Over simplified problem...
spuddy said:
For me, a too-low seat (anything lower than 10 or 15mm than i usually have) results in sore knees after 15 or so minutes in the hills.
5mm too low is very noticable to someone acclimated to riding, 15mm is totally obvious and would be uncomfortable as you won't get your full leg extension. When I head to the, uh, hills, I make sure my seat is already up to the right height. On a mountain bike which is (should be!) designed around geometry for a properly located saddle possition at full leg extension (slight bend in knee, just how much is another topic...), this matters. BMX bikes and urban/dirt jump bikes usually are not designed to put the seat in the correct possition for seated climbing anyway, whether that's too low or too far forward or backward, those bikes are not designed foremost with any seated climbing in mind... my Cannondale Chase dirt jumper had a seat tube so short you couldn't raise the seat high enough with a 350mm post, and my Evil Imperial has such a laid-back seat angle that the seat is too far back for a proper seated climbing possition, even with barely enough post with a 410mm post. I mention all this because when you start talking how high you have your BMX saddle, a lot of this is a moot point. I have a Mosh BMX saddle on my Evil, and it is about the most uncomfortable seat to climb on because it wasn't designed for it. BMX saddles don't have the propper sit bone support for seated climbing and this matters about as much has riding with your seat at the right height. I raced BMX for seven years. Those bikes are not designed to be ridden seated, much less seated climbing.
spuddy said:
My BMX saddle is a full 10 inches too low, but, alas
.
10" too low for what? Dirt jumping? (I'm assuming you have that saddle on a mountain bike of some type here or this is a moot point). I had no luck riding my dirt jumper type bikes as trail bikes, there is not much of a comparison here if your bike is not designed for seated climbing. My Chase wasn't bad with a seatpost that was longer than the frame's seat tube and with a setback head on the post to put it back in closer to the right possition to get the correct fit in correlation to where the bottom bracket and seat were in relation to each other, but it was a dirt jumper, not a cross country bike meant for a lot of climbing.
spuddy said:
I dont necessarily think SS-seatheight should differ to geared bikes either, only if the cranks are different lengths.
Well I agree with you here, as do I think most of us.
spuddy said:
Do you have problems (pains) when you run your seat lower than usual?
I think it's more of a it is a less effective and less comfortable for climbing issue where I wouldn't run my seat lower than usual anyway, there would be no reason for riding a bike that's not set up right when it could be. The only pain I have when I can't have my seat the right height is not in my knees, it's more of a pain in the @ss from picking and riding the wrong type of bike seated.
spuddy said:
I'm also wondering why longer cranks on your SS makes you run your seat lower than usual, care to elaborate? I don't mean to sound like a nong by asking, im truly interested.
Well, it's already been answered here and just above you almost answered it again yourself, but it's still a good question for those that may not be aware that once you find the proper seat hight for yourself for proper leg extension, your pedal and top of your seat should always be the same distance measurement when the pedal on that side is in its furthest possition away from the seat (down). So... if you put 5mm longer (lower) cranks on your bike, you would lower your seat 5mm to keep that distance the same.
This is assuming you ride enough to stay in about the same relative bike shape. Some people have an argument that during "bike season" (what's that?, I ride all year...), you should have your seat a bit higher, or at the correct possition for proper leg extension while in good physical riding condition as opposed to where you may lower it to if otherwise, but I think if studying that argument or opinion, 5mm of seat hight adjustment may not mean much to that in-less-than-riding-shape rider anyway.
BTW, I raced BMX on 180mm cranks, and the height of my seat in relation never crossed my mind, I never sat on that seat anyway unless I was waiting in line behind the gate for my moto to be called.
Cheers,
Dave