Suspension choice for NS - OME vs Ironman, and which one?

[pronvit]

I know there's a lot of questions about suspension choice, and I know it's hard to choose suspension for NS that would work best in all situations. But what I want is to get some general understanding what to expect in terms of ride comfort from different options below.

Being in NZ, I'm choosing mainly between OME and Ironman suspension kits which are easiest to get and fit here. Also, I'm mainly talking about rear suspension as in the front I will only have a set of Ironman bash plates so that won't require any heavy-duty upgrade.

Initially I was thinking about a 200kg constant load OME kit which sounds just right for our trips (two adults, two little kids, tents, fridge, water, extra fuel, gear - usual stuff, pioneer platform, towbar). It has 40/50 mm lift, rear spring rate 122-147 N/mm.

However I'm worried it might be too firm for the kids in the back when we're just going around the city. So then I decided to choose a "medium load" kit - 98-129 N/mm spring rate, different shocks. My thinking is that everyday kids comfort is quite important, and even with this kit it will still be better than stock with full load.

But then I noticed that even the "performance" Ironman rear springs have spring rate of 137-166 N/mm which is higher than constant load OME (but the lift is 40mm). And Ironman 200kg constant load rear springs have 162-196 N/mm rate which is higher than even 400kg OME springs (the lift is 40mm vs 50mm again though).

So now I'm totally confused as to what to expect from these kits. Maybe I should mix and match springs and shocks instead? E.g. will springs from 200kg OME kit work fine with their "medium load" shocks and provide softer everyday ride but still good enough performance with heavy load?

[old Jack]

Knowing the spring rate is only a small part of the equation, you also need to know the free length of the coil and the total rear axle weight of your Pajero. Once you have these I can give you an you an idea of what the unloaded and loaded ride heights will be as well as the actual effective spring rate within the springs normal operating range.


OJ.

[pronvit]

Ok, I get it. Although I'm still not sure what I'll do with that data. Due to this being the first time I upgrade suspension, I need to understand consequences of the choices. I.e. if I choose 200kg constant load kit, how bad will it be with just 100kg load? Is it worse or better then choosing a "medium load" kit and loading it with 350-400kg of stuff occasionally? Both in terms of performance and comfort. Can I combine constant load springs with softer shocks to improve comfort?

[old Jack]

Once you have the data, either post it, PM it or email it to me.
Selecting the correct coils can be very hit and miss and it is particularly difficult with independent suspension where the load is not constant, like the rear of a Pajero!


I have a Excel calculator that does over 1000 calculations per coil, all I need to know is the free height and the spring rate and I can give you a good idea of the unladen and lade ride heights depending on the axle weights.


There will never be a perfect solution and you will have to make compromises in either comfort, ride height or load carrying capacity to some degree. All I can do is to help you narrow down your choice.


OJ.

[pronvit]

OME medium load - 400mm, 98-129 N/mm
OME 200kg constant load - 410mm, 122-147 N/mm
Ironman performance 350/340mm, 137-166 N/mm
Ironman 200kg constant load - 335mm, 162-196 N/mm

[old Jack]

Based on the OME info I have on file and the Ironman info supplied by the OP, the calculations are;

OME PN 3110 Medium Duty, Free length 400mm, 9.99kg/mm to 13.15kg/mm progressive spring rate.
EBH 572mm Rear axle load 1421kg at 11.91kg/mm spring rate.
EBH 535mm Rear axle load 1566kg at 12.61kg/mm spring rate.

OME PN 3111 Constant 200kg, Free length 410mm, 12.44kg/mm to 14.99kg/mm progressive spring rate.
EBH 572mm Rear axle load 1795kg at 14.021kg/mm spring rate.
EBH 535mm Rear axle load 1965kg at 14.71kg/mm spring rate.

Ironman Performance, Free length 340/350mm, 13.97kg/mm to 16.93kg/mm progressive spring rate.
EBH 572mm Rear axle load 1314kg at 18.49kg/mm spring rate.
EBH 535mm Rear axle load 1542kg at 19.26kg/mm spring rate.

Ironman Constant 200kg, Free length 335mm, 16.52kg/mm to 19.99kg/mm progressive spring rate.
EBH 572mm Rear axle load 1331kg at 20.54kg/mm spring rate.
EBH 535mm Rear axle load 1584kg at 21.30kg/mm spring rate.

What is clearly obvious is the Ironman coils that are much shorter in free length and have a much higher spring rate and this translates into less load carrying capacity at the same ride heights (EBH) and with a much firmer spring rate than the OME coils.

