How much seat post exposure is too much

[dirttorpedo]

Interesting. I measured the c-c and c-top distance on the Rampage and my Zaskar last night. The Zaskar was about 18.25 c-c and 20.5 c-top. The Rampage is 17.5 c-c and 18.5 c-top which explains why the 18 inch Zaskar works for me and the 18.5 inch Rampage doesn't.

 

[dirttorpedo]

Picked up a Rocky Mountain Hammer Race last night for $20. It measures 19.5 which according to bikepedia was the large size frame. paint seems in pretty good condition, but it looks like it has a stripped crank arm on it. Oh, well. I've removed a few of those in the past. Just hope the bb isn't seized.

 

[cyclotoine]

picks or it didn't happen? What year?

 

[dirttorpedo]

Yup, I'll get some up ASAP. Not sure of the year - my best guess so far is that its a 97. Black paint with yellow text with red outline for decals. Decals are in terrible condition. I may start a new thread in the bikes forum for the build. Only downside is I need a new seatpost - it takes a 26.8 which I don't have in my parts box.

 

[dirttorpedo]

Re: How much seat post exposure is too much (now with RM pic

Junky cell phone photos are the best I have now. I managed to remove the stripped crank arm and have the plastic cup out of the bb. I tried to get the drive side out, but even with a breaker bar I was not successful. I've soaked the drive side in penetrating oil to see if that helps unstick it.

 

[ultrazenith]

Thinking about the physics of all this, I very much doubt the seat post height would be a major issue. Let's assume you've got the post not higher than the manufacturer's max line. Using a 400 mm instead of 300 mm seat post will increase the leverage by only 25%, which is practically nothing and almost certainly within the manufacturer's designed tolerances. A frame or seat post will be designed and tested to withstand being used at the max extension, by a significantly overweight rider doing some serious off-road riding (although probably not extreme downhilling / jumps, etc.).

 

[jimo746]

Thinking about the physics of all this, I very much doubt the seat post height would be a major issue. Let's assume you've got the post not higher than the manufacturer's max line. Using a 400 mm instead of 300 mm seat post will increase the leverage by only 25%, which is practically nothing and almost certainly within the manufacturer's designed tolerances. A frame or seat post will be designed and tested to withstand being used at the max extension, by a significantly overweight rider doing some serious off-road riding (although probably not extreme downhilling / jumps, etc.).

hmmm... how do you know this?

 

[ultrazenith]

Thinking about the physics of all this, I very much doubt the seat post height would be a major issue. Let's assume you've got the post not higher than the manufacturer's max line. Using a 400 mm instead of 300 mm seat post will increase the leverage by only 25%, which is practically nothing and almost certainly within the manufacturer's designed tolerances. A frame or seat post will be designed and tested to withstand being used at the max extension, by a significantly overweight rider doing some serious off-road riding (although probably not extreme downhilling / jumps, etc.).

hmmm... how do you know this?

Which part?

The leverage just scales with distance from the pivot (exposed seat post length).

Manufacturers need to cover themselves from being sued by riders whose seat posts or frames have snapped and been dumped on the ground / impaled up the bum. Bets are off when it comes to super light race components, though...

 

[cyclotoine]

Thinking about the physics of all this, I very much doubt the seat post height would be a major issue. Let's assume you've got the post not higher than the manufacturer's max line. Using a 400 mm instead of 300 mm seat post will increase the leverage by only 25%, which is practically nothing and almost certainly within the manufacturer's designed tolerances. A frame or seat post will be designed and tested to withstand being used at the max extension, by a significantly overweight rider doing some serious off-road riding (although probably not extreme downhilling / jumps, etc.).

The seatpost in question is so overbuilt that marginal testing has likely been done on it. Something like a syncros back in the day? Those carried warnings that they were racing products meant for a season of use by a professional rider (what 140-170 lbs usually?) and that they made no guarantees after that. These old steel frames are pretty sturdy it is true but they were racing frames and replaced often. Something like a rampage was made to be light not necessarily durable. true the OP could likely ride that bike with no problem. But the fact is, it's a lot of post and a lot of leverage and the possibility of something happening is greater.

 

[jimo746]

But the fact is, it's a lot of post and a lot of leverage and the possibility of something happening is greater.

This ^^^.

not sure that an extra 100mm = 25% more stress either, plenty of other things to affect it.

I'm not suggeting that the OP's life is in imminent danger, but personally I would look at that bike and think "hmmm... seatpost too long, must find correct size frame".