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Post Posted: Mon May 20, 2019 11:53 pm 
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So there's this driveway - it's about 50m long, 20 degree gradient, and the first few metres are covered in a fine layer of damp, tiny, flowers from the overhanging tree.

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Attempting to drive up in 2WD, the Jimny's rear wheels get a metre up from the bottom, before they start spinning.

Roll back down, and shift to 4WD-High. This time, I get maybe 3m, before all four wheels begin to spin, and I'm going nowhere.

So the choice is either stall, or spin the tyres.

Back down again, this time to 4WD-Low, and huzzah! It climbs all the way to the top. First time I've used 4WD-Low, and it's on a driveway in a city.

The left turn at the top is "weird", steering feel is heavier, which I assume is normal for 4WD-Low?

SO the question - the car's manual warns against engaging 4WD-Low on tarmac - how does that warning interact with this situation?

Thanks.

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Post Posted: Tue May 21, 2019 12:16 am 
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As long as the wheels can slip/spin you'll be fine - the problem with 4WD (both high & low) on a hard surface is that as you turn a corner, each wheel on the vehicle is required to rotate at a different speed (revolutions/minute), and this puts excessive strain in the drive train and can cause damage - if the tires can slip/spin, this strain is released.

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Post Posted: Tue May 21, 2019 12:35 am 
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fordem wrote:
As long as the wheels can slip/spin you'll be fine - the problem with 4WD (both high & low) on a hard surface is that as you turn a corner, each wheel on the vehicle is required to rotate at a different speed (revolutions/minute), and this puts excessive strain in the drive train and can cause damage - if the tires can slip/spin, this strain is released.


So the problem is primarily a front wheels issue, or does it also apply to the back wheels?

I think the corner (I'm parking behind the silver car on the right) at the top might be a problem then, because the wheels don't really have the opportunity to slip, since the concrete is nice and dry and grippy by that stage.

What would be the recommendation - stop before the corner, handbrake, go back to 2WD, then hillstart for the remaining bit?

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Post Posted: Tue May 21, 2019 5:17 am 
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It applies to all four wheels - front wheels rotate faster than rear wheels, wheels on the outside of the turn rotate faster than wheels on the inside of the turn, and yes, it might be an idea to shift back to 2WD before turning.

Two things puzzle me, first, wheel spin is primarily a traction issue - 4L provides no more traction than 4H, so, in theory, you should be able to make it up in 4H, which is a moot point because drive train wind up will occur in both 4H or 4L, and second, how is the car making it up the slope if you can't do it in 2H?

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Post Posted: Tue May 21, 2019 7:26 am 
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The “weird” steering feel is the effect of bind up. I’m surprised the car will shift out if 4wd once you’ve completed the turn- I would have thought it would be jammed in 4wd due to bind.

I generally don’t use 4H at all, but if you use 4H on hard dirt it’s not uncommon to have to reverse and work the steering left and right to release the bind to permit shifting into 2H

That focus must have a secret.

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Post Posted: Tue May 21, 2019 12:43 pm 
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fordem wrote:
It applies to all four wheels - front wheels rotate faster than rear wheels, wheels on the outside of the turn rotate faster than wheels on the inside of the turn, and yes, it might be an idea to shift back to 2WD before turning.

Two things puzzle me, first, wheel spin is primarily a traction issue - 4L provides no more traction than 4H, so, in theory, you should be able to make it up in 4H, which is a moot point because drive train wind up will occur in both 4H or 4L, and second, how is the car making it up the slope if you can't do it in 2H?


if 4L is a lower gear ratio, my assumption was that it was letting me keep the revs above stalling, while the tyres rotate more slowly - that seemed to be the problem in 2H / 4H I'd get some slip, ease off the throttle and then it would stall.

The driveway is slippery / steep enough that a hill start at the bottom would give you immediate wheelspin, and there's no room at the bottom for any sort of run-up (narrow single lane, and then an unfenced dropoff / embankment).

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Post Posted: Tue May 21, 2019 12:48 pm 
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Gwagensteve wrote:
The “weird” steering feel is the effect of bind up. I’m surprised the car will shift out if 4wd once you’ve completed the turn- I would have thought it would be jammed in 4wd due to bind.

