I’ve been meaning to do a footprint article for a number of years, looks like 2012 is it. In 2005 & 2006 I did quite a bit of work footprinting various tyres as we were looking to change onto Bias tyres for the expedition work and I wanted to know just how much difference there was in footprints between the various tyre constructions.
Breaking out the paint, paper, a brush & the vehicle jack away I went. Each tyre was "measured" at a number of pressures from 60 psi for highway to 10 psi for extreme mud & soft sand. Considering the myths about wide & narrow in regard to floatation I was pleasantly surprised by quite a few of the results over the next couple of years.
I didn’t footprint a range of normal 4WD tyres, such as Cooper & BFG, I just didn’t have any to test but I may do that later on sometime. I was far more interested in the tyres we need to do our work. However I did footprint tyres on the front end of a Ford F250, the V8 version that is very heavy on the front axle. I tested the standard Goodyear Wranglers against Bias 14 ply. It was quite simply astounding how little difference there was between the two, considering the supposed difference in width and great difference in construction.
Anyway, I learnt the following;
| Steel Belted Radial | 65 psi & 20 psi comparison (same tyre) | |||||
| Michelin 4X4 OR – 7.50R16C (10 ply) | Unladen rear axle of 130 Defender | |||||
| Cold pressures | approx. 600 kgs per tyre | |||||
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These are the 7.50R16 tyres fitted on 130 Defenders when you buy a new one, or at least they were in 2005. They are a tyre of exceptional quality and last for a very long time. The mud pattern is a bit noisy on the highway but apart from that they are hard to fault, so much for the "cheapest tyre on the new vehicle" theory! I included them in this testing simply because I didn’t have any other "normal" 4WD tyres on hand. Everything else in the shed was either an All Steel Radial or Bias.
You can clearly see the difference in footprint as the pressure changes in the left hand image. The picture on the right is a direct difference between a maximum pressure for weight carrying (65 psi) and a low pressure (20 psi) that you might need in soft going or corrugations. A 30cm ruler has been added for scale.
| All Steel Radial | 60 psi & 20 psi comparison (same tyre) | |||||
| MRF Steel Muscle L – LT7.50R16 (14 ply) | Unladen rear axle of 130 Defender | |||||
| Cold pressures | approx. 600 kgs per tyre | |||||
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These tyres (more than one set) have done almost half of our desert work over the last 9 years. To put it simply they are exceptional bush tyres for every occasion, I cannot think of another tyre that can do as much as the MRF All Steel Radial. It’s just a pity we can no longer get them here in Australia. As it has been explained to me the Indian domestic market along with the Chinese export market consumes every tyre that MRF produces in this 7.50R16 construction.
Despite being the thickest heaviest tyre in 7.50R16 I have ever seen the footprint was pretty much identical to the normal "party balloon" Steel Belted Radials. I was astonished to say the least. So in a nutshell you can run thick heavy Radials and they’ll footprint the same as the famous whizbang 4WD tyres that you "must have" to go bush..... (as long as they are the same width of course).
| Bias – 12 Ply | 60 psi & 20 psi comparison (same tyre) | |||||
| MRF Super Miler – 7.50-16 | Unladen rear axle of 130 Defender | |||||
| Cold pressures | approx. 600 kgs per tyre | |||||
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Our favourite tyre for western desert off-track work is the MRF Super Miler in 12 Ply. Several sets of these tyres have carried us through about 6000 kms of off-track meanderings. Now if you're thinking that Bias don’t footprint think about this. Half of our off-track work has been in the Great Sandy & Little Sandy deserts. Now these are sandhill deserts that run east west & expeditions cross virgin sandhills, no established tracks.....our trips went south north. Our vehicle loaded for a month will normally be 3.5 tonnes and maybe a bit more until the fuel, water & tucker starts to be used.
I guess what I’m getting at is this. How much footprint or floatation do you actually need ???? The other way to think about it is this. How did the vehicles get around before the advent of Radials ????
