it will have the same contact area as any tire with the same 5 psi.
Agreed.

What is your definition of "contact patch?"

A balloon with 5 psi would burst. A tire with 5 psi is pretty much being run flat.
A kart tire at 100 psi will deflect very little during cornering, so the contact patch changes very little.
That same tire at 5 psi will deflect considerably and the contact patch changes a bunch.

I get the physics, but I'm not sure we're agreeing on what that contact patch is (or looks like) statically and dynamically.
 

SoCo Tire Treatments

Well-known member
I wont get into the contact patch discussion, that is just making an easy thing much harder :)
What i will say is this, one of the best in the business, runs no air pressure split. He keeps things simple and wins, of course he probably has 1 of the best drivers in the country also.
I personally do a .5lb split anywhere i go, just habit more than anything. Keep things simple, do the same thing, and your results will show this.
 

95 shaw

Premium User
Contact patch is the portion of the tire actually in contact with the track surface.

A tire is simply a reinforce balloon, capable of pressures to support an actual load.

if you chalk up a tire and set it on a piece of paper., the resulting imprint would be the contact patch.

The patch will be very different for a tire supporting its own weight vs a tire supporting 100 pounds. And even moreso at speed on track, dynamically, with slip angles, tread and sidewall deflection, which are beyond the scope of this discussion. Even though they are pertinent.
 

CarlsonMotorsports

Premium User
Think of the balloon as a Burris tire.
Think of a semi-truck tire as a Maxxis tire. (The similarities are real. :) )

For sake of discussion:
Both are inflated to 5 psi (typical race pressure.)
Both are cut using the same template (ie profile.)

Changing the psi by 1# changes that profile so minutely, that you will not see any difference (chalk dust or not.)
And that's statically on the scales.
On the race track (dynamically) the change in contact patch is considerably different due moreso to sidewall strength than 1 psi. That is the exact reason that tire manufacturers experiment with different sidewall construction. While 1 psi does change the spring rate of a tire, it's certainly not as much as the actual construction of the tire.
 

racing promotor

Well-known member
Contact patches are still going to be different.

Size and shape.

Will handling change?
Thats a crapshoot depending on the charateristics of the tire being used, and where you are relative to optimum for track conditions to those charscteristics.
Sounds like were agreeing, your just pointing out there was still a very slight difference ON PAPER, which I acknowledged.
 

95 shaw

Premium User
Think of the balloon as a Burris tire.
Think of a semi-truck tire as a Maxxis tire. (The similarities are real. :) )

For sake of discussion:
Both are inflated to 5 psi (typical race pressure.)
Both are cut using the same template (ie profile.)

Changing the psi by 1# changes that profile so minutely, that you will not see any difference (chalk dust or not.)
And that's statically on the scales.
On the race track (dynamically) the change in contact patch is considerably different due moreso to sidewall strength than 1 psi. That is the exact reason that tire manufacturers experiment with different sidewall construction. While 1 psi does change the spring rate of a tire, it's certainly not as much as the actual construction of the tire.





Although the profile of the tire is relevant to the shape of the contact patch, it has little bearing on the area of that patch.
As pointed out earlier, a tire loaded with 100 pounds will see a 17% change in contact patch area.
In my book, that is neither negligible, nor minute.

And we see what we want to see. Lol.
 

JRCPerformance

New member
I wont read all the comments but read the first few...... Yes it will make a difference running the same exact numbers but having different psi from even split to 1 lbs split....

Ever do a tire sidewall test with a 1 pound difference?
Ever check the ride height of the left side frame rail going from 5psi to 6psi......?

Just because the numbers wont show a difference there is a difference..... especially on the track......

For example you can place a seat accordingly to what a kart maunf. says...... (ie. seat strut 4.5" off inner motor rail,have the recommended scale numbers, that kart will not handle the same with the rs seat strut at 5" off motor rail and the same set up numbers...... why? glad you asked...... you now have the driver sitting more over the lr..... I dont care what the numbers show..... the numbers dont show where the driver is sitting in the kart. so when weight transfers not just laterally but also vertically (VCG) it makes the kart react different.... Thus why seat placement is critical.....

take it for what its worth......

JR Curtis
JRC Perfomance (309)281-0266
Ultramax Racing Chassis
 

racing promotor

Well-known member
I wont read all the comments but read the first few...... Yes it will make a difference running the same exact numbers but having different psi from even split to 1 lbs split.... Maybe I'm reading this wrong, but I'm taking it like your stating running a 1lb split vs even would make a difference, I would agree but that's not the debate the debate is running the same air even up after the numbers were re-set.

Ever do a tire sidewall test with a 1 pound difference?
Ever check the ride height of the left side frame rail going from 5psi to 6psi......?

Just because the numbers wont show a difference there is a difference..... especially on the track......

For example you can place a seat accordingly to what a kart maunf. says...... (ie. seat strut 4.5" off inner motor rail,have the recommended scale numbers, that kart will not handle the same with the rs seat strut at 5" off motor rail and the same set up numbers...... why? glad you asked...... you now have the driver sitting more over the lr..... I dont care what the numbers show..... the numbers dont show where the driver is sitting in the kart. so when weight transfers not just laterally but also vertically (VCG) it makes the kart react different.... Thus why seat placement is critical..... This I would agree with 100% but big difference in comparison to the debate.

take it for what its worth......

JR Curtis
JRC Perfomance (309)281-0266
Ultramax Racing Chassis
 

95 shaw

Premium User
I guess the big takeaway from this discussion should be,
You can use whatever numbers, air pressures , etc, you like for a baseline. Then make adjustments from there. Being consistent would be the main thing.

If you are going to use someone else's numbers, or setup, You need to be sure you are using the same pressures, vcg, etc, to expect to get the same results. The trick is knowing you are different. Even trickier, knowing how different.
 
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