I was advised in one of my (many) asking for advice threads, to try a straight seat post on my Marin to help with back pain. Would you say that this is a bad idea (correct position over crank wise ) ?
Not wishing to pervert this thread, but yes imho. As far as i was shown years ago, setting up is bike is a fairly simple process but you need to follow a method. However, this was pre computers, digital slow mo cameras and 28 million pages of internet space dedicated to varying philosophies and arguments! I don't really want to add to that.
My route is pretty simple and works for me.....that's it ....and despite having serious back problems im still riding plenty of miles.
1. Put the bars roughly where you want them.....i assume your not 19, you have a bad back and your not racing the Norba series....so lets start grips level with the saddle top
. This stem might be all kinds of wrong, but you need a starting point!
2. Roughly set saddle height and tilt. I use the heel method then tweek from there later.
3. Set your position over the crank, so set saddle fore/ aft. Too far forward you will be all on your thighs....too far back and you will do your knees in. Aim for smooth full rotation. Try riding almost full effort, are you bumping the bottom of the rotation? Can you still pull up on the back stroke? Are you using all your muscles? If not move it a bit 5mm is plenty, try again....
4. Now adjust the saddle hight again, as moving back/ forwards changes the height a bit.
5. Then adjust your length via the stem / bars. Aim for slightly bent elbows and the ability to see where you going without twisting your neck up too much. Your spine is strangely all one bit.....pulling one bit effects the rest. Ive lost count the number of peoples bikes ive "fixed" purely by putting the stem up! Or swapping for a 20mm shorter ahead. I keep a few old cheapo stems of different sizes just for try out purposes.
I tend to drop my stem as low as I feel i can sensibly go, ride for a few hours and see how it feels. If I've got back ache from the bend I jam another 5mm on.
There is a point where you could end up too upright and actually be adding weight to yout spine....but im my humble experience your unlikely to get to that angle on an mtb!
Marins have a very upright seat tube compared with say a rocky mountain. Putting in a straight seatpost will certainly bring you closer to the bars and straighten your back upwards, but then your not in the right position to pedal, which will create you a whole new set of back, hip, leg issues. Yes, there is a fair margin you can get away with, for instance if you have wrist issues, push back as far as you dare, to lighten your grip. But, you need to stay in the zone that allows you to pedal efficiency and comfortably. I have used a minimal set back post once, but that was with a specific saddle that had some very odd characteristics.....my position over the crank stayed where it normally was...
The other issue with marins were they were designed long. My 20.5" has a 600mm top tube and came with a 10o x 150mm stem! Im now on a 25o x 110mm. Basically 2" further back...PLUS 4" further up than my mid 20s self! Bars are 30mm higher than saddle at the grip centre......But it means i can ride for hours and im comfortable. Yes, it looks a bit funky, but its that or not ride....I know which I'm going to live with!