EBH 535mm is factory unladen height and this is a good target height for when fully loaded.
EBH 572mm is a modest 40mm lift and is a good target height for the unladen but fitted accessorised weight.

There are over 1200 calculated cells in each coil calculation and a complex set of formulas used. Because I do not not have the actual rate of progression I have to assume the rate of progression is linear. Another limiting factor is the specifications supplied by the spring manufacturers are +/- 5% to 7% so this adds another variable I cannot compensate for when comparing coils. No manufacturer provides actual load vs compression graphs and my calculations are based on the coils being compressed squarely but in real life they are compressed unevenly by the independent suspension so unless the coils are actually fitted to the vehicle and load tested my numbers are theoretical. I have compensated for unsprung weights of factory alloys fitted with 10ply LT265/70R17 tyres and the leverage factors of the independent suspension.

So moral of the story is "Coils ain't Coils!" and this is why it is so easy to make an incorrect judgement when selecting coils.

Sorry OP I have just made your head hurt!



OJ.

[pronvit]

Sorry OP I have just made your head hurt!

That's ok, thanks so much. I'll think about it and probably come up with some dumb questions.

Just couple things I've noticed:

OME medium load springs should have PN 3110 and their catalogue shows a bit different spring rate 98-129
OME 200kg PN is correct, and spring rate is just a bit different from the catalogue value of 122-147

But 3111 axle load values look strange to me - they don't intersect with medium load value range, and most important, even 1795kg is higher than the max allowed rear axle load. Is everything right there?

Oh, also, is any data available for OE springs?

[old Jack]

That's ok, thanks so much. I'll think about it and probably come up with some dumb questions.

Just couple things I've noticed:

OME medium load springs should have PN 3110 and their catalogue shows a bit different spring rate 98-129
OME 200kg PN is correct, and spring rate is just a bit different from the catalogue value of 122-147

But 3111 axle load values look strange to me - they don't intersect with medium load value range, and most important, even 1795kg is higher than the max allowed rear axle load. Is everything right there?

Oh, also, is any data available for OE springs?

I have corrected the typo errors in the OME part number and starting spring rate for the OME 3110 coil.

Spring rates supplied in Nm/mm are correctly converted to kg/mm, they are not a 1:1 conversion rate. 1Nm/mm = 0.101971621 kg/mm.

The 3111 axle load values, I have looked at the calculations and cannot see an obvious problem.

The OME 3111 is a very tall coil so it has to compress 170mm to get to a compressed length of 240mm which is approximately EBH535mm and at a spring rate that ranges from 12.44kg/mm to 14.17kg/mm.

OME 3110 has the compress 160mm to be at a compressed height of 240mm and at a spring rate that ranges from 9.99kg/mm to 12.12kg/mm.

Tough Dog Performance has to compress only 110mm to be at a compressed height of 240mm and at a spring rate that ranges from 13.97kg/mm to 19.52kg/mm.

Tough Dog 200kg has compress only 95mm to be at a compressed height of 240mm and at a spring rate that ranges from 16.52kg/mm to 23.1kg/mm.

Data I can find on factory coils for the LWB IRS Pajero are free length 350mm and a spring rate of 11.61kg/mm. I have the rear axle load at 1800kg. I suspect the limiting factor to the axle load is the coils and their ability to maintain at least minimum legal ride height of 488mm at maximum axle load. One day I might get around to load testing a new factory spec Pajero.


OJ.

[pronvit]

Spring rates supplied in Nm/mm are correctly converted to kg/mm, they are not a 1:1 conversion rate. 1Nm/mm = 0.101971621 kg/mm.

Oh right sorry, just got used too much to people using g=10 in everyday calculations.

Anyway, seems like 3111 will just look ridiculous without enough load, that means I'll likely get 3110 for the rear. I've also spoken directly to an ARB guy (my dealer wasn't very helpful), and he mentioned airbags which I was thinking about myself, and then said medium load + airbags would be better for me. I've read you don't really like airbags, but looks like they would provide the required flexibility for me. After all, I will probably not carry THAT much, and we don't have THAT many bad roads here, so I think it should be fine for occasional situations where I do need to have extra support in the rear.

[old Jack]

Oh right sorry, just got used too much to people using g=10 in everyday calculations.

