[vet 180]

I know if I asked what’s the best tyre size for an ungeared jimny everyone in here would say 215/75R15, now I can’t get that size in my region but I will use a 215/75R15 BFG KM2 as a base for what’s acceptable.

Ok KM2 measures in at 27.9x8.5 inches and weighs in at 35.5lbs

What I am looking at measures up at 28.4x8.5 inch and weighs in at 31lbs.

Does AZ consider this acceptable?

I am thinking the small difference in diameter will be substituted by less weight and be fine. There is another option of 26.9x8.5 inch and 27lb in the same tire that I also have as an option. Both options are SL radial tires.


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[Gwagensteve]

I’m on my phone and will reply in more detail later but less weight doesn’t negate gearing. I understand the auto Jimny with stock gearing suffers from wide ratio gaps. Taller tires will exaggerate this by reducing torque at the ground.

[fordem]

The difference in diameters between 27.9 & 28.4 is 0.5 inch, less than a 2% increase in diameter and roughly the difference between a new tire & a well worn one - I'd consider that to be of no significance, going the other way, it's a one inch drop in size, also, in my mind, of no great significance.

Now, I've no personal experience with the Jimny, but, I've upsized & downsized tires on many other vehicles, it takes a bit more than what you're considering to impact performance, and more so if it's an automatic transmission, as the torque converter allows more leeway than you would get with a manual transmission.

[vet 180]

fordem wrote:

The difference in diameters between 27.9 & 28.4 is 0.5 inch, less than a 2% increase in diameter and roughly the difference between a new tire & a well worn one - I'd consider that to be of no significance, going the other way, it's a one inch drop in size, also, in my mind, of no great significance.

Now, I've no personal experience with the Jimny, but, I've upsized & downsized tires on many other vehicles, it takes a bit more than what you're considering to impact performance, and more so if it's an automatic transmission, as the torque converter allows more leeway than you would get with a manual transmission.

Yeah this is what I am thinking, it’s such a small difference that tire weights would have a bigger impact. I do drive a lot of sand so un-sprung weight is something I am conscious of. I expect I will have substantially less un-sprung weight than most, as most will run steel wheels and LT 215/75’s muds and this is considered acceptable. I’ll be running the 215/80 with alloys.

Ok we are comparing to what is normally accepted as the max, so the stock size is just over 2 inches less than what I am thinking, but still not a huge difference. I have done more before on different cars and it was fine.

In saying all that I would be interested to see Steve’s response.


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[Eddy]

This really comes under the heading of "overthinking" particularly in reference to the sanity portion of your thread title.
The differences in weights and diameters are inconsequential.
Un-sprung weight in soft sand is totally irrelevant, footprint is by far the most important consideration followed by ... umm ... floatability.

I grew up in the Mallee sand dunes out from the riverland area in SA, been driving soft sand for 50 years. One of the best sand tyres I ever had was a pair of old aircraft tyres. on a 16 x 10 inch rim these things were up around 80 -100 kgs. Ride over sand that you'd sink to your ankles like it was graded road.

[fordem]

Hold on a second - you're not looking at a replacement tire 0.5" larger than the OE size - you're looking at a replacement that is 0.5" larger than what is considered acceptable - that changes the picture significantly.

The OE tire is 205/70R15 which is a 26.3" diameter, the 28.4 is almost 7% larger, and the 26.9 is about 2% over OE - you'll probably feel the difference, but personally I'd still be tempted to go with the 28.4

[Eddy]

fordem wrote:

...you'll probably feel the difference, but personally I'd still be tempted to go with the 28.4

and that difference will only really matter at the lights on race day ...

[Gwagensteve]

The commonly referenced diameter for stock Jimny tyres is 26"
28.4" is almost 10%. That's quite a bit.

Fordem - I think you're confusing what vet 180 is saying - he's implying that 215 75 is the maximum tyre diameter before gearing correction is required, so is going another 0.5" beyond that a show stopper?

The obvious answer is no, but that depends how the car feels on stock tyres. If it's borderline on big dunes, or you're only barely holding high range currently, then I think you'd notice a substantial difference between the 75 and 80 profile.

Unsprung weight is ALWAYS a consideration except in pure technical rock crawling (although total vehicle weight is still relelvent there - it's about minimising overall vehicle weight and maximising unsprung weight)

light cars outperform heavy cars in sand. - we know that better than most.

As for your aircraft tyre example Eddy - I fully understand thats a result of lots of floatation, and the high weight was a trade off, but vet is on huge dunes where added weight is always going to be an issue.

