Sunday armchair mechanics to the forum please.

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Sunday armchair mechanics to the forum please.

15 posts & 10 voices | Started 4 months ago by matt_outandabout | Latest reply from coffeeking

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matt_outandabout - Member

So having spent the last hour arsing round with a new rocker on my ST4 that won't fit...

So the questions:

1. Why do we use bearings on pivots on FS bikes?
They only move a few degrees and are subjected to big side loads. As I understand it cassette style ball bearings are best used when the loading is purely rotational, and the few degrees they move is asking for premature wear of one area of the bearing surface / balls.
Would bushings (like Turner?) or needle bearings or even cup n cone be better? or a hybrid bushing at an angle or curve?

2. Why are they designed to be press fit / fit only when any of thdimensions are perfect?
All pivots I have seen get rammed in against the paintwork, not bare metal. Not accurate(?). Why do we not face pivot surfaces on frames?
Any slack means replacement, not adjustment - so why do we not have an adjustment system (e.g. my old Sub5 you could snug up the swingarm clamps a bit if needed. Surely there is a simple system (like cup n cone) that could take up slack / be adjusted to allow for a mm or so difference in frame size or alignment?

3. Why do we rely on a single bolt at each rotating pivot?
Again, my limited home mechanical knowledge say (like a cup n cone bearing) you should have opposing bolt / nut to tighten down onto and prevent the rotation of the bearing from undoing the nut or bolt.

Come on STW, can we redesign these things?

Posted 4 months ago #
richmars - Member

I guess the simple answer is cost. They use what's available, instead of designing the perfect solution.
That's engineering.

Posted 4 months ago #
MartynS - Member

well point 1..

I've a 8 year old turner 5spot, I've replaced the bushings/pivots 3 times in this time.. so I'd say bushings are a pretty good solution.

Point 2
Again the Turner I have is faced surfaces for the pivots, and bare metal contact.

Point 3
As you've pointed out pivots don't move that much so there isn't a need for a locking nut. Threadlock or one of those nylock nuts does the job

so the conclusion is buy a Turner..!!

Posted 4 months ago #
matt_outandabout - Member

But they do move - even with threadlock my ST4 pivots need checking every couple of rides.

Posted 4 months ago #
ade ward - Member

Not all ball bearings are the same, some have the balls caged and some are called full compliment these have no cage and are almost full of balls, these are good for high load but small angles of rotation ,
A bush can be lower friction but as the bearing or bush doesn't rotate much the point of loading is in the same place so wear can be a problem

Some companies have got good designs as they offer a life time garentee for their bearings

Posted 4 months ago #
scaredypants - Member

cheap, easily replaceable so don't need to be long-term useable - lucky, given the mud, shite and jetwashing they suffer

minimal movement means careful alignment is unnecessary (I made this up) and replaceable anyway

I've never had a bolt unwind

When I eventually replaced the bearings in my trance it was because I'd dismantled it to take abroad and noticed they were pretty much seized. It did feel better afterwards but I can't honestly say that I'd have noticed otherwise. I would never notice a mm or so misalignment unless it caused a noise

Posted 4 months ago #
matt_outandabout - Member

But then that is the point - replacing is long winded, a faff and as I am discovering, still not good enough.

An adjustable, maintainable system would be better...

Posted 4 months ago #
pedlad - Member

Matt. Do you have any documentation / torque settings for the rear triangle on the st4?

Posted 4 months ago #
matt_outandabout - Member

nope. none on Oranges sit either (all other models are listed). I am going on 'that is snug but not restricting movement'.

Posted 4 months ago #
bencooper - Member

Probably because bearings are more readily available off-the-shelf - there might also be a factor of designers not really thinknig things through.

Bushes are better - in things like Bromptons and Birdys, the bushes last a long time, and the failure is slow and predictable. A bush just gets sloppier and sloppier, a bearing can disintegrate.

Even better are flexible pivots - some use flexy titanium or carbon plates, and of course there's the Ibis/Castellano flexy chainstay idea.

Posted 4 months ago #
TandemJeremy - Member

1. Why do we use bearings on pivots on FS bikes?

Cheap easily available, long life. Bushes need mainenentce ( greasing( thus grease nipples fitting adn using

2. Why are they designed to be press fit / fit only when any of thdimensions are perfect?
Standard way of fitting bearings

Surely there is a simple system (like cup n cone) that could take up slack / be adjusted to allow for a mm or so difference in frame size or alignment?

Once a cup and cone needs adjustment it is worn. Given the high loadings and the high leverage rations on a sus bike you would never be able to adjust it to zero movement. also far more expensive to make

3. Why do we rely on a single bolt at each rotating pivot?
Again, my limited home mechanical knowledge say (like a cup n cone bearing) you should have opposing bolt / nut to tighten down onto and prevent the rotation of the bearing from undoing the nut or bolt.

Its not rotationg, its only back and forth movenmetn so unlikely to undo.

[/quote]

Posted 4 months ago #
bommer - Member

A bush reamer for a brompton is about £600 last time I checked. Bushes are better on paper, but need care fitting. Fine for smaller production runs (brompton, turner) but if you are making proper quantities (trek, Spesh) Its a helluva lot easier to push a bearing in.

Posted 4 months ago #
matt_outandabout - Member

hmmmm.

The issue that started me thinking on all this is now resolved:
I disconnected the chainstays and seat stays, then flexed the seatstays (quite a bit) which opened up the casting on the end a smidge, until I could just about nip in the edge of a spacer. Then a wee tap and it was in. A few wiggles and I could get the bolts in.
However, the seatstays now don't sit properly straight - a cm or so out of line at the rear. No problem in that they flex back, but not ideal to say the least. The back was about 2mm out of line before.
Going to speak to Orange in the morning.

Posted 4 months ago #
bencooper - Member

Mine didn't cost that much - about £250 I think - but you're right that it's essential to do properly. I've fixed several Bromptons where another shop has just hammered in new bushes and pivot pin, and then wonders why it loosens up all the time

It's not really all that big a deal for mass production, though. Bromptons are definitely not made in small runs any more.

Posted 4 months ago #
coffeeking - Member

My old Coyote main pivot was a teflon bush on the single-pivot swingarm, went strong for years with no maintenance other than casting an eye over keeping the area clean. Had a lot more friction than a ball bearing setup. As mentioned, there's lots of reasons to make a compromise and usually its cost vs performance.

Facing shock mounts etc - not much point - most pivots for shock mounts are CNC'd metal and anodised - about as accurate as you can want. All the pivots on my bikes that are not CNC'd are on joints that have enough compliance for the misalignment not to affect the performance any more than other irregularities.

Posted 4 months ago #

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