chagzuki

Posting here merely as I like the forum, though this is general technology.

I spent some time today reading/googling rubber/polymer properties just out of interest. I've been curious about rebound rate in elastomers; the Thudbuster website claims that "The elastomers provide the compression and rebound damping". . . I'm not really aware of any substantial dampening going on and of course it's rebound damping that would be desirable in a cheap and simple system such as the thudbuster. Googling brought up info on a range of polymers with unusual properties, mostly similar to memory foam. . . none of which seemed to have the desirable characteristic of compressing quickly but uncompressing not as quickly. . . instead compression and uncompression rate are tied. . .

And stuff like that. I don't really understand this stuff at all, I'm finding myself wanting to know more.

EM42

i've experimented with natural material[leather]+rubber on the brompton
so far the results are better than the stock elastomer which is too springy

here's how it looks:


you add leather or rubber rings to get the desired dampening

for the seat I prefer the damping properties of real leather like the brooks b17 or b67[with spring] I think leather[skin]
and human skin have about the same characteristic or properties that they work great together as opposed to other materials molded plastic, foam, metal springs or
polyurethane dampers which has different elasticity as our posterior. Latex/gum rubber is my next preference just my opinion.

shock absorbers using oil instead of polymer/spring are also great but they significantly add to cost and complexity, weight and maintenance.
Good ole leather material as ancient as human history is not obsolete in my book anyone wants to wear vinyl shoes instead of leather shoes ?

even with cycle clothing merino wool jerseys is my #1 preference but thats another story

chagzuki

Gosh, that's certainly a different way of going about things. . . is the leather running all the way through to the bolt/centre/i.e. I'm assuming you've chopped the brompton elastomer into layers? Doesn't it sog up in the wet?

Since posting here I've been prodding all things elastomeric. . . I've an eraser here that has a very slow rebound rate, and I'm left wondering why it's so hard to get hold of different types of elastomer for shock-absorbing useage?

jur

Rebound vs compress rate isn't it - it is basic loss that you're interested in, both for compression and expansion. Loss is what gives an elastomer its damping characteristics.

Natural rubber is more lossy than the poly-urethane ones that the Birdy comes with. It's pretty hard to come up with something better than rubber for approaching critical damping.

social suicide

I have a Thudbuster ST in combination with a Brooks 66 on my Raleigh Twenty. Today I took the Gorge-ous trail home from work instead of the highway. Gotta love the thudbuster. Magic carpet ride. https://www.noquetrails.org/wp-conten...ent-4.6.10.pdf

AEO

there's no material that is remotely similar to what rubber is capable of.

chagzuki

Incidently, on the Thudbuster ST manual it says this:
"The Thudbuster ST elastomer is made from natural rubber for its favorable damping characteristics but unlike the urethane LT elastomers, natural rubber can not be effectively colored and as a result all ST elastomers are black. To identify the stiffness of each ST elastomer, find the small dial that is molded into one side. The arrow indicates the elastomer’s relative stiffness. 1 is extra-soft while 9 is extra firm, 5 is medium."

I wonder 2 things: why natural rubber isn't used on the LT and whether this is out of date as I'm sure the ST elastomers are coloured?

chagzuki

Whilst googling rubber/elastomer things I was expecting to easily hit upon some type of official measurement for something like rebound rate, but I didn't find anything. Have I missed something obvious, after all this is a crucial property of this type of substance. ?

BruceMetras

Did you search on Shore Durometers ?

chagzuki

I'm messing around with some Poron insoles, the rubber seems to have appropriate qualities. Or perhaps it's too memory foam-like and would simply compress gradually under a constant load.

JulianEdgar

I think for long travel springs (ie a suspension with a low motion ratio) using rubber air springs as fluid displacers (ie the rubber spring filled with water), pressurised by an external plastic bottle and with damping achieved by restricting fluid movements works very well.

See https://www.autospeed.com/cms/A_111821/article.html and https://www.autospeed.com/cms/A_111868/article.html.

For a direct comparison of the ride quality of this system versus a sprung seat Brompton (same size wheels on both machines ) see https://www.autospeed.com/cms/A_111837/article.html.

For short travel springs (high motion ratio suspension) I prefer shaped rubber bushes.



I use this cut-down suspension bump rubber from a car on my Birdy rear. The shaping is very important as the spring rate can be made to increase much more progressively as the bush closes up.

I have never achieved good results from elastomers although I think the Thudbuster LT seat suspension works very well.

chagzuki

What's the thinking behind that shape? The narrower areas compress quickly until the wider areas touch?

AEO

my guess, is that shape allows the wheel to stick to the road over bumpy terrain without the tire taking most of the impact. On big bumps the rubber will bottom out on the narrow section, then compress less at the wide section and make the tire absorb more.

Or basically, it'll work as a damper for the majority of the ride, but keep the suspension from bottoming out on hard hits.

or if you want, you can look at this page. https://en.wikipedia.org/wiki/Bicycle_suspension

JulianEdgar

That's actually confusing the notion of springs and dampers.

The shape of the spring gives it a more progressive compression - the narrower bits compress more easily than the thicker bits. So you get a lot more travel before the spring becomes so hard it no longer compresses.