Thread: tyres & pressures for Fraser Is

All things being equal I'd pick the 7.50s, will be better in sand for the same tread pattern / tyre construction. Also your speedo will be more accurate on the highway. At the end of the day though they will both work and which set has a good spare and is in the best condition?

Hi Didge,

There is probably 25% difference in tread face width, that extra width doesn't just disappear in the sand, that's real increased contact patch that provides extra floatation, just the same as airing down to increase the contact patch increases floatation. This has been supported by glass plate testing.

Re length, absolutely a larger diameter tyre gives a longer footprint and more floatation. Your calculations of a tyre 30mm bigger in diameter giving a footprint 95mm longer really do surprise me though and I think they are worth checking. You'll also find that with the 31x10.5 that with the 15" rim you can run virtually the same contact patch length as the 7.50-16 as sidewall height is closed to being maintained, I would estimate you'd be within 5-10mm.

I think there are some flaws in the comparison however and I don't believe you should run the 7.50 at "just a lower pressure". If anything I think they should not be run at as low a pressure. They tyre has a high profile and that profile brings instability. I can feel the difference in lateral instability even with 255/85 through to about 30-35psi and as the pressure drops the instability just keeps increasing. The narrow section and the high sidewall are just not able to provide control and it leads to rolling under of the tyre and a lot of vehicle movement when pressures are dropped. I would seriously question dropping pressures to much because of this instability.

Re comfort, a stiff sidewall will not ever provide more comfort than a more pliable sidewall. There is a very fundamental flaw in any argument that suggests this is the case. Compare pressures, but also see above re pressures.

I think 7.50s have there place in outback and agricultural settings, but really they have been superseded by modern tyres for performance in all areas including rolling resistance, but with the one exception of puncture resistance. As I also said before, every sand specific tyre is wide, not narrow.

Anyway, all of this in reality is largely irrelevant as they will both work at Fraser. Ngarla is the only spot where see people getting stuck consistently, but if your tyre pressures are right you should go through even on a soft day.

Out of interest, the best tyre that Ive used at Fraser was my Simex jungle Trekkers. Big fat open tyres. Someone stopped and commented that ill have fun digging with those as I aired down before the barge. Not ever the case because they just never spun to dig. We came through Ngarla which was really soft and all of the 4wds were getting bogged and some just couldn't get through, but I kid you not with the Simex I drove around one of the bogged 4wds that was sitting smack in the middle of the place. (Yes, we were helping bogged 4wds at the time)

Originally Posted by Slunnie

Hi Didge,

There is probably 25% difference in tread face width, that extra width doesn't just disappear in the sand, that's real increased contact patch that provides extra floatation TRUE - NO ARGUMENT THERE, just the same as airing down to increase the contact patch increases floatation. This has been supported by glass plate testing.

Re length, absolutely a larger diameter tyre gives a longer footprint and more floatation. Your calculations of a tyre 30mm bigger in diameter giving a footprint 95mm longer really do surprise me YEAH IT WAS ONLY A SUPER ROUGH CALC AND WAS BASED ON THE PREMISE THAT I'D AIR DOWN THE 7.50S TO GIVE THE SAME DIMENSION FROM SAND TO TOP OF TYRE AS THE 31X10.5S GAVE WHEN AIRED DOWN AIRED DOWN - IF THAT MAKES SENSE - SO IT WAS BASED ON AIRING DOWN THE 7.50S MORE THAN THE 31S WHICH ON REFLECTION IS PROBABLY A PRETTY DUMB IDEA though and I think they are worth checking. You'll also find that with the 31x10.5 that with the 15" rim you can run virtually the same contact patch length as the 7.50-16 as sidewall height is closed to being maintained, I would estimate you'd be within 5-10mm.

I think there are some flaws in the comparison however and I don't believe you should run the 7.50 at "just a lower pressure". If anything I think they should not be run at as low a pressure. They tyre has a high profile and that profile brings instability.YEP, DIDN'T THINK ABOUT THAT ONE AND A VERY GOOD POINT YOU MAKE I can feel the difference in lateral instability even with 255/85 through to about 30-35psi and as the pressure drops the instability just keeps increasing. The narrow section and the high sidewall are just not able to provide control and it leads to rolling under of the tyre and a lot of vehicle movement when pressures are dropped. I would seriously question dropping pressures to much because of this instability. I AGREE AND THANKS FOR POINTING IT OUT - I DO NOTICE HOW SQUIRMY THE TYRES FEEL WHEN I GO FROM HIGHWAY PRESSURE TO AROUND MID 20'S WHEN HITTING BUSH TRACKS.

