Hypothetical suspensiion question.

[Seeker]

Let's assume we are only talking about a rear shock. You can achieve a rider sag of, say, 30mm. You now fit a softer spring, the sag goes to 35mm but you have sufficient preload adjustment to return it to 30mm. What happens to the ride quality?

[Dave20046]

And you return it to 30mm or just have sufficient rate to, but leave it at 35mm?

[Seeker]

Originally Posted by Dave20046

And you return it to 30mm or just have sufficient rate to, but leave it at 35mm?

you adjust it back to 30mm, so same sag but with softer spring. How would the ride quality change in this scenario?

[glang]

I reckon preload isnt really a good name, better is ride height adjuster. The sag can be set back to 30mm but the shock will now sit in a more compressed position so leaving less travel on it. This is why its important to have the right spring so you have the optimum amount of travel on the shock in both the rebound and compression directions. If the new spring is linear it will compress more easily so using up even more shock compression travel if riding with additional weight or when hitting bumps.

[Dave20046]

Originally Posted by Seeker

you adjust it back to 30mm, so same sag but with softer spring. How would the ride quality change in this scenario?

Well it won’t feel drastically more plush that the extra sag would have given but if it’s a linear spring and softer it will be a softer ride and be at a different rate of travel than the other spring would have. If it’s too soft for your weight it will be in the wrong place in harsher conditions and could even bottom out.

Having the correct sag does not mean that the spring is correct for your weight and the valving/dampening of the absorber.

[Seeker]

Originally Posted by Dave20046

Having the correct sag does not mean that the spring is correct for your weight

This is where I get confused because we use the sag measurement to determine whether to change springs. Racetech has a spring rate calculator but I've read conflicting responses to its accuracy.

[glang]

sorry I got it wrong in my post. Preload does adjust ride height but also allows the sag position of the shock to be kept in the right place to reduce the possibility of bottoming out in either direction of travel.

However the full sag test has two parts: no load->bike weight->total weight. The first movement can be adjusted with preload the second not as its determined just by spring rate...

[Dave20046]

Originally Posted by Seeker

This is where I get confused because we use the sag measurement to determine whether to change springs. Racetech has a spring rate calculator but I've read conflicting responses to its accuracy.

It’s an indicator, and you can’t get the right sag if the spring is completely wrong but two springs can be within sag tolerance but one will be better for your weight.
I use racetech’s calcs but in honesty that’s blind trust - I haven’t actually heard anything to the contrary .

It’s funny we (or I) don’t obsess over spring rates as much on the rear, not sure if that’s SV syndrome or because it doesn’t seem to effect braking as much (but probably does). On my SV I’ve ridden with a spring thats way to hard for me for years with little issue, on another bike I am looking to change the rear but mainly because the rebound is shot.

[Bibio]

Originally Posted by Seeker

Let's assume we are only talking about a rear shock. You can achieve a rider sag of, say, 30mm. You now fit a softer spring, the sag goes to 35mm but you have sufficient preload adjustment to return it to 30mm. What happens to the ride quality?

without altering the compression and rebound the slow (clickers) comp/rebound will need to be altered to compensate.


TBH unless the spring rate is really soft from the suggested rate its not going to matter that much.


if you have reduced 5mm on the rider sag then you automatically add this to the bike sag and in doing so reduce the bike sag so the damper rod now tops out inside the shock when entering such things as pot holes and will also let the rear of the bike rise too much when heavy breaking so lifts the wheel off the ground easier. remember if you take 5mm from 35mm to get 30mm you wind the collar down and you are actually raising the height of the valve in the damper tube.



as i have said before you adjust the bike sag then check the rider sag and leave be if its within(ish) spec or swap out the springs if out of spec. remember that most new motorcycles these day have rate linkages where lets say 10mm at the tail of the bike will = 1mm to 5mm at the shock shaft for movement depending on the linkage rate. (as a note this is why the pointy SV does not like a heavy spring as it has a rising rate linkage where the more you push down on the rear the less the shock shaft moves giving a very harsh ride.)



once the sag is set up then its down to the actual dampening settings to control how the bike behaves while riding. this can be the 'clickers' for slow movement like braking and cornering and the valves/shim stack adjustment for faster stuff like pot holes and bumps.


a note: the clickers on shocks do not control comfort to change that you must change the shim stack in the valve.


to answer your question. the ride will get plusher but you run the risk of topping/bottoming out.

[embee]

In the simplest terms, there are three issues to consider.

1 - The sag just puts the suspension in the best nominal static working position when the vehicle is not moving to allow the suspension geometry to work over its intended range of compression and expansion once it does move. Philosophy varies a bit, but something like 30% of travel for extension, and 70% for compression is fairly typical.
2 - The spring rate is selected to provide enough force to support the vehicle and provide suitable resisting force to limit the travel of the suspension taking into account the mass and the typical road inputs. Too soft and it will hit the end stops, too hard and you will use only part of the available travel.
3 - The damper keeps the dynamics under control, being selected according to the mass, the forces involved, and the amplitude and frequencies of the movement, and minimising overshoot or bounce.

The sag or starting point can be got right in pretty much any circumstance, it's simply an adjustment of a physical height of something.
The fundamental thing to get more or less right is the spring rate, according to the mass of the vehicle and the available suspension travel.
Damping needs to suit the spring rate and vehicle mass (and by inference the natural frequency of the system etc).
A common misconception is that you can compensate for an inappropriate spring by altering the damping (oil etc), you can't really, they do different jobs. Spring forces depend on displacement, damping forces depend on speed of travel. Front fork oil level (air gap) will influence the air spring effect, but that really needs to be taken into account when selecting the spring rate, the oil level doesn't affect damping as such, the viscosity does.
Too soft a spring paired with too firm damping results in a floaty suspension over undulations but harsh over rough surfaces and potholes. Too hard a spring with weak damping results in bounce and loss of tyre contact etc.

There are two sometimes slightly conflicting aspects of user satisfaction, one is technical performance of the suspension, the other is comfort, not always the same thing. A compromise is usually required. Track use might prioritise performance, road use might prefer comfort.