Bumpstops

trader · Postby **trader** » Thu Jan 03, 2019 10:42 am

While I can’t find my post on bumpstops I know Kevgoat Will be interested in this pic of my rear bumpstops which I replaced this morning with fat cats. If this is what can happen to OEM after 50000 I’m not concerned about the fmc’s longevity.

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No wonder I was complaining about the SE’s ride!

hks_kansei · Postby **hks_kansei** » Thu Jan 03, 2019 11:10 am

Wow, and I thought my old ones were bad!

At least mine were in one piece after ~160,000km, granted the rears had big splits down the sides, and the centre hole was flogged out.

weirdly, I cant find your old thread either.
Odd since I remember posting in it. Must just be the forum server having a fit and losing the odd thread.

KevGoat · Postby **KevGoat** » Thu Jan 03, 2019 12:11 pm

Holy moly that's crazy! Mine were also about 50,000 kms when I changed them out but they still seemed fine. Probably an indication of how much more your car is "properly dríven" than mine :roll:

bruce · Postby **bruce** » Thu Jan 03, 2019 12:48 pm

Someone explain what I'm looking at? It looks like two piles of dirt.

hks_kansei · Postby **hks_kansei** » Thu Jan 03, 2019 12:56 pm

bruce wrote:Someone explain what I'm looking at? It looks like two piles of dirt.

Crumbled bump stops.

The NB ones were a high density foam, unlike the rubber of the NA ones.

And yeah, these ones haven't fared too well, and have crumbled into high density dust.

bruce · Postby **bruce** » Thu Jan 03, 2019 12:57 pm

Oh, that is weird.

trader · Postby **trader** » Thu Jan 03, 2019 1:20 pm

Owned by “a little old lady” who drove it from Brisbane to Toowoomba regularly...just pulled the front ones - left hand side front has started to crumble. In the SE the front ones are yellow white and the rears pink.

hks_kansei · Postby **hks_kansei** » Thu Jan 03, 2019 2:07 pm

Standard NB are the same, rears a pink/red, fronts on mine were white, but they may have just faded.

Mr Morlock · Postby **Mr Morlock** » Thu Jan 03, 2019 3:44 pm

Its a bit odd - I would have guessed bump stops being moulded products - rubber or even plastic injection moulded- machining is not what you would expect for an OEM volume item. Foam products you would not expect to be used in a "shock" application.

hks_kansei · Postby **hks_kansei** » Thu Jan 03, 2019 3:51 pm

Mr Morlock wrote:Its a bit odd - I would have guessed bump stops being moulded products - rubber or even plastic injection moulded- machining is not what you would expect for an OEM volume item. Foam products you would not expect to be used in a "shock" application.

I agree that it's an odd choice.

especially since the foam (which is probably a foam rubber) isn't particularly dense or resistant to compression, you can squeeze them almost flat by hand.

Here's a diagram that shows the NA on the left, and the NB setup on the right.

The shock rod passes through a pair of rubber donut shaped bushes that sandwhich the top-hat (the ones with the left leaning strokes in the image)

I suspect these may form the final limit of the bump stop, being the last piece to stop the shock bottoming out in it's body.

Spac · Postby **Spac** » Thu Jan 03, 2019 3:57 pm

Foam bump stops work far better than rubber ones, because it foam is more progressive and compresses down more (meaning that they take away less shock travel).

Failure like this is usually due to exposure to UV and/or age, rather than usage.

hks_kansei · Postby **hks_kansei** » Thu Jan 03, 2019 3:59 pm

Spac wrote:Foam bump stops work far better than rubber ones, because it foam is more progressive and compresses down more (meaning that they take away less shock travel).

Failure like this is usually due to exposure to UV and/or age, rather than usage.

True, but the purpose of a bump stop is to halt movement before either the shock bottoms out and damages itself, or before spring bind.

Bump stops are supposed to take away a little bit of shock travel, to prevent the shock piston from trying to punch itself out the bottom of the body.

Magpie · Postby **Magpie** » Thu Jan 03, 2019 6:11 pm

This is the quandary. The MX5 was designed, at least in the case of the NA for the bumpstops to be an integral part of the suspension hence the low spring rates. The bump stops ADD to the spring rate and effectively give a progressive spring rate.

Hence when people go to 9/6 coilovers they complain because of a harsh ride.

In my case I had 4-5mm of bump travel before the bumpstops engaged on the rear

For a track car the bump stops should only limit the extremes of damper movement. Personally I would argue that any car should have bumpstops that limit extremes not add to the spring rate!

Spac · Postby **Spac** » Thu Jan 03, 2019 9:33 pm

hks_kansei wrote:

True, but the purpose of a bump stop is to halt movement before either the shock bottoms out and damages itself, or before spring bind.

Bump stops are supposed to take away a little bit of shock travel, to prevent the shock piston from trying to punch itself out the bottom of the body.

Of course, but a foam one can do its job and be fully compressed to (say) 10mm, rather than 30mm, then your whole shock can be 20mm shorter.
In reality, a rubber bump stop has to be very long to give the same progression a correctly specced foam one.

The late John Wills had a wall covered in tubs of bump stops, of varying lengths, shapes and durometers - there's a lot more to bump stops than "they should make them out of rubber"!