I generally don’t use 4H at all, but if you use 4H on hard dirt it’s not uncommon to have to reverse and work the steering left and right to release the bind to permit shifting into 2H

That focus must have a secret.


I recall a clicky sound as I pulled the tyres back to straight, and as I said, the steering felt very "heavy" is the only descriptor I can think of, but it went back in to 2H without any problems - I shifted it while stopped, wheels straight ahead.

The Focus gets some wheel spin as well, if the drive hasn't been swept, but it's an automatic so there's no stall issue. I suspect what's counting against me is the Jimny weighs less, and it's got BFG-AT tyres on it, so there's maybe less rubber contact-surface on the road than a standard road tyre would have?

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Post Posted: Tue May 21, 2019 1:08 pm 
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what tyre pressures are you running?

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Post Posted: Tue May 21, 2019 1:30 pm 
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It’s not the tyre tread pattern that’s the problem.
I imagine the focus has traction control and that’s probably making the difference.

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Post Posted: Tue May 21, 2019 1:46 pm 
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markmo wrote:
what tyre pressures are you running?


40psi. I've heard a zillion different versions of what the "correct" pressure should be - the tyre says 40-50 on its wall, the car's original tyres and the doorjam plate were ~24psi from memory.

I was running them at 24 (assuming that it would maximise the footprint on the road), but my mechanic recommended I stick with 40.

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Post Posted: Tue May 21, 2019 1:54 pm 
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matt_g wrote:
markmo wrote:
what tyre pressures are you running?


40psi. I've heard a zillion different versions of what the "correct" pressure should be - the tyre says 40-50 on its wall, the car's original tyres and the doorjam plate were ~24psi from memory.

I was running them at 24 (assuming that it would maximise the footprint on the road), but my mechanic recommended I stick with 40.


Give it a shot at 24 (assuming you don't have 500kg of cargo) and get a new mechanic.

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Post Posted: Tue May 21, 2019 2:01 pm 
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Gwagensteve wrote:
It’s not the tyre tread pattern that’s the problem.
I imagine the focus has traction control and that’s probably making the difference.


It's a 2005 focus, but it's definitely had "can't get up the driveway without sweeping it" moments, where the driver was spinning the tyres going nowhere and smoke was starting to appear (her first experience with anything remotely like this). The thing with the automatic is you can drop the revs as low as you like - that's the overwhelming feeling in the Jimny, you just need to inch up very slowly with the wheels turning as slow as possible, so they don't slip, but to have the wheels moving slowly enough that they don't slip, seems below the stall point for the engine.

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Post Posted: Tue May 21, 2019 2:04 pm 
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Time for a new mechanic.

If anything, the ideal pressure for road use would be lower than 24 psi- you’ve replaced a very lightly constructed tyre with a light truck tyre. It’s much stiffer. I run my load range C tyres at 17-20 psi on road.

The pressure on the sidewall is the pressure the tyre is rated at to carry its maximum load. You’re probably applying 25% of the rated maximum load. You don’t need 40 psi of air to support the weight of a Jimny. No wonder you have terrible traction.

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Post Posted: Tue May 21, 2019 3:01 pm 
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Just a crazy thought, but what about cleaning the driveway and pruning the tree? =) It also looks like there's a bit of mold or whatever on the driveway so perhaps a pressure clean too?

Side note: How terrifying is it when you're leaving and trying to stop before the road? There must have been a few pucker moments!!! haha

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Post Posted: Tue May 21, 2019 5:11 pm 
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Gwagensteve wrote:
Time for a new mechanic.

If anything, the ideal pressure for road use would be lower than 24 psi- you’ve replaced a very lightly constructed tyre with a light truck tyre. It’s much stiffer. I run my load range C tyres at 17-20 psi on road.

The pressure on the sidewall is the pressure the tyre is rated at to carry its maximum load. You’re probably applying 25% of the rated maximum load. You don’t need 40 psi of air to support the weight of a Jimny. No wonder you have terrible traction.


cool, i'll take that into account, cheers.

