Are bump stops meant to be an active part of the suspension?
Discussion
I’m just fitting some new lowered springs & shocks to my SE E93 & noticed the original rear bump stops must’ve been touching the shock
The mark where the dust cover stopped is clear, so its easy to measure up inside it & see that end of the bump stop inside the dust cover was touching the top of the shock.
That one in the picture is from the drivers side, that had sagged a bit, so maybe it didn’t quite touch when it was new.
BMW call it an auxiliary shock absorber rather than a bump stop https://www.realoem.com/bmw/enUS/showparts?id=WL72...
They show different ones for the SE & MSport, I guess my best option would be to get some of the MSport ones, as mine are bad, the other has broken up.
My kit only lowers the rear 25mm so I guess that’s not much more than the MSport.
The mark where the dust cover stopped is clear, so its easy to measure up inside it & see that end of the bump stop inside the dust cover was touching the top of the shock.
That one in the picture is from the drivers side, that had sagged a bit, so maybe it didn’t quite touch when it was new.
BMW call it an auxiliary shock absorber rather than a bump stop https://www.realoem.com/bmw/enUS/showparts?id=WL72...
They show different ones for the SE & MSport, I guess my best option would be to get some of the MSport ones, as mine are bad, the other has broken up.
My kit only lowers the rear 25mm so I guess that’s not much more than the MSport.
Which dampers are you fitting? Are the lowering springs a slightly increased/progressive spring?
The bump stops on the OE dampers are designed so that when they bottom out under extreme load it doesn't just smash the damper rod into the bottom of the casing and the suspension has no give. Instead the bump stop gives a more progressive cushion at the bottom of the stroke.
Fitting a different one leaves you open to letting the damper reach the bottom of its stroke, i.e. fitting the MSport ones to non-sport dampers for example (which will be softer).
The bump stops on the OE dampers are designed so that when they bottom out under extreme load it doesn't just smash the damper rod into the bottom of the casing and the suspension has no give. Instead the bump stop gives a more progressive cushion at the bottom of the stroke.
Fitting a different one leaves you open to letting the damper reach the bottom of its stroke, i.e. fitting the MSport ones to non-sport dampers for example (which will be softer).
ultimately the bump stop is there to stop the suspension from traveling beyond a certain point.
So that point should be before the tyres touch the insides wheel arches. Often on modified cars with bigger wheels and tyres the original bump stop isn't long enough and the wheels will frequently touch the insides of the arches. That's not good for tyres, rust prevention, interior noise or comfort.
You can check a bump stop with the spring removed and the wheel put back on the car. Jack up the hub where the spring has been taken out until the weight of the car is resting on the bump stop. If the wheel still turns freely then the bumpstop is correct and working. If the tyre rubs or the wheel is stuck and won't turn then the bump stop is too short or the wheel is too big for the arch. Worst case scenario is that the shock bottoms-out before the bumpstop takes the load as that'll put pressure on the shock mounting points.
If it is using up too much of the free travel on the shock then more likely it'll be the shock which should be changed as the bumpstop has to remain an effective length if you want your car to work properly. A shorter stiffer shock will have potentially have more room for a longer bumpstop as it won't be sitting at the end of travel all the time... but more in the middle so potentially more space available for the bumpstop. Other suspension designs have the bumpstop outside of the strut assembly so this problem doesn't come up. Just mounted to the chassis instead so the suspension arms hit the bump stop and it works like that.
So that point should be before the tyres touch the insides wheel arches. Often on modified cars with bigger wheels and tyres the original bump stop isn't long enough and the wheels will frequently touch the insides of the arches. That's not good for tyres, rust prevention, interior noise or comfort.
You can check a bump stop with the spring removed and the wheel put back on the car. Jack up the hub where the spring has been taken out until the weight of the car is resting on the bump stop. If the wheel still turns freely then the bumpstop is correct and working. If the tyre rubs or the wheel is stuck and won't turn then the bump stop is too short or the wheel is too big for the arch. Worst case scenario is that the shock bottoms-out before the bumpstop takes the load as that'll put pressure on the shock mounting points.
If it is using up too much of the free travel on the shock then more likely it'll be the shock which should be changed as the bumpstop has to remain an effective length if you want your car to work properly. A shorter stiffer shock will have potentially have more room for a longer bumpstop as it won't be sitting at the end of travel all the time... but more in the middle so potentially more space available for the bumpstop. Other suspension designs have the bumpstop outside of the strut assembly so this problem doesn't come up. Just mounted to the chassis instead so the suspension arms hit the bump stop and it works like that.
Thanks for the replies, I‘m fitting a matched set of shocks & springs (if we can keep the piss taking to a minimum over the brand name I’d appreciate it 
)
It a TUV approved kit & seems good quality.
https://blog.demon-tweeks.com/performance/who-are-...
My front shocks were totally gone, I could see one went just after I got the car & I found the other had gone after taking it off. The front is done now, it’s lower than I was expecting, I took it out for a quick test before starting on the back, the ride comfort is amazing for such a low ride hight, felt much the same as the the E39 with standard Sports suspension (both on non runflat tyres). Will be interesting to see how it feels with the rear end done.
As I said my right rear spring had sagged badly so it seemed to make sense to get a set of 4 shocks & springs.
As you can see from the picture the spring looks like it’s progressive. The shocks themselves are almost exactly the same length as the SE original shocks, the rears there are just a few MM shorter. I was expecting them to be a bit shorter than that as they are matched to the shorter springs. I guess with them being the same length I possibly should go for the SE bump stops then?
I left the SE bump stops on the front (they were still in perfect condition) & they are touching but it drives very nicely & the guy from Demon Tweaks said to leave them if its feels ok.
I wonder if the standard MSport shocks are shorter then & what the difference is with the bump stops for them, maybe just a harder?
)It a TUV approved kit & seems good quality.
https://blog.demon-tweeks.com/performance/who-are-...
My front shocks were totally gone, I could see one went just after I got the car & I found the other had gone after taking it off. The front is done now, it’s lower than I was expecting, I took it out for a quick test before starting on the back, the ride comfort is amazing for such a low ride hight, felt much the same as the the E39 with standard Sports suspension (both on non runflat tyres). Will be interesting to see how it feels with the rear end done.
As I said my right rear spring had sagged badly so it seemed to make sense to get a set of 4 shocks & springs.
As you can see from the picture the spring looks like it’s progressive. The shocks themselves are almost exactly the same length as the SE original shocks, the rears there are just a few MM shorter. I was expecting them to be a bit shorter than that as they are matched to the shorter springs. I guess with them being the same length I possibly should go for the SE bump stops then?
I left the SE bump stops on the front (they were still in perfect condition) & they are touching but it drives very nicely & the guy from Demon Tweaks said to leave them if its feels ok.
I wonder if the standard MSport shocks are shorter then & what the difference is with the bump stops for them, maybe just a harder?
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