Length of seatpost shim: is 6cm too short

[whojammyflip]

I'm building up a retro Specialized Rockhopper FS for my son, and the original seatpost was a 30.4mm boat anchor. So I bought a seatpost shim off Ebay, a Truvativ Husselfelt. I'm concerned that its only 6cm long. However, its nicely machined, and being from Truvativ, I'm guessing its been thought through. The bottom of the shim extends down to the bottom of the top tube weld, but no lower. Its a 17inch frame, and my son will probably have about 20cm of post showing above the top of it. Is this cutting it fine?

 

[bikeworkshop]

The post to frame interface needs to extend well below the lower weld for safety, especially aluminium.

What you've got would probably be fine for light, occasional use, but it's a gamble you don't need to take for the cost of the correct size post or a 100mm shim.

 

[whojammyflip]

I kind of suspected this, I've ordered a 30.4mm Cane Creek shim from BikeInn. The frame is steel, but its pretty light for a basic mountain bike frame at 2.2kg. Its the last of the steel frames Specialized made I think, double butted Ritchey Logic...seems similar to the Rocky Mountain Hammer. I'm surprised the fact its steel improve matters, I'd have thought that frames are all built to the same specs for strength. All it would take is for an unsuspecting uncle who weighs 100kg to have a go, and then break it.

 

[bikeworkshop]

I kind of suspected this, a 30.4mm Cane Creek shim from BikeInn is necessary, I think. Its steel, but its pretty light for a basic mountain bike frame at 2.2kg. Its the last of the steel frames Specialized made I think, double butted Ritchey Logic...seems similar to the Rocky Mountain Hammer. Does the fact its steel improve matters, I am going to get some new tyres and Enduro seals from BikeInn anyway, so I could just add a shim to the order, and put this down to experience.

Steel has a way superior fatigue life.

But Flex at the seatcluster weld will eventually break it, especially if the shim stops right by the weld itself, as the metal is often more brittle right by this point.

I'd like to see 2" minimum below the weld.

I'd suggest a half decent seatpost rather than a shim, but a shim is fine if l it's snug and long enough.

 

[whojammyflip]

Ha, overlapping posts, I edited mine above as I bought the shim from BikeInn just now. Makes sense now, regarding the fatigue characteristics of aluminium vs steel. I will keep an eye out for a 30.4mm Thomson. This is going to be a well dressed Rockhopper. I've bought some matching coloured Z3's in bright metallic blue, and am swapping over some Z2 internals.

The flex makes me wonder about my Voodoo Bizango which has 20cm of post showing as well. Feels like it has rear suspension, which is a tad unnerving.

Both these frames have externally butted seat tubes, about 1inch beneath the weld you can see they drop about 2mm in diameter.

 

[mk one]

Steel is a lot stronger, but agree with above, ideally below the top tube welds.

Shims are fine if long enough, I have run them on all my bikes and never had an issue, USE shims are the best I find, sjs has a good selection, I have linked one below for you.

https://www.sjscycles.co.uk/seat-posts/280-use-seat-post-shim-272-mm-id/

 

[whojammyflip]

Too late for the USE shim, I've ordered a Cane Creek! Thermoplastic would prevent stuck posts, which is an additional win for the USE shim.