I mountain bike and recently while climbing I would bottom out and my peadals and crankset would hit large rocks or even the ground. I only have the problems going up hill and not down. Is my seat to low? Is my frame to small? I cant firgure it out and its annoying. Thanks…

6 Comments »

1. 

   Your seat has nothing to do with the ground clearance of your bike and nearly all frame makers keep a consistent bottom bracket height across all their frame sizes so that wouldn’t impact it either.
   I’d venture to guess that you ride a dual-suspension xc type bike..??
   What I imagine is happening is that, as you’re climbing, the **** of your weight towards the rear of the bike actuates your rear suspension and compresses it into it’s travel. Additionally, most XC bikes use 175mm cranks arms and run a 13″ or lower BB height. Some even get as low as 12 inches… Now, those 175 arms are just a touch under 7 inches so, even if you’re just riding on flat pavement with uncompressed suspension you only have about 5.1 inches of clearance at the bottom of your pedal stroke. Let alone pedaling uphill on rocky terrain..
   Now, add in a bit of suspension sag… say 2 inches and a 4 inch rock on the trail and… SMACK go your pedals. You may consider getting new cranks in a 170mm or 165mm length or you may just have reached a point where the type of riding your doing is surpassing the capabilities of your current ride… It may just be time for you to step up to something with a bit more travel and a higher bottom bracket, or it could just be something you learn to live with. My XC bike is 100mm F/R travel with 175mm cranks and my pedals are beat up and there’s plenty of scrape on the crank arms too but I just keep pedaling and if it’s really rough terrain, take out one of my ‘bigger’ bikes that run higher BB, more travel, and shorter crank arms. The extra travel sucks it out of you on the way up but it’s just that much more fun coming back down, lol.
   As the other poster pointed out, coming back downhill you usually 50/50 your pedals since you dont need to actively pedal continually to keep moving downhill like you do uphill, and therefore you have much more clearance than when the cranks need to rotate.

   Comment by badbadle — February 5, 2010 @ 8:34 pm

2. 

   Yea, what you’re experiencing happens to everyone at some point….Petre answered it perfectly, it’s all based on the route you take…..if you see a rock coming up on the right, make sure you pedal down on the right just before you cross it so you can pedal down on the left to climb past it. If there are rocks on the left and right, well get enough speed to cross them without pedaling…if your crank is hitting the ground and your not manipulating a relatively sharp turn, but going flat and still hitting without obstacles in the way, you’ve got problem with that bike bud. might be a kids bike. Hope this helps….good luck and ride hard.

   Comment by flea — February 5, 2010 @ 8:45 pm

3. 

   you cant do anything about it except get a good new bike. this used to happen to me when I had a crappy bike, just because it had oittle ground clearence. if youre running a department store bike right now I suggest you go to the bike shop and get at least a 400 dollar bike, It is unsafe to ride those things on trails. I have never had this problem with my 800 dollar diamond back response comp, even my dads 300 dollar giant boulder does not do this.
   ok that i got youre email that is a pretty good bike, but i still somewhat stand by my anwser. its a cross country bike, and youve had it for four years so the suspension is probably fairly easy to max out so that would lower the clearence enough for youre cranks to hit the ground. I guess the whole crank stretch thing is possible too but i have never felt my cranks get nearly hot enough to stretch during a ride, never felt them got hot at all actualy.

   Comment by cromes — February 6, 2010 @ 12:18 am

4. 

   What you’re experiencing is “crank stretch.” As you pedal, the bottom bracket gets very hot; this heat then warms the cranks until the aluminum crank arms elongate, allowing the pedals to strike every object known to man. The best fix is to squirt some cool water on the bottom bracket every half hour.
   /sigh

   Comment by Peter Griffin — February 6, 2010 @ 5:09 am

5. 

   you are hitting things because you aren’t choosing the best route while you ride.
   you aren’t hitting things downhill because you’re riding with your cranks in a flat 9 and 3 o’clock position.
   to solve this problem, watch where you are going!

   Comment by petre — February 6, 2010 @ 7:32 am

6. 

   The best thing to do is steer around objects that are too high to ride over.

   Comment by Bob A — February 6, 2010 @ 9:08 am

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