Bias tyres in sensible ply ratings work well in sand !
| Bias – 14 Ply | 60 psi & 20 psi comparison (same tyre) | |||||
| MRF M77 – 7.50-16 | Unladen rear axle of 130 Defender | |||||
| Cold pressures | approx. 600 kgs per tyre | |||||
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OK, these are very strong tyres and see a lot of use with Kangaroo shooters. The M77's are quite close to being indestructible as a tyre, for stone country they are outstanding. The only problem we had with them was this; they are too stiff to footprint with the weight of our vehicle in sandhill country.
At the time we had a Ford F250 V8 travelling with us a bit. Fully loaded the F250 was much heavier than our Defender so we tried the M77 (the weight rating matched) on the big Ford & bingo, worked like a charm. The results of the footprinting on that F250 is further along in this report.
| Bias – 12 Ply | 60 psi & 20 psi comparison (same tyre) | |||||
| MRF Super Traction – 7.50-16 | Unladen rear axle of 130 Defender | |||||
| Cold pressures | approx. 600 kgs per tyre | |||||
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Another one of MRF's 12 Ply Bias tyres and it’s a good one too. We have had many vehicles on expeditions with these tyres and they have performed extremely well. They are not our favourite tyre for the simple reason that the tread pattern is fairly aggressive, which is not the best for sand work, but that has nothing to do with footprint.
I’ll point out that the tyre used for this was a new tyre and I believe new Bias tyres don’t flex like the same tyre that has done some work. I couldn’t prove this as I didn’t have a used Super Traction on hand at the time, I just suspected it.
OK, this is where we start to see how this footprint thing really works with the different tyres. The following images are direct comparisons and all of them are at 20 psi.
| 20 psi comparison | 20 psi comparison |
| All Steel Radial (14 ply) vs Steel Belted Radial (10 ply) | All Steel Radial 14 ply vs Bias 12 ply |
| As I mentioned above Radial tyres whatever the construction footprint the same at the same pressure, the difference is tiny, the above image confirms this. I found this result surprising but it confirmed what we had found in the desert travelling around. |
These are the two tyres we use pretty much all the time
in the desert, at a rough guess more than 30 tours & expeditions over a period of 9 years. Both tyres work exceptionally well in the desert and as you can see the footprint is not a great deal different. |
| Weight Comparison of the same tyre – Bias MRF M77 7.50-16 14 ply at 20 psi |
| Ford F250 front end vs Defender rear end (unladen) |
This is a very telling test of footprint. The exact same tyre at the same pressure but with different weight applied.
You can see the difference is quite noticeable and it is something to keep in mind when doing various things with 4WDs.
This little test produced the most surprising results. Both tyres were on the front end of a V8 F250. Now for those not familiar with these vehicles let’s just say with a big lump of a motor and a huge big transmission the front axle carries a lot of weight. The suggested tyre pressures for them have the front tyres as high or even higher than the rear tyres, I can’t really remember which it is.
Quite simply I’ll let the images explain the results. These were the two tyres tested;
| Top row – Goodyear Wrangler | ||||||||||||
| Bottom row – MRF M77 | ||||||||||||
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As you should be able to see, there is little difference in footprint until you reach 10 psi.
| 20 psi Comparison |
| Goodyear Wrangler RT/S – 265/75R16LT – 10 Ply VS MRF M77 – 7.50-16 – 14 Ply |
How could it be possible for a narrow tyre with 1950's technology on a split rim to have an almost identical footprint to a
modern wide tubeless tyre in a size renowned for floatation and 4WD ability – bit of a surprise eh?
| Goodyear Wrangler 265/75R16LT – 45 psi | MRF M77 7.50-16 – 45 psi |
So obviously these are both tyres inflated to 45 psi (above). I was interested to see how the sidewall flexed in both tyre constructions. I then let the pressures down to 15 psi (below), a pressure that can be needed every now and again when the conditions are wet & soft. Another time for lower pressure is climbing in steep country and also climbing very stony ground.
