Shocks vs Struts

[postoak_imp]

Are the rear shocks on motorcycles actually struts, since they include a spring?

[jtopiso_imp]

Are the rear shocks on motorcycles actually struts, since they include a spring?

For some, sure.

For most, no, as a "strut" not only controls how forces are transmitted from unsprung masses to sprung masses: it also controls the trajectory.

What this means is: a suspension system must have one degree of freedom for the vertical movement of the wheel. This degree of freedom is "controlled" by the spring&damper, but if you remove a shock, the trajectory of the un-sprung part (wheel...) remains the same.

On a car with a mcpherson suspension (strut suspension), the camber is controlled by the strut. If you remove the strut, the trajectory of the wheel is not well defined.

Let's go back to the CB (or most (all?) bikes): the rear wheel movement describes an arc imposed by the swing arm. The shocks are only limiting the travel, imposing a wheelrate, and adding damping, but if you remove them, the trajectory will remain the same.

[postoak_imp]

Maybe a stupid question but if you remove the shocks what prevents the swingarm from collapsing against the underside of the fender?

[jtopiso_imp]

nothing will stop it. That's not the point.

What I mean is that the trajectory will be the same with or without the shocks, as there is only one degree of freedom (bump or reboud)

What happens beyond the normal suspension travel is not another DoF, is just not the normal working range, but kinematically is the same movement.

Oposite case: front forks. With the fork in place, you have 1DoF for front suspension (and 1 DoF for steering). If you remove the fork... the wheel is completely unconstrained.

Another way of seeing it: shocks only work in tension/compression. Struts (as forks) support bending loads..

[PowerDubs_imp]

Pet peeve- in the car modification world it is very common for people to say they bought or installed 'coilovers'.

When in reality what they bought is a unit with adjustable spring perch to raise/lower/corner balance, and usually adjustable compression or rebound damping.

The vast majority of these people already had 'coilovers' from the factory- coil spring over the shock, just non adjustable in any way.

[postoak_imp]

I thought one definition of a strut was it was part of the suspension. You can remove a shock and the vehicle will sit there as normal. You remove a strut and the suspension will collapse onto the unsuspended part of the vehicle.

[the Ferret]

We always called struts a solid steel bar with a shock eye ae each end which means no suspension other than tire pressure and maybe seat springs

[jtopiso_imp]

Well, I assume informal nomenclature differs when you travel a few miles, so coilovers/struts/shocks/dampers are used almost at random

However, in automotive jargon, strut is part of the structure of the chassis. A shock (or damper) is only absorbing vibration. If it has a spring mounted around it, makes no difference: a "coil-over" doesn't make a shock become a strut.
In a strut, the "damper" body is not there only holding the shock pressure: it is actually holding the wheel alignment (camber under side loads, for instance).


There is a lot of "wrong" information on the internet, but these are not bad:

https://www.shockwarehouse.com/news/diff...struts.cfm

https://www.buyautoparts.com/blog/the-di...ks-struts/

On our bikes, if you removed the rear shocks, the alignment of the rear suspension wouldn't be affected. You mount them back, and everything goes back to the same position. This proves that's not a structural part of the suspension (even if the spring -which supports the weight- is mounted onto the shock).

With a strut (example: a mcpherson), if you take it form the vehicle, you lose all the alignment. The chassis will be missing some structural component, and the wheel movement will be under-constrained (moves up and down, but also will have camber and or caster freedom).

Another way to see the difference: if you have balljoints on both ends, thay can't be struts (as they won't bear bending loads through a ball joint). If one end is a "clamp"-bracket so it can't rotate around that end, then it will bear bending loads, so that is a strut.

[postoak_imp]

Okay, I'd heard the term coilover before but forgotten what it meant. Would it be correct to say that most motorcycles have coilovers in the rear?