Re comfort, a stiff sidewall will not ever provide more comfort than a more pliable sidewall. There is a very fundamental flaw in any argument that suggests this is the case. Compare pressures, but also see above re pressures.

I think 7.50s have there place in outback and agricultural settings, but really they have been superseded by modern tyres for performance in all areas including rolling resistance, but with the one exception of puncture resistance. As I also said before, every sand specific tyre is wide, not narrow.

Anyway, all of this in reality is largely irrelevant as they will both work at Fraser. Ngarla is the only spot where see people getting stuck consistently, but if your tyre pressures are right you should go through even on a soft day. I THINK (I KNOW) I'LL BE TAKING THE BYPASS TRACK THERE- AS GUYG SAID, THERE IS A LOT OF SAND MISSING AND EVEN AT LOW TIDE ITS NOW APPARENTLY A SWIMMING/ BOATING EXERCISE

Out of interest, the best tyre that Ive used at Fraser was my Simex jungle Trekkers. Big fat open tyres. Someone stopped and commented that ill have fun digging with those as I aired down before the barge. Not ever the case because they just never spun to dig. We came through Ngarla which was really soft and all of the 4wds were getting bogged and some just couldn't get through, but I kid you not with the Simex I drove around one of the bogged 4wds that was sitting smack in the middle of the place. (Yes, we were helping bogged 4wds at the time)

INTERESTINGLY ENOUGH I HAVE HAD AN IDEA IN MIND FOR A WHILE NOW - A LITTLE INVENTION YOU MIGHT SAY - THAT I'M HOPING TO GET FABRICATED AND TEST ON ON FRASER THAT MAY ELIMINATE THIS WHOLE TYPE OF DISCUSSION AS IT WOULD ELIMINATE THE NEED TO AIR DOWN OR THINK ABOUT TYRE SIZES. IF IT WORKS I HOPE TO MAKE SOME MONEY OUT OF IT. BETTER PULL MY FINGER OUT THOUGH, ONLY 3 WEEKS TO GO.

THE OTHER POINT AS 2STROKE POINTED OUT IS THE CONDITION OF MY SPARES, THE 31 IS BASICALLY A SLICK AND VERY MUCH A BOOKABLE TYRE IN THE EYES OF ANY LOCAL CONSTABULARY, WHEREAS I HAVE 2 GOOD SPARE 7.50S - MMHHMMNN - THE PREDICAMENT OF IT ALL.
SHALL LET YOU KNOW WHICH WAY I FALL AND THANKS AGAIN FOR ALL YOUR INPUT GUYS, IT IS REALLY APPRECIATED - THATS WHY I SUBSCRIBE TO AND LOVE THIS FORUM.

Originally Posted by Slunnie

Hi Didge,

There is probably 25% difference in tread face width, that extra width doesn't just disappear in the sand, that's real increased contact patch that provides extra floatation TRUE - NO ARGUMENT THERE, just the same as airing down to increase the contact patch increases floatation. This has been supported by glass plate testing.

Re length, absolutely a larger diameter tyre gives a longer footprint and more floatation. Your calculations of a tyre 30mm bigger in diameter giving a footprint 95mm longer really do surprise me YEAH IT WAS ONLY A SUPER ROUGH CALC AND WAS BASED ON THE PREMISE THAT I'D AIR DOWN THE 7.50S TO GIVE THE SAME DIMENSION FROM SAND TO TOP OF TYRE AS THE 31X10.5S GAVE WHEN AIRED DOWN AIRED DOWN - IF THAT MAKES SENSE - SO IT WAS BASED ON AIRING DOWN THE 7.50S MORE THAN THE 31S WHICH ON REFLECTION IS PROBABLY A PRETTY DUMB IDEA though and I think they are worth checking. You'll also find that with the 31x10.5 that with the 15" rim you can run virtually the same contact patch length as the 7.50-16 as sidewall height is closed to being maintained, I would estimate you'd be within 5-10mm.

I think there are some flaws in the comparison however and I don't believe you should run the 7.50 at "just a lower pressure". If anything I think they should not be run at as low a pressure. They tyre has a high profile and that profile brings instability.YEP, DIDN'T THINK ABOUT THAT ONE AND A VERY GOOD POINT YOU MAKE I can feel the difference in lateral instability even with 255/85 through to about 30-35psi and as the pressure drops the instability just keeps increasing. The narrow section and the high sidewall are just not able to provide control and it leads to rolling under of the tyre and a lot of vehicle movement when pressures are dropped. I would seriously question dropping pressures to much because of this instability. I AGREE AND THANKS FOR POINTING IT OUT - I DO NOTICE HOW SQUIRMY THE TYRES FEEL WHEN I GO FROM HIGHWAY PRESSURE TO AROUND MID 20'S WHEN HITTING BUSH TRACKS.

