Wide Vs Skinny Mud Tyres

Post by **Suspension Stuff** » Mon Jun 19, 2006 11:13 pm

If I do a mono on my bike does that mean my ground contact area exactly doubles

I have to do a mono now since I put the other one on the fourby

Post by **christover1** » Mon Jun 19, 2006 11:31 pm

I remember getting the theoretical tyre question at high school during a physics course, but don't actually remember my answer.

Practical experience has taught me much more, but its built on the theory I learned in my youth.

It's good to stretch your brains, without it being a ##it fight.

10 points to Shadow for being polite and calm.

christover

Post by **Shadow** » Mon Jun 19, 2006 11:32 pm

4WD Stuff wrote:Example only not accurate.

Bicycle Tyre pumped up to 32PSI has a Ground contact area of 2 inches wide x 20 inches long which = 40 square inches of Contact area.

750kg/40Sq Inches = 18.75kg per square inch

4wd Tyre pumped up to 32 PSI is 12.5 inches wide x 10 inches long = 125 square inches of contact area

750kg/125Sq Inches = 6kg per square inch.

Does this help

where did you get your contact areas from?

Post by **Shadow** » Mon Jun 19, 2006 11:33 pm

Beastmavster wrote:
So let me check this: are you truly saying if there is no tyre pressure, then there is no force on the ground? Holy sh1t batman, we have a hovercraft.

no because the rim would be loading directly on the road (albeit through a layer of rubber or 2).

Post by **Shadow** » Mon Jun 19, 2006 11:34 pm

4WD Stuff wrote:
If I do a mono on my bike does that mean my ground contact area exactly doubles
I have to do a mono now since I put the other one on the fourby

probably. try it.

Post by **Suspension Stuff** » Mon Jun 19, 2006 11:36 pm

Back at school I did Maths 1, Maths 2, Physics etc but I have to rely on common sence these days because I barely remember a formula from school.

TO BE CONTINUED

Post by **christover1** » Mon Jun 19, 2006 11:43 pm

4WD Stuff wrote:Back at school I did Maths 1, Maths 2, Physics etc but I have to rely on common sence these days because I barely remember a formula from school.

ditto,
Straight A student, much to the dislike of my classmates.
But all formulas in brain are gone, too

Post by **Suspension Stuff** » Mon Jun 19, 2006 11:45 pm

Dragsters use wider tyres for the large contact area to the ground.

Post by **christover1** » Mon Jun 19, 2006 11:47 pm

4WD Stuff wrote: Dragsters use wider tyres for the large contact area to the ground.

I had a Dragster, it had 24" x 1:75" tyres

an 80's treadly with chopper handle bars

Post by **sudso** » Tue Jun 20, 2006 2:00 am

OK here it is (reworded) from the 4wd Monthly test:

Tested was 285/75/16 Maxxis Buckshot mudders (33x11.7x16 - 8 ply)
the other was 7.50R16 Dunlop Roadgrippers (not sure of their diameter)

The contact pressures were measured as the weight on each tyre, in kg per square cm in the ground contact area of each tyre at different pressures.

To sum up, the wider Buckshot mudders had a lower contact pressures at both 35psi and 10psi than the Roadgrippers. This is probably because the Buckshot has a larger diameter and is a lot wider than the Roadgripper.

e.g. Buckshot = 0.99kg/cm.sq. @ 10 psi

Roadgripper = 1.18kg/cm.sq. @ 10 psi

Not much in it really but it equates to a bit over 40% less pressure per square cm on the ground because the weight of the vehicle is spread over a larger contact area by deflating the tyres.
The increase in contact area on both sets of tyres when deflated from 35 to 10 psi was around 74%. The longer footprint gives extra traction which means more forward travel and less bogging.

Post by **ISUZUROVER** » Tue Jun 20, 2006 2:14 am

sudso wrote:OK here it is (reworded) from the 4wd Monthly test:

Tested was a 285/75/16 Maxxis Buckshot mudder (33x11.7x16 - 8 ply)
the other was a 7.50R16 Dunlop Roadgripper (not sure of their diameter)

The contact pressures were measured as the weight on each tyre, in kg per square cm in the ground contact area of each tyre at different pressures.

To sum up, the wider Buckshot mudder had a lower contact pressure at both 35psi and 10psi than the Roadgripper. This is probably because the Buckshot has a larger diameter and is a lot wider than the Roadgripper.

e.g. Buckshot = 0.99kg/cm.sq. @ 10 psi

Roadgripper = 1.18kg/cm.sq. @ 10 psi

Not much in it really but it equates to a bit over 40% less pressure per square cm on the ground because the weight of the vehicle is spread over a larger contact area by deflating the tyres.
The increase in contact area on both sets of tyres when deflated from 35 to 10 psi was around 74%. The longer footprint gives extra traction which means more forward travel and less bogging.

