General questions about springs and shocks height.

Hello!

These days I was thinking about the different suspension options available for the Jimny and some other similar 4WD with rigid axles and I ended with a few questions that are quite general in nature. I looked around but I could not find a satisfactory answer, so I was wondering if you could shed some light about this.

It is my understanding that for the car to have the maximum flexing capacity the shocks should be compressed just 50%. This way, one of the wheels can go down half of the shock length while the other can go up the same distance before reaching the compression limit. When maximum flex is reached, one of the shocks would be fully extended and the other fully compressed.

However, I see that it is very common in the 40mm lift kits to sell shocks that are just 20mm longer paired with springs that are 40mm longer than the stock. At the same time, it is written in many places that regular height shocks are fine for springs that are up 20mm longer than the stock ones. However, if I was to combine a 40mm extended spring with a 20mm raised height shock, the shock would not sit in the middle, it would be a bit extended all the time and maximum theoretical flex could never be reached. Would it not make more sense to extend shocks and springs the same length? In other words, to combine a spring that is 20mm longer with a shock that is 20mm raised height, so maximum flex can be reached. I was thinking that maybe the reason for the other combination is that the people designing the lift kits are installing 40mm springs with 20mm shocks to compensate for the spring compression when the driver sits in the car and the vehicle is partially loaded, but in that case would it not be better to install higher rate springs?

Cheers!

Good questions. Although I personally feel there is no definitive answer. It really depends what you are wanting to achieve and use the vehicle for.

At the extreme ends of spectrum you get mud trucks, where ride height and clearance are often the most important factors.

https://truckandute.files.wordpress.com/2013/03/img_1286.jpg?w=768&h=511

They tend to have little extra droop and often limited up travel, although most will allow compression as they sometimes get air borne.

Other vehicles like Formula Off Road ride low to the ground for stability reasons. They tend to have more down travel than up travel.




What I'm trying to say is, it really depends.

For road modified 4x4s there is a trend to 'lifting' often with the belief that it allows you to fit over sized tyres. So this is where the market mostly aims kits at.

But most standard'ish road 4x4s often have limits in their design.

For instance, to get more droop a longer shock should typically allow this. But a longer open length also means a longer closed length. If the closed length is longer than the bump stop, you will top the shock out under full compression instead of hitting the bump stop. Which is uncomfortable and can break things.

So there is a limit to how much longer a shock can be. A way around this is to fit extended bump stops, which effectively just limit up travel. Reducing the total travel of the shock. Depending on many things, you may end up with the same or more overall travel. It will depend on what can be done within the dimensions of the upper and lower mounts between full droop and compression.

The next option would be to change the shock mounts to allow for a much longer shock absorber. But on the Jimny this is not so easy. The upper shock mount is inside the same cone mount as the spring on the chassis.



Which makes it very difficult to put a really long shock in there. You'd need major surgery or even relocate the shock to a new position.

On some vehicles like a Land Rover this is a lot easier as the shock mount is open and seperate.




To summarise. There is a physical limitation to what can be done on the Jimny. But also more importantly many kits are not aimed at improving flex or suspension travel (true with many vehicles, not just the Jimny) and are focused on lifting instead, while retaining a similar amount of travel as standard.

Also on the Jimny there is the front cross member brace that gets in the way if you have too much droop and the standard length of the brake lines. So some kits do not want to address these, allowing them to remain stock. But still wanting to offer a lift.

Therefore, when picking out what components you want to buy, it very much depends on what you want the suspension and vehicle to be able to do.

for the Gen2 Leaf sprung Jimny I have installed both OME and Traimaster 50mm lifts and they differ. The OME has shorter shocks, and the trailmaster longer shocks and bumpstops. The OME would not flex as much but had other qualities like a softer ride.

For the coil spring Jimny this might not be the same of course.