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Post Posted: Tue May 21, 2019 5:14 pm 
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alien wrote:
Just a crazy thought, but what about cleaning the driveway and pruning the tree? =) It also looks like there's a bit of mold or whatever on the driveway so perhaps a pressure clean too?

Side note: How terrifying is it when you're leaving and trying to stop before the road? There must have been a few pucker moments!!! haha


unfortunately, that was the day after the drive was swept clean - one rainstorm and the tree drops a carpet of flowers.

going down the hill is fine, car didn't seem to give any indications of slipping in 2WD, so it's just sllllooooow, and stop at the bottom to listen for drivers, cause it's a pretty blind exit from the driveway.

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Post Posted: Tue May 21, 2019 11:55 pm 
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Gwagensteve wrote:
Time for a new mechanic.


X2 - I was going to say your mechanic is an idiot, but Steve beat me to it, and he's explained the reasons why.

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Post Posted: Wed May 22, 2019 3:19 pm 
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Gwagensteve wrote:
If anything, the ideal pressure for road use would be lower than 24 psi- you’ve replaced a very lightly constructed tyre with a light truck tyre. It’s much stiffer. I run my load range C tyres at 17-20 psi on road.


One question, would you mind posting a picture of how your tyres look at that pressure? At 24psi, the tyre looks a bit flat to me, ie the underside is obviously narrower than the top.

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Post Posted: Wed May 22, 2019 3:54 pm 
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It’s not relevant- my tyres are 35” diameter.

Radial tyres are supposed to look like that. That’s where the traction comes from.

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Post Posted: Thu May 23, 2019 7:30 am 
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If you have a concern with the tire pressures here's the quickest way to sort it out.

Get yourself some chalk and find a straight level quiet stretch of road - mark a broad band across the tread of the tire and then drive a short distance in a straight line and then look at the chalk mark - if it's rubbed away in the center of the tread, the pressure is too high, if it's rubbed away at the edges of the tread, the pressure is too low - adjust the pressure as required and re-test. If the chalk rubs away on one side/edge of the tread only you have an alignment issue, either camber or toe. What we're doing here is simulating a hastened tread wear test - the chalk rubs off in the same way as the tread rubber would wear out, just quicker.

If you want another way, google the 4psi rule - I can't explain why this works, but I've fooled around with it on vehicles with tire pressure monitoring displays enough to know that it does work.


Last edited by fordem on Thu May 23, 2019 10:58 am, edited 1 time in total.
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Post Posted: Thu May 23, 2019 8:08 am 
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I’m a little reluctant to advise the 4psi rule on a light car with LT tyres fordem. The OP is concerned 24psi is too low- the 4psi rule would probably result in a road pressure well under 20psi in a Jimny on all terrains. I’m sure it works fine for a Jimny on stock tyres though. Light cars on stiff tyres can’t generate enough heat in the air in tyre to add 4psi of pressure.

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Post Posted: Thu May 23, 2019 9:00 am 
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In my Grand Vitara I run 24psi (BFG AT's and BFG MT's). In my Sierra I used to run 18-22 psi. It makes the ride on the road so much more pleasant.

I do not understand tyre 'professionals' suggesting you should run at 40 psi on a car that weighs 1 tonne. It's no 2.5 tonne Landcruiser, or a 40 series profile performance car tyre. You have plenty of sidewall and a car that would hardly make it flex to any damaging degree on the road.

The first thing I do when I get new tyres is let out half the air they put in!

I have been running these pressures for over 30 years (in Suzuki's!!) and have noticed no discernable increase in tyre wear and have rarely had a tyre issue (puncture) - certainly not one that could be contributed to 'low' tyre pressures.

Do it - lower your pressures. Use the suggested 24psi as a starting point and run them for a week. You'll be surprised at how much better the car feels, and have a go on your driveway in 2wd @ 24psi, you might be surprised at the difference.

Believe me, your car won't blow up, and your tyres will not explode with these lower pressures!

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Post Posted: Tue May 28, 2019 7:24 am 
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Keep it pegged in 2H tyres will spin a bit and find grip under the flowers.


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