Air or Coil suspension & map gearbox or re gear

[Mike WK2]

Any knowledge / experience on a couple of things I’m considering for the next stage of my WK2 build, now it’s my permanent toy and not required for work. 
 

1 ) currently I run the air suspension which I think I have improved as much as is possible with the bilstein shocks, air links level kit etc: but ride off road is still quite harsh even when aired down and flex isn’t great. 
 

I have been looking at the air suspension delete that’s available in the states, which converts to a coil spring giving a permanent 2 1/2” lift. Which enables 33’s to be fitted. 
 

Does anyone have a comparison between air and coil in the sense of would this improve articulation as well as ride off road ? 
 

2 ) leads on from the above, I may be wrong but I don’t believe you can re gear the WK2 so if I went with the 33”s I would be increasing the tyre diameter from stock by approx 2 1/2”. Which I suspect if it was possible the recommendation would be to re gear ? If that’s not possible would a map on the auto gearbox raising the gear change points so that the change in gear doesn’t make the engine labour with the larger tyres, would this help ? 
 

Thanks for any thoughts? 

[V]

I guess air-springs that raise the height of a vehicle over it's static ride height on coil springs must be at least equal in spring rate or higher in spring rate than the coil springs which would explain why the ride height feels harsh.

 

With coils you can tune spring rate up or down. Either way you need to calculate the bound spring height when it is fully compressed to make sure you are not actually losing suspension travel by adding too many coil turns. This is why off-road coil-overs often stick out higher than the hood line, because if they didn't they would bind.

 

Does your Jeep have an 8 speed auto or a 5 or 6 speed? If it is a 5 speed do you know if it is a Mercedes transmission or a Chrysler transmission?

 

At a guess, 33" tyres on an 8 speed would be OK without regearing because of the deeper 1st gear. You may lose 8th gear but it should still give you overdrive in 7th. 6th is 1:1. The first gear on 33" tyres would likely feel like a Chrysler 545RE on stock tyres. It will have a negative effect on fuel economy but I am guessing that it shouldn't be any worse than a 5-speed WK2.

 

You will also need to change the tyre diameter in software. I don't know if one of the Wrangler gizmos can do this or if it has to go to a Jeep dealer to be set.

 

[Mike WK2]

Thanks V, 

 

I believe it’s the Mercedes gearbox and it’s 5 speed. 
 

I had anticipated the need to change the tyre diameter in the software, I have a really good local garage and it’s in for MOT in the next few weeks so he’s going to plug in the magic computer and see if his kit will allow him to change tyre size. 

[V]

I have not done any calcuations yet, but it appears that the Mercedes transmission has a lower 1st gear which would help with 33" tyres if keeping the R&P stock. However, you would likely lose the ability to use 5th gear much. Reverse would become similar to the 545RE with stock tyres.

 

The 545RE would lose some mechanical advantage in 1st gear but I don't know if you would notice on the street, but you would off-road using engine braking. I think 5th gear may become unuseable but as the transmission has two overdrive ratios, 4th would become 5th. In this transmission it's more like losing 1st gear. Reverse gear will also rely on torque converter slip.

 

Gear  W5A580     545RFE
1st      3.59     3.00
2nd a  2.19     1.67
2nd b  n/a      1.50
3rd     1.41      1.00
4th     1.00     0.75
5th     0.83     0.67
Rev     3.16     3.00

 

If you let me know the stock metric tyre size and the R&P ratio I can calculate an effective final drive table to compare stock tyres with 33" tyres.

[jeepstertim]

Does the WK run solid axles? if so, there will be a way of changing the diff ratios.  Google WK regear and I'm sure it will have been done in the US, I'd be surprised if the petrol and diesel models ran the same diff ratios.

 

Re articulation, I'd expect you to be able to achieve better with coils, albeit lifted, not stock.

[Mike WK2]

Hi Jeepstertim, 

 

No the WK2 is IFS & IRS. 

[Mike WK2]

Standard tyre size is 265/50/20, will have to do a google search on the R&P ration not one I know off the top of my head.

[V]

I need a couple of extra items of data:

 

1. Your current tyre size
2. The engine rpm with your current tyres at a known speed in 4th
3. The engine rpm with your current tyres at a known speed in 5th

Number 2 is important because the transmission is in 1:1. It doesn't matter what speed or rpm E.g. 61mph @ 2,450rpm just that the transmission is in 1:1 and both road speed and rpm are known.

Number 3 is just a safety check to confirm that the transmission in (2) was in 4th and not 5th. Same test just at around 70mph.

 

I am prepping for an expedition on Monday so take your time on doing the rpm tests.

 

 

Edited July 13 by V

[V]

I took a long lunch. Hopefully the PDF is self explanatory. Top table is engine rpm at road speeds for stock gears and stock tyres, bottom table is 33 tyres on stock gears. I looked around and the ZF axles appear to have a 3.45:1 gearing. I don't know your redline limit so guessed 5,000rpm.

 

Jeep WK2-final drive-rpm.pdf

[V]

Mercedes ML has 3.7:1 and 3.9:1 R&P options. If the ML's ZF units are the same as the WK2's, it is worth checking to see if they are swappable with yours.

[V]

I was curious, I did the calculations for the Mercedes ML 3.7 differentials and it works almost perfect correction for 33" tyres. The ML also came with 3.27 and 3.45 diffs so you would have to find out which model came with 3.7 that will fit your subframes and driveshafts. The ML calcs are on page 2 of the PDF.

Jeep WK2 with ML 3.7 differentials-final drive-rpm.pdf

Edited July 13 by V

[Mike WK2]

Thanks V, really appropriate the time and effort great help. 
 

I’ll do some digging around the ML’s and see what I can find. 
 

Thanks again.