Rear Suspension - Ohlins or Penske?

[670cc]

Nice post L.B.S.! As sort of a thought experiment, think of a stiff spring out in the garage, not in a bike. If we step on it, balance all our weight on it, we compress it to a degree based on the springs rate and how much we weigh. Same thing happens on the bike, it is just the total portion of the bike and rider that compresses the spring. Add "preload" and the spring is not compressed any more (as long as there is still some "sag"). What the "preload" adjustment will do is change the position of the suspension within its practical travel. We want to target some number like one third of the full travel so that around two thirds travel is usable for absorbing a bump and one third is there to allow the wheel to "droop" into a roadway depression, for example. All this is a very simplified and basic model of the total dynamics involved. Often, the dynamics dictate that there is much less than all the bikes weight on the wheel (think about going over a steep ramp to railway tracks where it would be easy to have a negative weight on it (like when the bike jumps). I think it is this factor that makes my butt feel a higher stiffness ;-)

Yes! I like the analogy of stepping on the spring. Now to add preload, we slip a block of wood under the spring. Our height went up; our weight did not change. The spring length is the same (the spring is not compressed any further). The spring rate is also the same.

Applying this to the motorcycle, when we increased preload, all that happened was the bike was raised. Now this all works fine until we reach the end of the shock or the suspension travel, then things change. So to your point, we simply use preload to position the working portion of the spring where we want it within the shock/swingarm limits. Is that correct?

 

[NC Rick]

Perfectly correct! I had to laugh since when explaining the fact in the shop, I often grab a bit of 2x6 wooden cribbing, place it on the spring and stand on it.

 

[670cc]

Perfectly correct! I had to laugh since when explaining the fact in the shop, I often grab a bit of 2x6 wooden cribbing, place it on the spring and stand on it.

That's a relief to hear you agree. I get so frustrated since people seem to make more out of preload than what it is. Perhaps the name "preload" is not the best choice, and leads to confusion or misinterpretation. I'd prefer it was just called "ride height", or "spring position adjustment".

 

[NC Rick]

Yep, with a shock ( or forks) out of the bike, it totally is preload of the spring. The damping, compression or rebound is only a function of the hydraulic and or friction conversion of motion into heat. Our suspension components are predominantly designed to be sensitive to velocity with the occasional position relative modification. The damping setup on a bike is heavily dependent on the target spring rate. The spring supports the weight and the damping controls the spring and the sprung as well as the unsprung mass.

 

[670cc]

Yep, with a shock ( or forks) out of the bike, it totally is preload of the spring. The damping, compression or rebound is only a function of the hydraulic and or friction conversion of motion into heat. Our suspension components are predominantly designed to be sensitive to velocity with the occasional position relative modification. The damping setup on a bike is heavily dependent on the target spring rate. The spring supports the weight and the damping controls the spring and the sprung as well as the unsprung mass.

Yep, and yep. With the shock off the bike, the preload is just that, it preloads the spring against the limit of the shock travel. I can see why it has that name. Then when it's installed on the motorcycle, the sprung weight of the bike pushes it down off the limit of the shock travel, so it's somewhere in the middle, based on the desired sag.

There. Now it's all settled.

 

[Antarius]

And to make it fully complicated -- one must realize that the preload and valving in the front will also affect how the rear reacts during actual riding.

Back on to suspension settings

I've begun experimenting with a new setup and so far I like it (for commuting). I am at 60mm of rider sag in rear, 65mm of rider sag up front. That's about 2.5 and 2.75 inches of sag, respectively. Out of 6 inches of travel.

Definitely more than the 35/45 I generally run, but it's so far been a much smoother ride and still holds a line relatively well. I'll tinker more and finalize my results when I get them.

I should note that I added compression and rebound damping on the rear shock as well, when I increased sag.