There are several potential reasons for this:
1. No one's body is symmetrical. It is common for people to have one leg longer than another and our bodies compensate for this by tilting the pelvis. As you sit on the saddle it makes your pelvis straight and you end up pressing more on one side than the other. Most people can compensate for this with practice.
2. Wheel may be dished to one side. This is not easy to check and just looking at the frame/wheel positioning can give you misleading information. Here is how to check it:
- Mark the side of the wheel that is closest to the frame with a marker or tape.
- Remove the wheel from the frame and re-insert it the other way around (so the left crank is on the right - remember to put it back afterwards).
- If your taped mark is still closest to the frame then the problem is the dishing on the wheel which is out.
- This should be corrected before going any further. (you can do this yourself if you are confident by using a spoke key or take it to a bike shop). The tolerance for wheel building is typically +/- 1mm so if it is less than 2mm it is within specification and wont be effecting your riding.
3. Bent saddle or seatpost. Saddes and seatposts can get bent in use. Many saddles have steel plates inside, these can get bent with jumps or falls. Seatposts can get bent in use. A visual inspection should show this. If bent, replace.
4. Frame is off centre. Frames can get bent by various means or sometimes they just need fitting to a new hub. A bent frame can be caused by doing kick-up mounts in the gym or by jumping down loads of stairs and landing badly. On the larger frames the forks can widen after doing lots of turning, this then causes the bearings to be pulled off towards the cranks. Sometimes it can be that the frames when supplied from the manufacturer are just not straight or are too wide for the bearings and just need some tweaking for optimum performance. This is not a hard process but does require a little patience. It is easy to correct the straightness of most frames. If the frame is still under warranty contact the suppler before attempting to remedy.
- Check if the wheel is not buckled (if yes, straighten or replace).
- Check that the wheel is not dished. (if yes, get straightened).
- Check that the frame has the same length legs. There has been a problem with some frames being manufactured with one leg longer than the other, but this is very, very rare but is worth checking. Without the wheel in place measure from the bottom of the seat tube to the edge of the bearing housing. This distance should be identical. If it is not then the correction should be made with a thin metal shim placed above the bearing in the bearing holder, a soft drinks cans can be used but be careful when cutting them.
- If the wheel is consistently closer to one side than the other after you have done the 3 tests above then your frame needs adjusting.
- Place the wheel in the frame and mark the rim side that is closest to the wheel with a marker or tape (do not use the tyre as they are often distorted, for true measurement use the rim only)
- Take the wheel out and place the frame on the ground with the marked side to the top. Place your foot on the frame between the crown and the seat and then apply a gentle pressure on the frame pushing it towards the ground.
- Turn the frame over. Place your foot on the frame again and this time pull the leg upwards.
- Place the wheel back in to the frame and check the positioning of the wheel. You will probably need to repeat this process several times until the wheel is central in the frame and the frame slips over the bearings easily. Ideally you should aim to have your frame about 1mm smaller than you require so that there is a slight inward pressure on the bearings.
It is worth noting that the aligning of the frame to the wheel using the frame legs is a good general test, but for truly accurate centring of the wheel you should align with the seatpost stem. To do this you need to remove the tyre and align the centre of the wheel with the seatpost pushed through the frame to where it touches the rim.
5. Distorted tyre. This is one of the most common problem with 29" unicycles. There are two things that can cause them to be off centre:
- The tyre is not fitted on to the bead correctly. To check this look at the tyre near the rim and you will see a small ridge in the side of the tyre a few mm from the rim. This should be the same distance to the rim all the way around the tyre. If it is not, deflate the tyre and refit.
- The manufacturing process to make tyres is relatively crude and unfortunately are sometimes made non symmetrical. This is most noticeable on road tyres. By removing the tyre and refitting the other way around you can determine if this is the case. If the tyre is the problem then look to replacing it.