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Tuesday, April 14, 2009

Seats and Seat Posts

BICYCLE SEAT TERMINOLOGY

Integral Seat Clamp - A seat clamp that is built into the seat post.

Non-Integral Seat Clamp - A seat clamp that is separate from the seat post.

Seat Lug - The part of the bicycle frame where the seat post is inserted.

Seat Rails - The rods/wires under the seat, for attaching to the seat clamp.

Seat Post - A metal shaft that the seat is mounted on and inserts into the seat tube.

Seat Post Binder - A clamp or device that secures the seat post inside the seat tube.

COMMON PROBLEMS WITH BICYCLE SEATS/SEAT POSTS

1. The seat post is bent.
2. The seat post is the wrong size (too small), not secure and needs to be replaced with a thicker seat post.
3. The seat post is old or needs to be upgraded.
4. The seat post is corroded due to lack of grease and needs to be greased up as preventive maintenance.
5. The seat rails are bent.
6. The seat is torn or worn out.
7. The seat is just plain old, smells funny, has a bird shit stain on it and needs to be replaced.
8. The seat is uncomfortable and needs to be realigned or replaced.
9. The seat post is stuck, possibly permanently if its rusted right to the interior of the seat lug.
10. The seat post is deformed or dented (and possibly stuck).
11. The seat post keeps sliding down too easily and/or is the wrong size.
12. The seat post rattles because its the wrong size.

You could use a caliper to measure a seat lug, but if its deformed the hole won't be round and it won't be very accurate. The proper way to size a seat post or seat lug is using a Stein SZ-1 sizing rod (simply insert the rod into the seat lug and read off the dimension once fully inserted). If not available, use a caliper but take several measurements and find the average.

Seat posts are always undersized compared to the inside of the seat lug, so that it fits smoothly. HOWEVER, sometimes the manufacturer or person doing maintenance on the bicycle uses a seatpost which is too small which can cause it to become loose or even break while trying to clamp it.

ADJUSTING THE BICYCLE SEAT

Depending on the type of seat clamps, it can be very difficult or very easy to adjust the angle of the seat. Some seats require that you unclamp it before you can change the angle, or they may not be adjustable at all.

Always check the seat angle and direction before and after clamping it. You want the seat to be facing straight forwards and not leaning too much backwards or forwards. If there is a limited number of settings, its better to have the setting which has a slight upwards tilt on the nose of the seat.

REMOVING A SEAT POST

1. Mark the seat post with tape or washable marker where the height needs to be restored to.
2. Loosen the binder bolt on the seat post. Don't completely remove it.
3. Using a gentle twisting and pulling motion, remove the seat post.
4. Inspect for scratches or dents.

In the event of a stuck bicycle seat post, follow these steps:

1. Remove the seat post binder bolt completely.
2. Spread the compression slot so its wider at the top.
3. Drip penetrating oil between seat post and seat tube.
4. Wait 5 minutes for the oil to penetrate.
5. Place one foot in the crotch between the seat tube and the down tube for leverage.
6. Using your hands twist the seat back and forth and pull upwards for 45 seconds.
7. If its still stuck, add more oil, wait 15 minutes, repeat steps 5-6 multiple times.
8. If no progress is made, disable the seat clamp and remove the seat.
9. Place bicycle upside down in a stand, fasten it firmly and hit the bottom of the integral clamp (if any) with your plastic mallet repeatedly. (DO NOT USE A HAMMER!)
10. If failure, remove the bicycle from the stand, clamp the seat post to a vice and get a friend/coworker to help you pull on the bicycle frame (you may want to remove the wheels so they're not in the way).
11. If this has all failed, take a hacksaw and cut off the seat post approx. 1 inch above the seat lug.
12. Take a Jabsaw and cut 3-4 vertical slots in the sides of the seat post, without scratching the seat lug.
13. Crush the seat post in a vice and remove using the Twist-and-Pull method.

INSTALLING A NEW SEAT POST

1. Check to see if the seat post is corroded and hone/ream it if necessary to remove any corrosion. Clean it afterwards with emery cloth.
2. Grease the section of the seat post that is to be inserted.
3. Grease the seatpost binder threads.
4. Oil the quick-release (if any).
5. Insert the seat post past the minimum mark (for safety reasons). If there is no minimum mark on the seat pos, insert it at least 2.5" or 7 cm.
6. If the rider needs a lower height, insert the seat post to the desired height.
7. Gently secure the post using the clamps.
8. Align the nose to the center.
9. Secure the clamps more firmly using a torque wrench to 60in-lbs, but don't use excessive force. It doesn't have to be perfectly immobile (which could make the seat post stuck).
10. If there is a quick release, use that instead and adjust it so it releases/locks into place at approx. the 45 degree angle.
11a. Test the seats security by pushing on the nose of the seat with approx. 50 lbs of force. If this fails repeat step 9 using additional 5in-lb increments until the test passes.
11b. If the test fails when using a quick release, adjust it so the clamping force begins 15 degrees earlier, until test is passed.

REMOVING THE SEAT

If you're removing a seat for a customer or friend you should measure and record the angle and its fore-and-aft positions first. Use of a protractor is recommended. This way you can restore it to its original angle later (no matter how unorthodox the position is, because otherwise they WILL complain).

If you're setting up a bicycle for sale, adjust the seat to the mid-range so the average person can get on easily. Tall or short people can still reach the pedals with some effort and they're used to adjusting things to their height anyway.

When removing and reattaching bolts on Non-Integral Seat Clamps, remember to grease the threads and tighten the nuts equally to approx. 130-170in-lbs using a torque wrench.

When using a single bolt integral clamp, grease the threads and tighten to approx. 120-145in-lbs.

Re-check the angle of the seat after setting the clamps.

Double-bolt integral clamps have two bolts that work in unison, but when one is loosened the other becomes tightened, thus changing the seat angle. To deal with this you need to grease the threads and alternate tightening the bolts to reach the desired seat angle. Once that is done, tighten both bolts equally to approx. 85-95in-lbs using a torque wrench. Re-check the seat angle, loosen and repeat the last steps if necessary.

Some seats have angle-adjustment screws, often a second smaller bolt or screw at the front or back of the seat, which when loosened allows you to adjust the angle of the seat. Loosen both the main bolt and adjustment screw. Find the desired angle and then tighten the main bolt to 120-145in-lbs in torque and then secure the adjustment screw.

Lastly, test the security of the seat by exerting at least 75 lbs of downward force to the nose of the bicycle seat, and at least 50 lbs of sideways force to the nose from either side. If it doesn't budge, good work.

2 comments:

  1. Hi! I just saw this blog, it's very informative. I am trying to change my bike seat; it has an integral seat clamp. So far I got the seat and clamp off of the post, but I would like to change the seat out completely. Can you help?

    ReplyDelete
  2. Charles excellent blog here. I ride only paved trails and shoot lots of video with my camcorder mounted on my bicycle handlebars for my Trail View Mount website, so a comfortable seat is crucial for me. Lots of great info here to help me out. Thanks and all the best.

    ReplyDelete

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