[Gwagensteve]

The temperature of the tread surface has very little to do with the temperature of the carcass, which is related to the friction of the belts as they deform. The point of using a highway drive cycle to determine the 4psi pressure is because carcass friction is high but there's sufficient airflow to cool the tread surface and therefore negate the effect of road surface temperature.

That's why the 4psi "rule" works regardless of ambient or road temperature.

By your reasoning, the 4psi rule couldn't be valid because it's a measure of road surface not carcass friction.

[Eddy]

Not my reasoning .. my recollection of Beaurepaires training course for tyre fitters a while back.

As the tread surface is usually attached to the carcass, it must all be related.
Internal carcass movement, tread movement, tread temperature, road surface temp, ambient air temp, rim temp, all must contribute, as they are all in direct contact with each other, even brake temp will contribute as heat is radiated out onto the rim.
If it were solely on carcass friction, then it would only work when aired down on uneven rocky surface where the casing is being punched and shoved every which-way.

[Gwagensteve]

It certainly does work when aired down and the carcass is being shoved every which way. My tyres generally gain 2psi from airing down to airing back up due to heat in the carcass. It's not valid in this case though because obviously deformation will be enormous and forces are in all sorts of directions.

If we applied the 4psi rule in urban driving (average speed of the vehicle in abnormal urban cycle is ~40km/h) I'd be running about 12 psi on road.

[Kailes]

OK , so we are straying away from traditional question , which is fine as I'm sure I'm not the only noob learning stuff here ,
So could people please throw me their sizes o. Their stock size rims , please not aftermarket rims . I'm just curious to see what y'all are running. To give me ideas for mine haha .

[Eddy]

Gwagensteve wrote:

If we applied the 4psi rule in urban driving (average speed of the vehicle in abnormal urban cycle is ~40km/h) I'd be running about 12 psi on road.

... which no-one will ever advocate and I did not suggest .. my point was that the rule need not be ignored completely in urban / suburban driving. In high traffic situations, the road temperature will definitely contribute enough to show up in tyre pressure variation.

I wrote:

It is possible to have a warmer tyre at 60 kph on a busy city road than on a wide open country highway.

[Gwagensteve]

You can run a 215 75 15 or 235 75 15 on a stock rim.
Beyond that you're outside of approved fitments, but 33 9.5 fit stock rims well (but are no longer available) as would 35 10.5 KR2 race tyres. (Which are nowhere near 10.5" wide)

Obviously anything other than 215 75 15 isn't a legal fitment on your vehicle, if that matters.

I have seen all sorts of tyres squeezed on to stock rims, generally these make baby Jesus cry.

[Eddy]

Currently running 235/75R15 on stock Vit rims.

[Kailes]

Eddie : on a jimny?

[Eddy]

Kailes wrote:

Eddy : on a jimny?

oop .. sorry on Vitara ...
just been over to the neighbour's place, and looking at his jim decided that 235s won't work but 225/75 may be ok.

[Kailes]

Hmm OK , thanks Eddie .
Please feel free guys to keep posting your size tyres on your jimny .. someone like me might find them useful .

[Gwagensteve]

How common is 225 75 15? Can't say I've ever seen it.

[Kailes]

Looks like wrangler makes a set of them

[Kailes]

Seems to be a few around on that size . Am kind of surprised .

[kenzook]

I've always wondered why u skip from 215 to 235 Steve. Now I Know,


Sent from my iPhone using Tapatalk

[fordem]

Eddy wrote:

Not necessarily ... Urban road surfaces can get warmer than country road surfaces simply because of the amount of traffic using that road. This will be transmitted to the tyre. It is possible to have a warmer tyre at 60 kph on a busy city road than on a wide open country highway.
Companies involved in resurfacing roads use different mixes for different road types and locations for exactly this reason.

It's in my nature to be curious, and this concept did not jive with my personal experience, so I did a little experiment...

I went to an intersection of two roads, one being a major highway and the other a side street with limited traffic - I took several measurements of roadway surface temperature with an IR gun, on the black surface of the roadway, on the painted white surface of the road markings, and also on black surface where it was virtually impossible for a the wheel of a vehicle to pass under normal circumstances (because of the adjacent curb). The distance between measurement points ranges from 15cm to 3 metres, the time between measurements was no more than 5~10 minutes (I had to wait for a break in traffic to be able to measure the traffic lanes of the highway).

Reference temperatures - black surface indoors - 95*F, light colored surface indoors - 93*F
Averaged roadway temperatures - black highway traffic surface - 113*F, black roadway surface - 110*F, white traffic markings - 101*F

The difference in temperature between the major highway and the side street adjacent to the curb was approximately 3*F, the difference in temperature between the black surface of the roadway and the immediately adjacent white painted line was approximately 10*F - these readings confirm my suspicion that the heat of the sun has a significantly greater impact of road surface temperature than passing traffic.

I also measured the roadway temperature in a residential community with very little traffic approximately two miles from the highway and perhaps 10 mins later and found the black surface temperature almost 15*F higher than those of the highway measured previously, and a similar variation with the white road markings being some 10*F less than the surrounding black surface - this difference is probably due to cooling effects of wind out along the highway.

[fordem]

Eddy wrote:

In high traffic situations, the road temperature will definitely contribute enough to show up in tyre pressure variation.

I think I have established (in my earlier post) that traffic volume is not a major contributor to road temperature, I now want to address the impact of road temperature on tyre pressure variation.

First - I don't think there is anyone who will argue that seasonal weather changes in a temperate climate (the south-eastern United States of America if you need to be specific) will result in significant changes in both ambient and road surface temperature. I also think there is little doubt that ambient temperatures affect tire pressures and that as the ambient temperatures fall, the tyre pressures also reduce - what I am addressing is not the drop in pressure associated with the change in ambient temperature but rather, emphasizing that the variation in pressure will still occur whether the road surface/ambient temperature is cold or hot - ie. the 4 psi rule.

