OEM Bump Stop length?

[cp[mike]]

Anybody know the lengths of the stock bump stops, Front and Rear? I don't have any available to measure, and I can't seem to find the information anywhere. Thanks

[johnl]

I don't know, but it's not very important, just so long as there is something soft (not too soft) to prevent the damper from bottoming out. Too long (within reason) probably isn't a big deal since the 'rate' of most damper mounted bump stops is very non linear, i.e. they are very soft for most of their compression and then suddenly become much stiffer for the last few cm of potential rod motion. These things become more of an issue the lower the ride height is...

[cp[mike]]

I don't know, but it's not very important, just so long as there is something soft (not too soft) to prevent the damper from bottoming out. Too long (within reason) probably isn't a big deal since the 'rate' of most damper mounted bump stops is very non linear, i.e. they are very soft for most of their compression and then suddenly become much stiffer for the last few cm of potential rod motion. These things become more of an issue the lower the ride height is...

Koni offers two versions that would fit our shocks (Koni shocks for a CB have a wider 12mm stem, not sure what stock stem diameter is):

Part Number...........Length ..............Type
70.34.05.000.0 ..... 45mm (1.8") ..... Linear soft
15.34.20.000.0 ..... 55mm (2.2") ..... Progressive soft

So based on your comment, I would guess that you would recommend the Progressive version (summitracing.com). Like you said, being progressive, the very beginning of the compression is extra soft, so I can't imagine the 1cm of length would make a huge difference at that point. Thanks for the info!

[johnl]

Koni offers two versions that would fit our shocks (Koni shocks for a CB have a wider 12mm stem, not sure what stock stem diameter is):

I don't know the damper rod OD either. IMO it would be somewhat better if the bump stop were a bit too tight than a bit too loose. If too loose then the bump stop will slide down the rod and always be sitting on the top of the damper body, and be rubbed by the damper rod with every rod movement. I doubt it would be a significant problem, but there is at least some possibility that this might gradually wear the hole even larger.

If it's somewhat too tight then I think the bump stop would (probably) still stretch enough to allow fitment, and the tension would hold the bump stop at the top of the damper rod. It might be more of a problem if trying to stretch a small ID bump stop onto a much larger OD rod (as you find on Mac struts), but as long as you can get it on I don't think it's going to cause a problem.

So based on your comment, I would guess that you would recommend the Progressive version (summitracing.com). Like you said, being progressive, the very beginning of the compression is extra soft, so I can't imagine the 1cm of length would make a huge difference at that point. Thanks for the info!

It's my understanding that elastomer springs (which in effect is what bump stops are) are inherently 'progressive', even if the shape is uniform along the length of the 'spring' (i.e. not tapered / 'shaped' etc. in any way). It's possible to make 'spring' X less or more linear than spring Y, by changing the shape / diameter of the spring along it's length, but not possible (as far as I understand it) to make the spring truly linear. So, personally I'd take the 'linear' description with a grain of salt.

I doubt it would matter which of those you used. If the ride height is substantially lowered then maybe use the shorter one. Either way, all you need is something that prevents the damper piston from bottoming out against the bottom of the damper with metal to metal contact (potentially damaging valves and possibly bending the rod).

My understanding is that most bump stops are very non linear, i.e. they compress for most of their 'compress-ability' with very little resistance, then suddenly become very stiff, which causes a harsh bottoming out of the suspension (but still protects the damper itself from bottoming out internally, which is the main thing). For this reason I think that an inherently stiffer bump stop (of X length) may counter intuitively provide a less harsh 'bottoming out' than a similar length but softer bump stop, i.e. the initial resistance will be somewhat higher (but still soft relative to the metal spring stiffness), but the rate at which the bump stop becomes stiffer will probably be less abrupt, meaning the change from softer to stiff will occur over a longer range of rod stroke and be less abrupt. If so then a stiffer bump stop may result in a less harsh bottoming out impact.

Some cars (e.g. Mazda MX-5, aka Miata) use bump stops that are intentionally in 'constant contact' with the top of the damper body (i.e. at normal ride height the chassis weight is at least partially resting on the bump stops as well as the steel springs). This means that the bump stop contributes a degree of 'progressivity' to the suspension rate when the wheel travels upward in 'bump'. At least for the MX-5 it's possible to purchase aftermarket 'bump stops' that are stiffer than the stock items, to stiffen up the springing of the car, also making the suspension rate more progressive (at least in bump, in droop the 'bump stops' lose contact with the damper body, so are doing nothing).

I'm not sure how long this actually works for because the constant working of the elastomer by wheel motion must cause the 'bump stops' to wear out more quickly than if the bump stop were only acting as a bump stop and only occasionally being compressed.

[Fleetw00d]

Probably a little late, but fronts are about 3.25 inches.

[johnl]

Probably a little late, but fronts are about 3.25 inches.

The first half or so of the length doesn't do much.

Sit one of those style bump stops on the bench, and then compress it by applying body weight with your hand. It will compress a lot just with the force a human can apply to it, let alone the force of the suspension bottoming out onto it. The 'bellows' section (the first three 'lumps' from left to right in the photo) has little significant stiffness, it's mostly the last third of the bump stop (the last 'lump') that does the real work. I.e. that style of bump stop is VERY non progressive (it will have a slowly rising rate that abruptly becomes a severely rising rate), mostly because of the shaping of the tube wall, even if the material itself is inherently progressive as it compresses (though it's still not a smoothly rising rate).

If you look inside the bore you'll see that it 'opens out' inside for more than half the bump stop length (like three 'voids', one inside each of the first three lumps, i.e. they are effectively hollow), so that squashing the bump stop mostly just bends the sides of the tube wall (quite easily), until it collapses onto the last part of the stop which is more 'solid' (no 'void' in the last lump).

It's only after the first part of the tube has 'collapsed' that the material itself actually starts to compress (rather than just change wall shape). Even when the material compresses, it's still fairly soft until the air in cells in the plastic foam is nearly fully compressed.

IMO rubber is a far better bump stop material, more consistently progressive as it compresses, and can be shaped to fine tune it's progressivity. Despite the material being harder than the polyurethane foam used in the above style of bump stop, rubber bump stops can give a less harsh bottoming out because they aren't soft soft soft HARD. The bump stop can be a lot shorter for the same or better shock absorption.

The downside is that the rubber is prone to eventually disintegrating if the damper leaks any fluid onto it. Also, a rubber bump stop should be prevented from sliding down the damper rod and sitting on top of the damper body because the resulting constant rubbing on the rod (as the suspension moves up and down) tends to rub some rubber off onto the rod, and this rubber adhering to the rod can then cause a problem with the upper seal. This doesn't seem to be such an issue with poly foam bump stops, which can sit on the damper body with no real problem.

I've made quite a few bump stops using spring shackle bushes (the kind often used in leaf spring eyes, with no steel sleeves). Note that some spring shackle bushes are made from rubber that is too hard for this purpose, but some are soft enough and can also be tapered down to improve the progressivity of the bump stop.