Is it really necessary to move the bump stops after a 2" lift??

[kiwi1973]

I was having an initial chat with an engineer that I may use to issue my engineering certificate in the event that I decide to go the route of fitting 33 inch tyres. I was surprised when he mentioned that he would also need to inspect my now long ago installed 2" lift. He mentioned that he will be specifically looking for the bump stops to have been raised by approximately the same amount as the lift - i.e. 2".

This surprised me, because the Pajero's total suspension travel length is completely unaffected by fitting lifted coil springs. All that happens is the new coil springs cause the Pajero to sit up higher within its normal suspension travel range. The engineer explained that because of the bigger springs, on full suspension compression there would be greater potential for the coil springs to contact themselves - i.e. the coils crashing together - which he explained could cause them to break. This is why he wants to see raised bump stops that would limit travel before the coils could crash together.

I can see his perspective, but equally it doesn't seem right that raised bump stops should be used to reduce the Pajero's range of suspension travel. It seems a little simplistic to assume that lifted springs automatically have more coil windings - sometimes they are just thicker with a higher spring rate.

Thoughts on this?? Anyone else been told they need to raise the bump stops following their suspension lift?

[old Jack]

The engineer is correct, you do not want your coils to bind/bottom out before the bumpstops are at full compression. You can calculate this by measuring all the gaps in the coil at the centreline of the coil and coming up with a total spring compression distance, measure the distance along the transevrese suspension arm from the inboard pivot the the centreline of the coil. Then measure the gap between the top of the compression bumpstop and add 30mm for compression, and also measure the distance along the transverse suspension arm from the inboard pivot to the centreline of bump stop. Once you have the 4 measurements then it is just a mathematical calculation to compare the maximum compression angles which will check if the coil will bind/bottom out or if the bumpstop will do the job it is designed to do.
If you want to PM me the measurements I can double check your calculations for you.

OJ.

[kiwi1973]

Thanks OJ. I am not opposed to being required to adjust the bumpstops if there is merit for the rationale, which you have confirmed for me.

[nj swb]

OJ is spot on (no great surprise there). The engineer is correct to be concerned about coils binding, but very incorrect to say the bump stop needs to be moved the same distance as the lift.

1. The "fully compressed" height of a "taller" spring won't increase by the same amount as the increase in length at its "normal ride height".

2. There is a multiplying factor between spring compression distance and wheel travel, due to the suspension geometry.

I would be concerned that the engineer made such a basic over simplification. Either he hasn't really thought about your job, or he doesn't really understand what he's talking about.

When I had Shorty engineered I spoke to three different engineers, and went with the most expensive - because he gave me the greatest confidence that he understood off-road 4wd modifications, and wasn't all about race cars and "hot rods". He was also happy to listen to me, answer my questions and explain the process - which is probably where the extra money really went.

I would speak to a few more engineers.

[conquistador]

Agree with others on coils bottoming out being something to avoid. Heavier and or linear rate are more likely to do this. A few of the current spring manufacturers has reduced the diametre of the wire used over the years due to improvements which would limit this posibilty.



Tell this engineer to use his time positively and contructively to design some A arms, cv and ball joints to get us 14inches of travel....... with hydraulic bumpstops if he must.

[kiwi1973]

I got this picture when reversing out of a difficult track - it shows my left rear suspension at full contraction and right rear at close to full extension. I checked and noted that my left upper control arm was at its bump stop, while the coil spring still was not that close to binding. Hence it seems that in my case I wouldn't need to adjust the bump stops.

Incidentally, although Paj's sometimes come in for criticism of their suspension travel, it's not that bad, in the rear at least. Perhaps not that bad of a compromise considering how nice and high the diffs are tucked up when in deep ruts. Besides, when the terrain gets really rough all 4wd's will have wheels in the air.