| Goodyear Wrangler 265/75R16LT – 15 psi | MRF M77 7.50-16 – 15 psi |
When you consider the sidewall strength of Steel Belted Radial tyres you don’t want the sidewall hanging out to much, but we all know that! I found the pictures above very interesting. When you look at the footprint pictures and then the above image of the Bias sidewall, you’d have to wonder why you’d use Steel Belted Radials at all in tough country with sharp bits that you have to drive over. Yes, I do realise that 15 psi is too low for normal work, but it was a good way to illustrate the difference in sidewall constructions.
Anyway, I hope folks have got something out of this little report. That old saying "a picture is worth a thousand words" is still correct even now.
Safe travelling,
MICK HUTTON
Copyright : December 2012
This information contains the figures which demonstrate the "Footprint" ability of the different tyre constructions and were tested on our Land Rover Defender rear axle, 1.5 Tonnes.
Tyre @ 90% wear
[Construction – Tread = 3 steel plies / Sidewall = 2 polyester]
| PSI | FOOTPRINT LENGTH |
INCREASE FROM 180 mm |
AREA cm2 | INCREASE FROM 228 cm2 |
| 65 psi | 180 mm | 228 cm2 | ||
| 50 psi | 192 mm | 7% | 234 cm2 | 3% |
| 40 psi | 200 mm | 11% | 268 cm2 | 18% |
| 30 psi | 207 mm | 15% | 274 cm2 | 20% |
| 20 psi | 237 mm | 32% | 322 cm2 | 41% |
| 10 psi | 280 mm | 56% | 364 cm2 | 60% |
Tyre @ 55% wear
[Construction – Tread = 4 steel plies / Sidewall = 1 steel ply]
| PSI | FOOTPRINT LENGTH |
INCREASE FROM 170 mm |
AREA cm2 | INCREASE FROM 208 cm2 |
| 60 psi | 170 mm | 208 cm2 | ||
| 50 psi | 185 mm | 9% | 236 cm2 | 13% |
| 40 psi | 195 mm | 15% | 255 cm2 | 23% |
| 30 psi | 210 mm | 24% | 276 cm2 | 33% |
| 20 psi | 240 mm | 41% | 318 cm2 | 53% |
| 10 psi | 270 mm | 59% | 363 cm2 | 75% |
Tyre – New unused, should be driven on before testing.
Bias tyres need to be "broken in".
| PSI | FOOTPRINT LENGTH |
INCREASE FROM 165 mm |
AREA cm2 | INCREASE FROM 220 cm2 |
| 60 psi | 165 mm | 220 cm2 | ||
| 50 psi | 175 mm | 6% | 229 cm2 | 4% |
| 40 psi | 195 mm | 18% | 250 cm2 | 14% |
| 30 psi | 200 mm | 21% | 264 cm2 | 20% |
| 20 psi | 205 mm | 24% | 297 cm2 | 35% |
| 10 psi | 225 mm | 36% | 315 cm2 | 43% |
Tyre @ 85% wear
[Construction – Tread = 8 plies / Sidewall = 6 plies
Data from unladen rear end Land Rover - too light to footprint well]
| PSI | FOOTPRINT LENGTH |
INCREASE FROM 160 mm |
AREA cm2 | INCREASE FROM 205 cm2 |
| 60 psi | 160 mm | 205 cm2 | ||
| 50 psi | 175 mm | 9% | 251 cm2 | 22% |
| 40 psi | 185 mm | 16% | 270 cm2 | 32% |
| 30 psi | 185 mm | 16% | 270 cm2 | 32% |
| 20 psi | 200 mm | 25% | 288 cm2 | 40% |
| 10 psi | 215 mm | 34% | 325 cm2 | 58% |
Safe travelling,
MICK HUTTON
Copyright : December 2005
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