The thing with rigid axles is how the axles pivot around the bump stops when flexing. At full compression on one wheel the axle will push up against the bump stop creating a force that that push the other wheel down against the surface, giving you better traction.
Seems to work fine on the coil sprung Jimny

Great answer from 300bhp, only one slight addition - remember that lift won't improve under axle clearance, unless you fit bigger tyres (and I'd argue that there's little point lifting a vehicle unless you upgrade the rubber) - in which case the upwards compression could be limited by the bump stop and/or the spring compression and/or the bigger tyre fouling.. whichever comes first.

Thank you for the really useful answers!

So I think I understand now why many kits raise the springs 40mm but the shocks just 20mm, possibly to prevent tyre fouling when people fit large rubbers on the Jimny. But, also, according to your answers, it seems to me that it should be perfectly reasonable to fit 20mm longer shocks with 20mm longer springs, so the original proportions are kept. This would allow one extra 1cm of flexing up and down, which is minimum, but still. The setup should be also fine to fit 215 75 R15 tyres, since these are like one extra centimeter in diameter, and this falls well in line with the body lift. At the end of the day, you would end with 1cm of extra clearance (due to the tyres) and 2cm body lift due to the combination of shocks and springs. Am I right?

You can fit 215 75 15's on standard suspension
On standard wheels without spacers 
 

To complicate things further, some shocks such as those made by Gabriel have shorter compressed length than standard as well as longer extended length to suit a lift.

wrote:

Thank you for the really useful answers!

So I think I understand now why many kits raise the springs 40mm but the shocks just 20mm, possibly to prevent tyre fouling when people fit large rubbers on the Jimny. But, also, according to your answers, it seems to me that it should be perfectly reasonable to fit 20mm longer shocks with 20mm longer springs, so the original proportions are kept. This would allow one extra 1cm of flexing up and down, which is minimum, but still. The setup should be also fine to fit 215 75 R15 tyres, since these are like one extra centimeter in diameter, and this falls well in line with the body lift. At the end of the day, you would end with 1cm of extra clearance (due to the tyres) and 2cm body lift due to the combination of shocks and springs. Am I right?

A lift kit generally doesn't allow bigger tyres. Think about it. If the suspension compresses off road, all of your lift is gone. The wheels need to fit regardless of lift, unless you are going to be 100% Chelsea Tractor/Mall Crawler.

A lift can make it rub less often, as you'll need more suspension compression before the tyre may impact an area. Stiff/HD lifts also don't usually flex very well, which again may limit rubbing, but simply because the wheels don't move up and down much.

Extended bump stops may also reduce or prevent rubbing, because they will limit how far up the wheel can go under coimpression.


As for +40mm springs and +20mm shocks. The lift will reduce the amount of available down travel from ride height. As the shock will be extended by 40mm. A +20mm shock will return some, but not all of the droop because the shock is longer. In theory your total suspension travel should have increased (would depend on the closed length of the new shock vs a standard one). But the ratio of droop vs compression travel will have changed. Probably not enough to worry about. Other things to consider, a longer shock may cause the spring to dislocate and fall out of its seat. So you need a spring long enough to work with the shocks and vice versa.

A common practice on Land Rovers....


A +40mm spring and +20mm shock should be fine on a Jimny. I'd expect similar levels of flex to standard though and possible more prone to becoming cross axled. Improved dampers however may be worth the swap alone. And the lift will improve many clearances including approach, departure and breakover angles.

Thanks again for the answers!

Considering all you said, it seems that a 40mm longer spring with a 20mm raised shock should be fine then! It also happens that even if the spring is initially 40mm higher, it will be a bit compressed with the weight of the accessories and the people inside the car... What a mess, I have been mumbling all this for a few months by now and I am still not sure about what to choose. I already went through all the possible combinations in my head multiple times! Good news is that I learnt many things, which is great.

I definitely know those Jimnies jlines! I saw them yesterday night in a Youtube video and I think that it was you in there explaining that one has a Pedders kit because you could not find a good supplier for the first brand you tried. How is it that being from UK you did not go for the classic Bilstein B6 + H&R combination and you got the kits from here, the other side of the world?