I've been fortunate enough to have driven considerable distances, at highway speeds, in TPMS equipped vehicles that allow continuous monitoring of the individual tire pressures whilst driving, via a dashboard display, and I have been able to watch the way the tyre pressures change as the tyres warm up and eventually stabilize - the same patterns hold true in both hot (summer) and cool (spring & fall) temperatures - tyres with a lower starting (cold) pressure saw a greater increase in pressure - what this suggests is that the tyre with the lower pressure would have had more carcass flex whilst driving than one with a higher pressure, and that carcass flex is the primary cause of tyre warming and pressure change.

[alien]

Epic effort fordem!

[Eddy]

Gwagensteve wrote:

How common is 225 75 15? Can't say I've ever seen it.

TYB ran a set of Mickey Thompson Baja A/T on a Lada Niva. The Daughter decided they were cool, so she ran the same on her stock 2 door Vitara.
I also had a set of BFG on a Datto 720 and some s/h Mickeys again on the Vitara (2"body lift) when the 235s got to be past their prime (back to 235s now)

[Gwagensteve]

Just did a quick tirerack search.

16 tyres available in 225 75 15, most are OEM replacement (highway) winter and trailer tyres.

46 tyres available in 235 75 15, including proper all terrain and mud patterns including BFG.

It's not by accident I skip over 225 75 15.

Reminds me of the kiwi guys that used to be in the Suzuki club 20+ years ago. They put 215 85 16's on their cars because that's what they used to run in NZ. Even though their cars could run 235 85 16's which was (is) 363637363 times more common, offers more clearance and a bigger choice of tyres.

[Kailes]

Makes sense Steve , However I can't run 235/85/16s On my stock jimny rims ...can I ?

So 235/85?75?/15 ?

[MrRocky]

I think 245/75r16 is a decent size for a jimny and easy to gear on the newer jims via the auto tcase swap. Gv or xl7 rims with a set of spacers is about the perfect offset

[Kailes]

I thought as a collective we decided for me to stay on stock rims for now ...

[Eddy]

Kailes wrote:

I thought as a collective we decided for me to stay on stock rims for now ...

makes us sound like the Borg ...

[MrRocky]

How very 90's

[Gwagensteve]

Kailes wrote:

Makes sense Steve , However I can't run 235/85/16s On my stock jimny rims ...can I ?

So 235/85?75?/15 ?

235 85 15 isn't a real size.

I wasn't advocating 235 85 16's- (or 215 85 16) Or 16" rims at all really. I was just using it as an example of people ending up on a very uncommon size because in theory it was a good option, but in reality it meant they were running practically unique tyres with no advantage over more common sizes. These guys were running patrols etc.

[Kailes]

Ahh OK .

[ZUZUKI]

Another vote for 235/75R15 on your stock rims, with a little plastic trimming (front bumper especially)

Off topic for the OP
The 215/85R16 is an appealing size, IMO, Tall enough to be of useful increase over a 235/75R15 (carcass size calculation 235/75R15 = 733.5mm / 28.9" where as the 215/85R16 = 772mm / 30.4") and narrower than the 245/75R16, which is 773.9mm / 30.5".

This is relevant when a 15" rim just won't fit (on a SQ625 GV) & the 245 on a std 16 x 7" rim will have chassis rub issues.
However, as Steve says, the 215/85R16 is an orphan size - more common in the US as OEM tyre in 'winter tread' patterns at best.
Hankook used to list & supply in Aust. both Dynamic RT01 & RT03 M/T patterns, but recently discontinued.

Back to 245/75R16 for me (BFG M/T KM2), on modified GV wheels (GV centres (because nothing else fits any better) with 6.5" wide hoops.
Having said that, BFG list the M/T KM2 in 245/75R16 as 749mm - only 5mm bigger than the 225/75R16 listed at 744mm.
The BFG A/T KO2 is shown as 744 (29.3") for the 225/75 & 774 (30.5") for the 245/75 - might be a mistake in the 245/75R16 listing.

Rgs, Michael

[MrRocky]

The two sets of maxxis 245/75r16 i have had measured 30.7 and were pretty much the same size as a few 31" tyres on the market just fit a bit easier on a jim due to the width.

[Gwagensteve]

I think you need to step back and have a really good look at what you want to achieve Kailes. Many of us drive quite and/or have built highly modified cars and whilst there's tremendous appeal and satisfaction in doing that, it might not be what you need (or are prepared for, in terms of labour, compromise, or cost)

I would suggest that any tyre size above 215 will require gearing correction. This can be expensive and difficult in a Jimny, because their gearing is very differently organised compared to a sierra. You can swap in a Sierra transfer relatively easily but this is often provide excessive reduction. An automatic Jimny transfer swap is also an option

However, once you have the road tyres back on you're going to be revving the car too hard on the road due to the lower gearing.

If you don't correct the gearing you'll be very hard on the clutch and ultimately the car by being forced to slip the clutch in technical terrain, and travelling too fast at all times. If you add a locker, the increased traction makes this situation worse.

If you're driving with other well setup suzuki's, I completely understand the need/desire to move well beyond a 215 and into the realm of gearing reduction etc. If you're driving with patrols etc I can't really see the point. I'd choose some aggressive all terrains like radial treps(?) or BFG AT's in 215 and run a rear locker. If you want more traction at this point put a front locker in. This doesn't compromise the car on road and prevents gearing correction being a problem.

Just my 2C.

[greyghost]

............and you'll out drive the patrols