So both tyres were fitted to the same vehicle and had the same weight placed on them? - at both 35psi and 10psi?

If that is the case then it shows that Shadow's "theory" is incorrect by about 20% in this case.

Post by **sudso** » Tue Jun 20, 2006 3:10 am

So both tyres were fitted to the same vehicle and had the same weight placed on them? - at both 35psi and 10psi?

Yep. Both sets tyres were fitted to the same Troopy.

Post by **ISUZUROVER** » Tue Jun 20, 2006 3:42 am

So - doing a quick conversion:

The wider tyre had 14.1psi ground pressure at 10psi, the narrower 16.8psi.

Post by **-Scott-** » Tue Jun 20, 2006 9:30 am

Yep, I came up with 1.0 kg/cm^2 =14.2psi.

Damn, I thought the theory had some merit. I guess the sidewalls are carrying more weight than I expected.

Which issue was the test? I'd like to dig out my copy and have another read.

Cheers,

Scott

Post by **bazzle** » Tue Jun 20, 2006 9:58 am

All the above is irellevant, theroretical and hpyothetical.
It depends on many more variables.

Carcass construction as to shape over whole contact area, (some bulge in in the middle of spot under load.)
Tread pattern, void ratio, angle of blocks to ground, angle of blocks to carcase, type of rubber composition, angle and denier of plies chemical mix, etc etc.

Bazzle

Post by **CRUSHU** » Tue Jun 20, 2006 10:10 am

christover1 wrote:
4WD Stuff wrote: Dragsters use wider tyres for the large contact area to the ground.
I had a Dragster, it had 24" x 1:75" tyres

My Dragster has 17 x 1.75" tyres

On the Front!!!
33 x 15 x 15 on the rear

They actually measure 33" x 19"

Post by **Suspension Stuff** » Tue Jun 20, 2006 11:12 am

Shut up Bazzle

Yeah all these are valid points and worth discussing but we are comparing same tread pattern(Although changed a bit to fit on a thinner tyre) just skinnier tyres. Your variables still do apply but to a lessor degree than what is worth really mentioning. I still feel the above garbage is a bit more relevant with comparisons of tyre width then all these other variables.

Feel free to elaborate about these new variables.

Shane

Post by **Beastmavster** » Tue Jun 20, 2006 5:46 pm

4WD Stuff wrote:Example only not accurate.

Bicycle Tyre pumped up to 32PSI has a Ground contact area of 2 inches wide x 20 inches long which = 40 square inches of Contact area.

750kg/40Sq Inches = 18.75kg per square inch

4wd Tyre pumped up to 32 PSI is 12.5 inches wide x 10 inches long = 125 square inches of contact area

750kg/125Sq Inches = 6kg per square inch.

Does this help

The thing I'm struggling to see here all along.

How does a bicycle tyre get a contact patch 20 inches long? How does evena big 4wd get a contact patch longer than about 6" long?

To achieve some of these theoretical contact patch lengths the tyre has to be flat against the rim, which will last maybe 20 feet before the tyre comes off the rim (even with beadlocks the carcass can't take that kind of pressure long).

Post by **HotFourOk** » Tue Jun 20, 2006 6:10 pm

Yeh, a bike tyre cannot get 20" of contact, the tyre does not have enough sidewall to begin with. Maybe 2"? lol
Even if they tyre was completely deflated, it would only be a few inches.

A 4x4 would be able to get 6" at lower pressures.. would be good to do some meaurements next time.

This has gone so far off topic its not funny

Post by **Suspension Stuff** » Tue Jun 20, 2006 6:44 pm

Yes I was exaggerating about the contact area of the bicycle wheel to try and prove a point.

Post by **Beastmavster** » Tue Jun 20, 2006 6:49 pm

4WD Stuff wrote:Yes I was exaggerating about the contact area of the bicycle wheel to try and prove a point.

Yeah as I was when I was using the same sort of logic to suggest that 18" of a 33" muddy could touch the ground at once.

Post by **sudso** » Tue Jun 20, 2006 10:10 pm

Beastmavster wrote:
4WD Stuff wrote:Yes I was exaggerating about the contact area of the bicycle wheel to try and prove a point.
Yeah as I was when I was using the same sort of logic to suggest that 18" of a 33" muddy could touch the ground at once.

I think thats called a flat tyre

Post by **sudso** » Tue Jun 20, 2006 10:31 pm

-Scott- wrote:Yep, I came up with 1.0 kg/cm^2 =14.2psi.

Damn, I thought the theory had some merit. I guess the sidewalls are carrying more weight than I expected.