Also don't underestimate how much extra heat that ~10% increase in tyre diameter will put into your auto. Taller gearing = more converter slip= more heat.

[fordem]

Gwagensteve wrote:

Fordem - I think you're confusing what vet 180 is saying - he's implying that 215 75 is the maximum tyre diameter before gearing correction is required, so is going another 0.5" beyond that a show stopper?

No - the comment had nothing to do with vet 180 - I just realized that the original poster was not looking at a 2% increase over the OEM size - the way I feel is that once you get over 5~6% increase you can start running into issues, and at that point an additional 2% does make a difference ...

[alien]

Just as a point of reference, running 31x10.5r15 on my sierra and I weighed my tyres on rims:
Kumho KL71 are 30kg EA
Grabber SRL are 38kg EA

The difference in braking and acceleration between the two was incredibly noticeable, enough so that I sold the Grabbers!!

[vet 180]

Gwagensteve wrote:

The commonly referenced diameter for stock Jimny tyres is 26"
28.4" is almost 10%. That's quite a bit.

Fordem - I think you're confusing what vet 180 is saying - he's implying that 215 75 is the maximum tyre diameter before gearing correction is required, so is going another 0.5" beyond that a show stopper?

The obvious answer is no, but that depends how the car feels on stock tyres. If it's borderline on big dunes, or you're only barely holding high range currently, then I think you'd notice a substantial difference between the 75 and 80 profile.

Unsprung weight is ALWAYS a consideration except in pure technical rock crawling (although total vehicle weight is still relelvent there - it's about minimising overall vehicle weight and maximising unsprung weight)

light cars outperform heavy cars in sand. - we know that better than most.

As for your aircraft tyre example Eddy - I fully understand thats a result of lots of floatation, and the high weight was a trade off, but vet is on huge dunes where added weight is always going to be an issue.

Also don't underestimate how much extra heat that ~10% increase in tyre diameter will put into your auto. Taller gearing = more converter slip= more heat.

Yeah there are plenty of dunes I drive that are boarder line already, or need to be run in low range so at the top when loosing wheel speed I can bump it back to first to keep the wheels spinning enough to get it over.

Most common gear in the dunes is high first locked in trying not to drop below 4000rpm. In this gear it will often be borderline and if the revs drop due to lack of torque you can be in trouble. On those cases I’ll switch to 4L run second and bump down to first when needed.

The other times I’ll run 4L is when bouncing off limiter in first gear isn’t enough speed to get what I need done. I only ever use 2nd high as an engine rest (and trans cooking [emoji1787]) gear. If first high doesn’t give me enough speed I will use 3rd low and kick down through the gears manually mid dune.

Now one thing I will mention and it refers to Your initial post is that originally I thought 3rd low would be a golden gear for a lot of stuff I drive, mathematically it makes sense when looking at speeds, but in reality I find it doesn’t have the torque mid dune for much and it’s only used as a ramp up gear on the flat or downhill of a previous dune or really fast flowing dunes. For example there is a bunch of dunes I drive sometimes and the car feels more capable in 1st high as 2nd low is too short and 3rd low is too long. Even though first high on paper is not much longer than 2nd low it just sits in that 4000-6000rpm sweet spot. It seems that with the bigger tires 2nd low will be the the exact same gearing as 1st high currently. So I am thinking I will be committed to low range a bit more often. 3rd low I am sure I will feel the loss of torque especially at the start of a dune where I am trying to make it half way up before bumping back to second.

In terms of the rocks I drive I am more ground clearance limited than torque/gearing limited so no real need to discuss.

Highway is a concern I drive a 40km round trip daily at 120-130km/hour. So the car does needs to maintain those speeds. I know first gear will be a slug as it’s tall with the tiny stock tyres and that’s fine but need to maintain speed on the highway when it eventually gets there.

To Eddy, I understand floatation is very important, but have felt first hand in the terrain I drive the difference in wheel weight on much more powerful cars than the jimny.

To Alien, that Is big difference for the same size tyre, I would assume the grabbers measured up a lot larger? I am aiming for under 22kg (combined) for a 28 inch tall tyre, which imo is a good height to weight ratio for a radial.


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[alien]

Grabbers and KL71 both measure around 30.5" - the Grabbers were just FAR heavier constructed, though interestingly, both bagged out the same at 8-10psi... so the Grabber was 'soft' but obviously carrying a lot more steel/rubber/both. My BFG KM2 were also heavier than the KL71 but I never weighed them. You could just really feel it when swapping between sets.

