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Old 2017-08-31, 09:47 AM   #8
Join Date: Jun 2014
Location: Murwillumbah, NSW, Australia
Age: 59
Posts: 942
Originally Posted by ScaredOldKid View Post
All the advice says to have your back straight up and in line with seat post angle.
That advice is utter nonsense. While riding steadily forwards, the frame is always leaning slightly back.

This orientation stabilises the uni by introducing "trail" to the geometry, placing the tyre's point of contact with the road behind the axis in the same way as the front forks of a bicycle or motorcycle put the contact point behind the steering axis.

Secondly, aligning the frame and back would mean the vertical movements caused by every bump in the road would be transferred straight up into the rider while the horizontal forces would push the uni out from under them without much means to stop it. With the frame leaning, the upward forces tend to rotate the frame while the horizontal forces can be resisted using a component of the rider's weight. This is precisely the way forces are handled in a leading arm suspension system.

Obviously, the centre of mass of the body must be above the point of contact of the wheel with the road or it would not support the rider. Hence the rider must lean slightly forward to compensate for the backwards lean of the frame.

The rougher the terrain the greater the lean to enhance the suspension effects. Greater lean (and hence trail) also helps overcome the tendency for the wheel to autosteer on cross grades.
Triton 36" + 29" | KH 29" | KH 26" | Nimbus eSport Race 24" | Torker LX 24" | Qu-Ax Luxus 20" | Qu-Ax Profi 20" | KH Onza 19"

Last edited by OneTrackMind; 2017-08-31 at 09:48 AM.
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