Minimum Seat Post Insertion

gambeta_fc

I'm in the process of buying a new bike and seem to have ended up between frame sizes (56 v 58). The advice from my bike shop was to go for the 56 and they would replace the stem if needed. Playing it safe, I decided to go for a Retul bike fit at the weekend and the bike shop gave me the 56 version to take along, idea being if it did turn out to be the better frame size I could at least get the saddle height and setback done.

I got a position I was happy with on the Retul bike by going with a 130mm stem. When the saddle position was mapped across at the end though to the 56 bike the seatpost ended up just a few mm above the minimum insertion line.

Just wondering if there's any feedback on implications of running like that? The lower end of my concerns would be hoping to avoid creaks developing over time but mostly I'd be worried about any possiblity of affecting the integrity of the frame if say I were to hit a bad pothole. For info it's a carbon frame with carbon seatpost, I'm fairly slim build (<80kg).

Basically, is the minimum insertion a guideline, bang on where it needs to be or am I better erring on the side of caution and staying well away from it? From the frame geometry table, the top of the seat tube would be 25mm higher on the 58 frame.

Craig06

Why not just get a longer seat post?

C3PO

I have seen a number of MTB frames cracking as a result of the seat post being extended too far out of the frame. Admittedly there are probably greater forces at play when riding off-road! However by the sound of what you're saying you would probably be better on the larger frame anyway.

Alek

The minimum insertion line on the seatpost refers to the seatpost safety. To be safe about the frame, the seatpost must be inserted at least an inch below the top tube joint.

quozl

If you're saying that the minimum insertion line is in the seat-tube but only by a few mm then you're fine. I think this is what you're saying.

If you're saying that the minimum insertion line is NOT in the seat-tube, ie it's a few mm above the seat-tube, then I really wouldn't.

As with all engineering there will be a margin of error built in. So if you've got the minimum insertion line in the seat-tube then I'd be perfectly happy with that. Especially as you're adding a few extra mm of added margin!

If the bike shop will swap for an equally good seat-post that's longer then I'd do that now though - just in case you ever need to raise the seat-post, for example if you get shorter cranks. A few mm isn't much to play with.

Brian?

I'd absolutely get a longer seatpost. I think they go up to 450mm.

For the sake of €30 it's not worth the risk

http://www.wiggle.co.uk/gusset-lofty-450mm-seat-post/

gambeta_fc

Thanks for the helpful advise folks.

>If you're saying that the minimum insertion line is in the seat-tube but only by a few mm then you're fine. I think this is what you're saying.

Yip, that's how it is, the minimum insertion line is inside the seat tube by a couple of mm if I set the saddle to the recommended height from the fit.

The bike is a Cannondale Synapse, it's slightly different from normal where the seat tube doesn't go above the top tube. It comes with a 25.4x350mm seat post, I don't have it to hand here at the minute to measure but from a post on another forum the minimum insertion line is marked at 80mm or just over 3 inches. I'll ask at the bike shop about a longer version, if not I'll go up a frame size.

Cee-Jay-Cee

With a 130mm stem and the seat post at it's limit I'd be going for the 58cm frame and shorter stem and the seat post slightly less raised.

outfox

When I saw the thread title, I thought I had strayed into the after-hours forum...

check_six

Rode a small frame MTB with the seat post just beyond the min insert line for years without trouble.

Then it snapped. It was an alloy post and it sort of felt like the seat was melting away as I was getting closer to the back wheel. I got a surprise but it wasn't like a sudden snap like you may get with a carbon tube.

Replaced the seat tube with an extra strong extra long seat tube. I'd advise you do the same!

gambeta_fc

> With a 130mm stem and the seat post at it's limit I'd be going for the 58cm frame and shorter stem and the seat post slightly less raised.

Exactly what I've done. On the 58 the stack increases by 21mm and the reach by 8mm, I can get the same position by removing spacers and going with a shorter stem.

Ordered the 58 this morning, hopefully have it in time for a weekend spin.

Have to say the bike shop have been great, definitely gained a long term customer.

Raam

What height are you?

gambeta_fc

Just over 6 foot, think it's about 184 in metric.