IMO, after having driven on tyres as heavy as the Grabber followed by the lightness of the KL71, I'd advocate for that weight saving every single time if struggling to choose between 2 tyres.

Someone somewhere worked out the equivalent difference in weight in terms of load in the car and if I recall correctly it's 10x the weight. Ie; 32kg of tyre on the wheels = 320kg in the tray if you're correlating to how it feels.

[fordem]

alien wrote:

Someone somewhere worked out the equivalent difference in weight in terms of load in the car and if I recall correctly it's 10x the weight. Ie; 32kg of tyre on the wheels = 320kg in the tray if you're correlating to how it feels.

There are so many variables that affect this calculation that actually setting a number on it is misleading - gearing and wheel radius both have significant impacts on the math - you can also use a rim/tire combination that adds no weight and still have a significant impact.

Many years ago I swapped out the OEM 155/70R13 wheels & tires on a Swift GLX for a set of alloys shod with 185/60R13 tires - rolling diameter is within 0.25" so immaterial, weight of the assembled wheels was also virtually identical - what I added in rubber was cancelled by the lighter alloy rims, I don't recall noticing a significant loss in acceleration, possibly because I was no longer breaking the tires loose, I was absolutely delighted with the handling improvements of the wider rubber, braking was where the impact was most felt - increased rubber mass, further away from the wheel center acted like a flywheel - there were some scary moments, and I eventually upgraded the front brakes to use the GTi calipers & disks.

[vet 180]

alien wrote:

Grabbers and KL71 both measure around 30.5" - the Grabbers were just FAR heavier constructed, though interestingly, both bagged out the same at 8-10psi... so the Grabber was 'soft' but obviously carrying a lot more steel/rubber/both. My BFG KM2 were also heavier than the KL71 but I never weighed them. You could just really feel it when swapping between sets.

IMO, after having driven on tyres as heavy as the Grabber followed by the lightness of the KL71, I'd advocate for that weight saving every single time if struggling to choose between 2 tyres.

Someone somewhere worked out the equivalent difference in weight in terms of load in the car and if I recall correctly it's 10x the weight. Ie; 32kg of tyre on the wheels = 320kg in the tray if you're correlating to how it feels.

Yeah I have heard 4x is the calc to use for sprung to unsprung but prob feels more like 10.

I am still on the fence. If I go the 27 inch baby tire I save 4lbs a wheel, so say 2kg more or less. Time by 4 and you have 8, times by 10 it’s only 80kg I guess, but then you get the gearing advantages also. Both tires are same and relatively light for their size though.


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[vet 180]

fordem wrote:

Gwagensteve wrote:

Fordem - I think you're confusing what vet 180 is saying - he's implying that 215 75 is the maximum tyre diameter before gearing correction is required, so is going another 0.5" beyond that a show stopper?

No - the comment had nothing to do with vet 180 - I just realized that the original poster was not looking at a 2% increase over the OEM size - the way I feel is that once you get over 5~6% increase you can start running into issues, and at that point an additional 2% does make a difference ...

Yeah I made it complicated by comparing to what’s normally accepted. The raw numbers for you:

The 215/80 is a 8% increase over stock
The 215/70 is a 3% increase over stock


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[alien]

Yeah but as above, diameter plays a HUGE part in that equation, as to how much stored energy is in the wheel (flywheel). Less weight and smaller will store less energy than the larger diameter, heavier one.

Try:
http://hpwizard.com/rotational-inertia.html
(scroll down for a tyre specific calc)

If i throw my 31" tyres in I get equivalent mass of:
30kg = 56.9kg x4 = 227.6kg
38kg = 72kg x4 = 288kg

That's significant!

[vet 180]

alien wrote:

Yeah but as above, diameter plays a HUGE part in that equation, as to how much stored energy is in the wheel (flywheel). Less weight and smaller will store less energy than the larger diameter, heavier one.

Try:
http://hpwizard.com/rotational-inertia.html
(scroll down for a tyre specific calc)

If i throw my 31" tyres in I get equivalent mass of:
30kg = 56.9kg x4 = 227.6kg
38kg = 72kg x4 = 288kg

That's significant!

Here is what mine came up at in that calculator. This is after inertia:

Rims at 8.5 for both scenarios
215/70 23kg
225/80 26kg

Combined:
215/70 31.5kg
215/80 34.5kg

31.5x4 =126kgs
34.5x4 = 138kgs

Yes I plan to have a light set up as it suits what I drive.

As a comparison Your typical 215/75R15 KM2 that majority of people would recommend on a stock jimny steel wheel:

Tire 30.5
Wheel 14.5
45x4 =180

So a good 40kgs heavier than the big tire I am thinking of.

