[prolecs]

Found a few mentions of a Jimny rear sway bar in the forum but would like to know if anyone has actually fitted one??
I changed from MT tyres to AT to get a bit more comfort on the crappy local roads, and the more flexible side walls make the Jimny feel like a jelly pudding. It seems to me it's all in the rear so I started thinking why not a rear anti-sway bar? In fact I had to stick my head back under to make sure I was not suffering a mental lapse. There is no rear anti-roll bar.
Yes I know a team of engineers have worn out many pencils designing the suspension, but with Suzuki that was back in about 1890, well maybe 1990, without body lift, big tyres etc.

Anyway, like to hear some comments esp if someone has tried it!

[Gwagensteve]

I typically recommend adding a rear sway bar to cars with front radius arms in order to raise rear roll stiffness, in order to balance articulation. I'm not sure anyone pays attention because flex is hektik and roll stiffness must be a bad thing, but whatever.

However, in your case I don't think the problem is the sidewalls of the tyre and I'm not convinced it will be resolved by a swaybar.

MT tyres don't typically have softer sidewalls than AT tyres, and assuming both tyres are radials, tyres are quite laterally stiff assuming you are running them at correct pressures. It's most likely the change in feel is due to tread squirm because of the increased void of the MT tyre. I'm willing to bet your AT's and MT both have a very similar load rating, implying the construction is pretty much identical.

Adding a rear sway bar will reduce body roll, and increase oversteer, but it won't make the tread stop squirming.

There's a few other things that are going on that aren't going to help the feel of the car on road though.

obviously, 80mm of lift. I assume that's spring lift, but this isn't all that clear as you mention "with Suzuki that was back in about 1890, well maybe 1990, without body lift," That's going to significantly raise the centre of gravity, as is the "custom aluminium roof rack on fitted tracks"

Then, you've raised the roll centre (I assume the raised panhard mount is on the chassis end) raising the roll centre contributes to the suspension "locking up" during body roll which correspondingly increases lateral load on the outside tyre and unweights the inside tyre, pretty much doing the same job as a swaybar. A High rear roll centre contributes to instability at speed, oversteer and poor traction. (if you want an interesting primer on roll centre, watch the episode of Homebuilt by Jeff on YT where he's putting a panhard rod on the rear of his Alfarrari.) I have a VERY high roll centre in the back of my car, which I did deliberately for off-road stability. It's extremely good, but its no road car. With a spool and the roll centre where it is, It's quite lively on fast dirt.

Here's my guess - you've adapted to the compromises you've made to the car's handling, and swapping to more aggressive tyres has focussed your attention on them, at the same time as you've reduced lateral grip and increased tread squirm. I think you should consider a more wholistic view of the way the car is setup. for example - 215's fit a Jimny with no modifications, so you don't have to have 80mm of lift. how often is the roof rack used? are the shocks in good condition and are they properly valved for the height and weight of the car? For example, OME shocks are very soft and with roof load a Jimny sways alarmingly with OME shocks installed.

[vet 180]

Gwagensteve wrote:

I typically recommend adding a rear sway bar to cars with front radius arms in order to raise rear roll stiffness, in order to balance articulation. I'm not sure anyone pays attention because flex is hektik and roll stiffness must be a bad thing, but whatever.

However, in your case I don't think the problem is the sidewalls of the tyre and I'm not convinced it will be resolved by a swaybar.

MT tyres don't typically have softer sidewalls than AT tyres, and assuming both tyres are radials, tyres are quite laterally stiff assuming you are running them at correct pressures. It's most likely the change in feel is due to tread squirm because of the increased void of the MT tyre. I'm willing to bet your AT's and MT both have a very similar load rating, implying the construction is pretty much identical.

Adding a rear sway bar will reduce body roll, and increase oversteer, but it won't make the tread stop squirming.

There's a few other things that are going on that aren't going to help the feel of the car on road though.

obviously, 80mm of lift. I assume that's spring lift, but this isn't all that clear as you mention "with Suzuki that was back in about 1890, well maybe 1990, without body lift," That's going to significantly raise the centre of gravity, as is the "custom aluminium roof rack on fitted tracks"

Then, you've raised the roll centre (I assume the raised panhard mount is on the chassis end) raising the roll centre contributes to the suspension "locking up" during body roll which correspondingly increases lateral load on the outside tyre and unweights the inside tyre, pretty much doing the same job as a swaybar. A High rear roll centre contributes to instability at speed, oversteer and poor traction. (if you want an interesting primer on roll centre, watch the episode of Homebuilt by Jeff on YT where he's putting a panhard rod on the rear of his Alfarrari.) I have a VERY high roll centre in the back of my car, which I did deliberately for off-road stability. It's extremely good, but its no road car. With a spool and the roll centre where it is, It's quite lively on fast dirt.

Here's my guess - you've adapted to the compromises you've made to the car's handling, and swapping to more aggressive tyres has focussed your attention on them, at the same time as you've reduced lateral grip and increased tread squirm. I think you should consider a more wholistic view of the way the car is setup. for example - 215's fit a Jimny with no modifications, so you don't have to have 80mm of lift. how often is the roof rack used? are the shocks in good condition and are they properly valved for the height and weight of the car? For example, OME shocks are very soft and with roof load a Jimny sways alarmingly with OME shocks installed.

I read it as he changed FROM MT's. So he ia trying to get some comfort back.

To the OP as you have increased COG and the front runs a swaybar and resr doesnt you will feel more in the rear. My jimny was way more balanced with the front swaybar disconnected, i ended up reconnecting after drivibg with a year off as my wife drove the car regularly. It did body roll a LOT more with it off, but the car was more balanced. You will just go the other direction adding one to the rear.

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[Gwagensteve]

Ah, I see, I don't know how I missed that- I read it a couple of times.

Regardless, pretty much everything I posted stands, the difference being the increased lateral grip of the tyres is highlighting the cars handling shortcomings. With less lateral grip, the tyres would bleed off traction before the car could wobble as far.

[vet 180]

Gwagensteve wrote:

Ah, I see, I don't know how I missed that- I read it a couple of times.

Regardless, pretty much everything I posted stands, the difference being the increased lateral grip of the tyres is highlighting the cars handling shortcomings. With less lateral grip, the tyres would bleed off traction before the car could wobble as far.

Yeah still solid info.


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[prolecs]

Thanks for that very detailed analysis and comments guys.
I suspect the grip might be the answer, nothing else has changed. The tyres are Hankook 215-75R15, the MT and now the AT2. Both have same load index and approx diameters. The MT's were quite old when they got low on tread, so maybe they got stiffer with age? The lift and shocks were done when I bought the vehicle (a suspension shop I believe), and although I would have gone for 40mm lift max, changing the springs etc is $$$ I can't justify now. Well not yet.
Will try taking the roof rack off and see how it goes. I tried that some time back with the old tyres and the body roll certainly changed but not dramatically (I had a roof top tent up there as a trial and that was a bit scary lol).
BTW the roof rack was taken off to see if fuel economy improved, but it made no noticeable difference to the terrible 10L/100km usage
Nor has cleaning sensors, opening the air box up, new air filter, new plugs, and changing tyre pressure. I guess now that it is a factor of the tall front profile having a lot of wind resistance.