Which issue was the test? I'd like to dig out my copy and have another read.

Cheers,

Scott

It wasn't exactly an issue mate, my copy came as one of those free supplements, with issue no.?? think it was with an issue early last year.
Anyway it's titled MODIFIED 4WHEELER Tips, Techniques and Accessories Volume 9
I think it was a selection of articles from previous issues as it had "A selection of the best from 4WD Monthly" on the front cover too.
Has a good Diff Lock comparo in it too.

cheers

Post by **Beastmavster** » Wed Jul 19, 2006 8:35 pm

Ok did some measurements tonight using the 285/75/16 tyres.

Sorry for the dealy but it's the first night I've got to use the garage since then....

Tested at 40psi, 20psi (1/2 of original) and 13psi (1/3 of original).

OK.... Here's how I went. I will do these again at some point with A3 paper and tyre black, since it's very hard to get data on plain A4 paper.....

Testing method. Jack up front (drivers side). Place paper under and slowly lower until it is standing on the tyre.

Other tyres approx 24psi during test.

40 PSI:

Tread mark 6.5" wide by 7.5" long, oval shaped. Very faint. It's amazing how little indentation caused on the paper. Good thing i never run at 40psi - it feels unsafe and gripless to me.

20 PSI

Tread mark 7.5" wide by 10" long, oval shaped. Treadblock markings clearly visible.

13PSI

Tread mark 8" wide by 13" long, oval shaped. Treadblock markings very clearly visible.

Overall the width didnt change much, only the length.

Will try again with some tyre black now.

Post by **-Scott-** » Wed Jul 19, 2006 9:07 pm

Beastmavster wrote:Ok did some measurements tonight using the 285/75/16 tyres.

40 PSI:

Tread mark 6.5" wide by 7.5" long, oval shaped. Very faint. It's amazing how little indentation caused on the paper. Good thing i never run at 40psi - it feels unsafe and gripless to me.

20 PSI

Tread mark 7.5" wide by 10" long, oval shaped. Treadblock markings clearly visible.

13PSI

Tread mark 8" wide by 13" long, oval shaped. Treadblock markings very clearly visible.

Interesting. I wouldn't have expected as much difference in width as you're seeing - have you measured your tyres shoulder to shoulder?

Roughly (ignoring the rounded corners - very rough) areas are 50in^2, 75in^2 and 104in^2. So the doubling I would have predicted between 40psi and 20 psi doesn't happen.

Alternatively, the figures don't make a lot of sense either - 50in^2 x 40psi would be 2000 pounds on one tyre. I presume you're using your Mav?

So, we need you to use graph paper, and calculate the area of the rubber on the road - minus the voids in the tread pattern.

Isn't science fun?

Scott

Post by **Beastmavster** » Wed Jul 19, 2006 9:11 pm

ok... with the tyreblack efforts it's a little more accurate on the lonnger patches.

13psi ends up only about 11.5" long, and 20psi about 9.5" long.

For those who want to see pics:

Left is 40 psi, then 30, then 20 then 13psi last.

Post by **Beastmavster** » Wed Jul 19, 2006 9:14 pm

If you wish to use graph paper you're welcome. You should clearly be able to see the difference, and also the darkness of the black area getting darker as we go along........

We're mainly looking at the proportional change versus the pressure change anyway, but you can clearly see the result.

If you really want me to mail them to you I can... but anyone could do this at home in half an hour, so long as they have an air compressor.

Post by **christover1** » Wed Jul 19, 2006 9:20 pm

thats impressive
can feel difference,
but have never seen it b4

christover

Post by **Beastmavster** » Wed Jul 19, 2006 9:27 pm

By the way the circles are "best guess" of the first effort. obviously the peper was not in exactly in the same spot for round 2.

Since you know the exact length of the A4 sheet, working out the area of tyre contact should be reletively easy.

Obviously this deflection will vary from tyre to tyre.

Anyone got a SWB Mav/GQ who can do this for 33x12.5x15 Muddies to compare? To get similar it would have to be a two ply muddy.

The Tyres are Cooper STT (old school) Light truck D load rating 2 ply poly sidewalls. Despite the two ply sidewalls I have to say the sidewalls in these are relatively tough - they DONT bag that easy.

Post by **Beastmavster** » Wed Jul 19, 2006 9:37 pm

sudso wrote: The increase in contact area on both sets of tyres when deflated from 35 to 10 psi was around 74%. The longer footprint gives extra traction which means more forward travel and less bogging.

Thats pretty close to what I got.... the 40psi is clealy half a page worth or less, and the 13psi one pretty much is the full width of the A4 page and the full length.

The potential maximum width of tread contact is about 10" wide at the most.

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