So here they all are together:
Option 1: 27x8.5 total 126kg
Option 2: 28.5x8.5 total 138kg
Majority: 27.9x8.5 total 180kg

P.s. thanks for that calculator. It’s awesome.

Yes Eddy I most likely am overthinking, especially since we are talking baby tyres, but still the process is the same of maximizing your equipment for the terrain you drive whether it be a jimny on 27’s or a JK on 40’s


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[alien]

What about keeping the current tyres and spend all the money on a rear airlocker?? =)

[vet 180]

alien wrote:

What about keeping the current tyres and spend all the money on a rear airlocker?? =)

need tyres regardless.


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[alien]

Fair call.

If I had a Jimny I'd run 215/75r15 muddies and twin lock it with CRMO front axles and full float rear disc.

[vet 180]

alien wrote:

Fair call.

If I had a Jimny I'd run 215/75r15 muddies and twin lock it with CRMO front axles and full float rear disc.

With those axle mods I would run 30’s min. Any reason for the 215’s with the axle mods?


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[alien]

Reliability/confidence in being able to use the lockers in anger.. plus, rear discs to remove soggy drum brakes.

I've never owned a Jimny though!

[vet 180]

alien wrote:

Reliability/confidence in being able to use the lockers in anger.. plus, rear discs to remove soggy drum brakes.

I've never owned a Jimny though!

Jimny driveline is weak, but not that weak. Never heard of any one having an issue with the front end on 33’s after beef up parts added, however the rear really starts to become a problem at that size, but full floater sorts that out. The sweet spot for cost and use is 30x9.5 with HD front, lockers and gears. Remove the swaybar and keep the lift low and it will be very capable.

I am still on the fence, but starting to lean towards the smaller 215/70


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[BlueSuzy]

Weights thread. Just for cross reference
viewtopic.php?f=5&t=23448&hilit=weights

[vet 180]

BlueSuzy wrote:

Weights thread. Just for cross reference
http://www.auszookers.com/forum/viewtop ... it=weights

In reference to this, the tyre model I was referring to in all my weights was the Yokohama AT-S

[SierraDan]

I went to 215/75’s on my Jimny.
It still felt short geared on the sand and I would happily have another 0.5” of tyre.

[vet 180]

SierraDan wrote:

I went to 215/75’s on my Jimny.
It still felt short geared on the sand and I would happily have another 0.5” of tyre.

Thanks for the input. I assume you drove in low range? Manual or auto?


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[SierraDan]

vet 180 wrote:

SierraDan wrote:

I went to 215/75’s on my Jimny.
It still felt short geared on the sand and I would happily have another 0.5” of tyre.

Thanks for the input. I assume you drove in low range? Manual or auto?


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I drove in 2H and 4H at one stage as it was firm and smooth.
But yeah, I spent most of the time in 4L. Manual G13

[sideways]

FWIW I went from extremely heavy 28.5" tyres to lighter 30s and I think it might have actually been a little peppier with the 30s, though that could be anti-placebo from how bad I thought it was going to be.

Interesting to hear your thoughts on gear choice. For any sort of decent dune my pre-VVT auto jimny was firmly stuck in 1st low, it couldn't pull anything higher though it wouldn't have needed much more power to do so. You just kind of slowly dropped revs until it dipped low enough that it had no power. Obviously the VVT auto is lower geared in low but I still couldn't pull 1st high. Maybe our sand is softer. Also the lesser flexibility of the pre vvt engine didn't help, it had nothing under 4000rpm and made max power at the highest revs. It would have been much nicer with a better dune climbing gear, it was pretty capable on sand but more momentum would have been great, it was really just relying on traction. You'd go up at like walking pace doing 6749rpm because the limiter is so aggressive you'd lose to much momentum if you touched it.

I found the lack of top speed in low pretty annoying (it wont shift into 4th in 4wd), VVT would be even worse!

[alien]

When my sierra was bog stock, I ran 215/75r15 cooper htt tyres and 5psi and it went EVERYWHERE in lancelin powder sand. The car was floating. We'd step out into the sand and sink to our ankles but the car was sitting up on top beautifully.

With stock tcase gears, I used to be able to change between high and low range on the fly too, so long as I rev matched it. This meant I could be going along in 2nd HI, change down to 1st HI, clutch it, change to 3rd, drop to Low, blip the throttle and carry on. This said, low range in a stock sierra still sees you doing around 80km/hr in 5th. I'm sure that's not unlike a Jimny!