Anyway, seems like 3111 will just look ridiculous without enough load, that means I'll likely get 3110 for the rear. I've also spoken directly to an ARB guy (my dealer wasn't very helpful), and he mentioned airbags which I was thinking about myself, and then said medium load + airbags would be better for me. I've read you don't really like airbags, but looks like they would provide the required flexibility for me. After all, I will probably not carry THAT much, and we don't have THAT many bad roads here, so I think it should be fine for occasional situations where I do need to have extra support in the rear.

All good. I do not trust airbags when they are the only suspension upgrade or they are fitted to compensate for under size coils. At some time they will fail, and most times it is the most inconvenient. I also do not see the value in spending $300 to $500 on airbags when the correct coils can be purchased for under $300. Coils are slightly quicker and easier to replace than removing the coil and fitting an airbag. On some coils it is possible to fit the airbags whilst the coils are still fitted but the you also need to insert the top of coils mount spacer and route the airlines.


OJ.

[pronvit]

So I've got my new suspension last week, and also installed all the bash plates (engine bay, transmission), and a hidden winch mount. In the end I chose 3110 medium load springs for the rear and 2914 springs for the front. 2914 are supposed to be used with a steel bullbar, I figured bash plates+winch mount+winch are about the same weight as full size steel bullbar alone.

However the resulting ride height numbers are worrying, unless I measured incorrectly - yesterday I got only about 520mm front and 540mm rear (with 1.5m roof platform, recovery and some everyday gear).

[old Jack]

So I've got my new suspension last week, and also installed all the bash plates (engine bay, transmission), and a hidden winch mount. In the end I chose 3110 medium load springs for the rear and 2914 springs for the front. 2914 are supposed to be used with a steel bullbar, I figured bash plates+winch mount+winch are about the same weight as full size steel bullbar alone.

However the resulting ride height numbers are worrying, unless I measured incorrectly - yesterday I got only about 520mm front and 540mm rear (with 1.5m roof platform, recovery and some everyday gear).

Double check the measurement first.

Also if you can get the car on to a weigh bridge and weight front axle weight, rear axle weight and total weight, then I can do some calculations on what ride heights you should have got with the coils you have fitted and what your options are.


OJ.

[rgrubby]

So I've got my new suspension last week, and also installed all the bash plates (engine bay, transmission), and a hidden winch mount. In the end I chose 3110 medium load springs for the rear and 2914 springs for the front. 2914 are supposed to be used with a steel bullbar, I figured bash plates+winch mount+winch are about the same weight as full size steel bullbar alone.

However the resulting ride height numbers are worrying, unless I measured incorrectly - yesterday I got only about 520mm front and 540mm rear (with 1.5m roof platform, recovery and some everyday gear).

Hi Pronvit
Sorry it's off topic, but I'm interested to hear about your hidden winch mount. Was it off the shelf or custom built? I'd be keen to see some photos if you have some handy

[pronvit]

Double check the measurement first.


Also if you can get the car on to a weigh bridge and weight front axle weight, rear axle weight and total weight, then I can do some calculations on what ride heights you should have got with the coils you have fitted and what your options are.

The numbers are right, maybe it's a few mm more but not much. Either I grossly underestimated the weight of my accessories or I don't know what. I'll find out the total weight but have no idea where to measure axle weights separately here.

Hi Pronvit
Sorry it's off topic, but I'm interested to hear about your hidden winch mount. Was it off the shelf or custom built? I'd be keen to see some photos if you have some handy

It's a winch mount from Asfir http://www.asfir.com/mitsubishi-Pajero%20/503020 I've seen some mentions of it here on the forums. I don't even have a winch yet so obviously can't say how well it will actually perform but so far I can highly recommend it - fast shipping, easy installation and looks strong. It fits very very well to all the stock parts and holes (for some reason I didn't expect it to be that good). The only problem is that it comes without installation instructions whatsoever, they only provide a detailed drawing how to put together the mount itself which has quite a few parts. On the other hand, once you've removed the bumper it's pretty clear what to do.

Sadly I forgot to make any useful photos after fitting it as I wanted to put the bumper back first thing in the morning, and with the bumper on it's, well, hidden.

[old Jack]

Axle weights are easy to get.
1. Just drive onto the weigh bridge as far as you can but leave the rear wheels off the weighing platform, get this weight recorded, this is your front axle weight.
2. Drive forward until both the front and rear wheels are on the weighing platform, get this weight recorded as your total weight.
3. Drive forward until, your front wheels are just off the weighing platform, get this weight recorded as your rear axle weight.

Once you have these figures either post, PM or email these, along with the part numbers of the coils you have fitted, so I can do some calculations.

OJ.