Re comfort, a stiff sidewall will not ever provide more comfort than a more pliable sidewall. There is a very fundamental flaw in any argument that suggests this is the case. Compare pressures, but also see above re pressures.

I think 7.50s have there place in outback and agricultural settings, but really they have been superseded by modern tyres for performance in all areas including rolling resistance, but with the one exception of puncture resistance. As I also said before, every sand specific tyre is wide, not narrow.

Anyway, all of this in reality is largely irrelevant as they will both work at Fraser. Ngarla is the only spot where see people getting stuck consistently, but if your tyre pressures are right you should go through even on a soft day. I THINK (I KNOW) I'LL BE TAKING THE BYPASS TRACK THERE- AS GUYG SAID, THERE IS A LOT OF SAND MISSING AND EVEN AT LOW TIDE ITS NOW APPARENTLY A SWIMMING/ BOATING EXERCISE

Out of interest, the best tyre that Ive used at Fraser was my Simex jungle Trekkers. Big fat open tyres. Someone stopped and commented that ill have fun digging with those as I aired down before the barge. Not ever the case because they just never spun to dig. We came through Ngarla which was really soft and all of the 4wds were getting bogged and some just couldn't get through, but I kid you not with the Simex I drove around one of the bogged 4wds that was sitting smack in the middle of the place. (Yes, we were helping bogged 4wds at the time)

INTERESTINGLY ENOUGH I HAVE HAD AN IDEA IN MIND FOR A WHILE NOW - A LITTLE INVENTION YOU MIGHT SAY - THAT I'M HOPING TO GET FABRICATED AND TEST ON ON FRASER THAT MAY ELIMINATE THIS WHOLE TYPE OF DISCUSSION AS IT WOULD ELIMINATE THE NEED TO AIR DOWN OR THINK ABOUT TYRE SIZES. IF IT WORKS I HOPE TO MAKE SOME MONEY OUT OF IT. BETTER PULL MY FINGER OUT THOUGH, ONLY 3 WEEKS TO GO.

THE OTHER POINT AS POINTED OUT WAS THE CONDITION OF MY SPARES, THE 31 IS BASICALLY A SLICK AND VERY MUCH A BOOKABLE TYRE IN THE EYES OF LOCAL CONSTABULARY, WHEREAS I HAVE 2 SPARE 7.50S - MMHHMMNN - THE PREDICAMENT OF IT ALL.


SORRY ACCIDENTAL DOUBLE POST

Something I found interesting when I was researching new tyres to buy: Beadell Tours - Tyre Footprints by Mick Hutton
Makes it easy to see, you can make you own choice then. Heaps of other tyre stuff on this site, geared towards outback off track travelling but interesting nevertheless

Very interesting article that Bigblackdog - thanks

Originally Posted by BigBlackDog

Something I found interesting when I was researching new tyres to buy: Beadell Tours - Tyre Footprints by Mick Hutton
Makes it easy to see, you can make you own choice then. Heaps of other tyre stuff on this site, geared towards outback off track travelling but interesting nevertheless

Am I being biased or is this what I've been saying all along...?

There are a small number of fundamental issues in play, but the misconception of the benefit of wider tyres seem to hang around...

Just look at the Camel Trophy's choice of tyres - point and case

Cheers,

Lou

It's exactly what you've been saying Lou. I can see valid points in both camps "arguments". The Camel tyres look like they're in the vicinity of 235s to me. With the tubeless tyres I worry about popping a rim and not being able to reinflate properly whereas with the 7.50s they have tubes which is a bonus in that regard. I'd like to wear out my 31 x 10.5R15s and replace them with 33 x 10.5R15s. I measured the sidewall height of the 7.50s today and will do the same with the 31s tomorrow (just out of interest). Oh, I was watching for a brief moment today a show on 7 or 9 called "Styles in steel" or something similar and it was on the Model T for and the subsequent Model A ford. And the old black and white footage showed them being given an hiding and what I did find interesting was one clip where a Model T Ford was hacking its way through some rather deep mud with the equivalent of motorbike tyres (more like postie bike tyres actually) and he just kept going and going.