How does this thing work – Rear shock DU bush sizing tool?

Just seen this thing on Rockshox’s website – https://www.sram.com/en/service/models/tl-rst-bush-a3

I’ve got a SIDluxe rear shock that’s been a PITA to fit bushings and mounting hardware (see other threads started in the past few months) and I’m wondering if this might be of use?

I’m aware of the process of burnishing tight fork bushings and know how the tools work, but how would one go about using one of the above for dear shock bushings / eyelets? There doesn’t appear to be any documentation on the Rockshox website, nor any videos of the process on YouTube.

Never seen that tool but with forks you are just pushing tools of increasing size through the lower leg bush until you hit the desired size/feel.

Unless you get a couple of taper pins I am guessing that you only have one “slacken it up” option, though maybe there is something I am missing.

It is much like swaging, if you have a google of that term you will see lots of examples such as pipes in plumbing etc. You are just changing the shape of the part you are swaging.

I suspect that tool will only work with metal/split bushes (lower leg bushes are the same for the most part). They are frequently a little flat at the split and the tool may resolve that. Can’t see it working with poly bushes.

Check the service manual for the shock, maybe it comes up in there?

When you ‘size’ a fork bush you don’t just size it, what you do is make it actually round – the reduction in friction comes mainly from the ability of the oil to film properly with a perfectly round bush as opposed to an ovalised one. With shock bush friction its down to bore toleracing mainly. Its a dry system so you can’t affect the quality of the lubrication like you can on a fork. I have shocks in stock where the bush fitment at each end is so different you need to use different brand’s hardware to make it work right. Madness, its a pet hate of mine.

To that end, Frank from Bounce ^ and I have actually been discussing offering a sized bush service, where we produce hardware in tiny increments of diameter, including a special low friction DLC type coating on the bushes. Because the tolerance of the shock should not change (without damaging it) and poly bushes are generally pretty consistent if you stay in brand & model, while the hardware would be expensive compared to generic options, it would be the case that you’d likely only ever need to buy one set per shock, job done, and the low friction coating combined with proper fit gives bearing-like performance even when using bushes. You would have to send us the shock in order to size the bushes to the shock, but that means you’d never waste money on mis-sized hardware again, so swings and roundabouts.

If we think there’s enough demand we might put them into production…

@benpinnick super interesting description, thanks.

From what I understanding you are saying, your proposed solution is a custom fit mounting pin / hardware that’s machined for each individual shock and eyelet, that would be compatible with a generic poly / igus bush because the manufacturing tolerances of the bushes themselves are pretty much always bang on?

Have you heard of this guy? https://huber-bushings.com/produkt-kategorie/gleitlager/ I ‘think’ his solution is to machine the poly bushes themselves to different tolerances, whereas the mounting hardware remains the same.

Sort of @Devash, what we’re talking about is a series of axles made to a series of small but significant enough diameter differences that with shock in hand we can pick the optimal hardware for each end (as opposed to custom made). Its easy enough to test whether you have the optimal fit or need to move up or down an axle size. We already do this at our factory for shocks where we don’t have hardware pre-installed, but its not a system, its trial and error with the axles we have to hand + a selection of bush types. Its a PITA.

Ive found that bushes from the same ranges from IGUS tend to fit within a narrow enough size window (with some tolerance for different sizes as they’re the softest component in the system) that you can generally reliably swap bushes without ruining a set up. and even if you had to try a few to get it perfect again if you’re at the edge of a tolerance, in bulk they’re cheap enough to be able to do that. The trick is to get the right set up to start. If we found there was batch variance in the bushes, we could supply a whole bunch at once.

Im not aware of the guy in your link – that seems viable I guess, but Im looking for something that can be made once and then with minimal effort fitted all the most common frames & shocks giving the best performance, as opposed to something custom each time. Also the average £10 vernier caliper + average home user isn’t likely to give accurate enough readings for what we’d want. Between the difficulty in measuring inner bores, added to that the precision of most verniers is only 0.1mm, before worrying about accuracy, its easy to be out enough to make a real difference. Better to send us the shock and do it right first time. It wouldn’t be for everyone, but if you have a £500+ shock it probably should have buttery smooth bushes not some notchy disaster.

Interesting. Is there a reason you don’t just ream the eyelets to a very slight oversize so they are consistent and supply a highly accurate bush and hardware kit in one size?

@tthew Because you’d invalidate the shock warranty, plus you’d machine right though the anodising thus exposing raw aluminium to the elements which would corrode.

I believe Intend do similar with the Hovershock. They make several pin diameters and select the appropriate one during assembly

So how do we tell at home whether we have a “good” bushing-shock-hardware interface or not?

I must admit I don’t tend to wear them out between services, so I let the suspension servicer change them.

My Bounce bushing tool is still sitting unused from a few years ago.

(Nice to see you here BTW Frank, hope things are going well in the world of bushings & tools)

Because you’d invalidate the shock warranty, plus you’d machine right though the anodising thus exposing raw aluminium to the elements which would corrode.

Two excellent reasons! 😁

Hey @chakaping thanks! Hope all is well with you.

So how do we tell at home whether we have a “good” bushing-shock-hardware interface or not?

Simple way is to put it in a vice sideways, and it should slowly fall down like its damped. Too fast and its too loose and will wear out too quick, too slow (or probably no movement more likely) and its too tight.

I should mention obviously over the life of the bush speed will increase until a point where there’s no resistance + play.