What a difference some front preload makes

dwalby said:

It doesn't matter if the spring rate is linear or not, coilbound or not, putting a preload spacer into the chamber does one thing and one thing only. It changes the amount of travel the front wheel (axle) applies to the forks before the spring pressure reaches equilibrium based on the weight applied to the front wheel. It doesn't even matter if you have two different straight weight springs in each fork, or a straight rate in one and a progressive in the other. The spring compression distance will always be the same for a given weight applied to the front wheel, the spacer simply determines how much fork travel is used before equilibrium is reached. The spacer doesn't change the spring rate at all, that's baked into the coils, and the spring rate doesn't change based on where in the fork tube its being compressed. For a progressive spring the rate does change based on compression distance, but again, that distance is constant regardless of what length spacer is installed. The distance is the sum of the spacer + fork tube travel.

I can demonstrate this with some diagrams if you want further proof.

Click to expand...

Not necessary. Empirical evidence works for me.

Dave, I think I see where we're diverging.

What I'm talking about is measuring rider sag only (bike+rider). For that single measurement, spring rate doesn't matter.

I think you're also considering free sag (bike only) and comparing the two sag values. In that case I agree with you that a progressive spring is more confusing to calculate than a straight rate spring to get from one sag number to the other. But I guess I'd also claim that even if you know your weight, without knowing the front/rear weight distribution of the rider's weight, knowing the exact spring rate isn't all that valuable either. Also, I've seen many sag setting articles that don't even quote a free sag spec, just the rider sag. The main thing about free sag is you want a little travel to remain without your weight on the bike so the suspension won't top out too easily, but I don't think an exact number is terribly important.

So in my comments I'm thinking along the lines of setting rider sag by adjusting the spacers as I described, then after that is set take the free sag measurement, and if its in a reasonable range, leave it alone and don't mess with anything else. I suspect you're trying to dial in both numbers a bit more accurately than I am, which may require a bit more fiddling and calculations, in which case you're right about the spring rate having an effect.

dwalby said:

...The main thing about free sag is you want a little travel to remain without your weight on the bike so the suspension won't top out too easily...

Click to expand...

Yeah, but that only applies to those Tesla self-driving motorcycles that you can summon to come to you with your iPhone, right? If I have an old-fashioned motorcycle such as the ST 1300 that requires a rider to be on it before it will travel down the road, I shouldn't need to worry about the suspension topping out when there is no rider on it? [/poke fun and be silly mode = off now]

CYYJ said:

Yeah, but that only applies to those Tesla self-driving motorcycles that you can summon to come to you with your iPhone, right? If I have an old-fashioned motorcycle such as the ST 1300 that requires a rider to be on it before it will travel down the road, I shouldn't need to worry about the suspension topping out when there is no rider on it? [/poke fun and be silly mode = off now]

Click to expand...

Yeah, I know you're joking, but the point of checking the free sag is to make sure that your rider sag is set at a point where you're well within the normal range of fork travel, and have room for the forks to compress and also extend without topping out. I think you said earlier that you weigh about 180lb + some gear, so its likely you're fine with the stock springs and after setting the rider sag the free sag will take care of itself. But if you were in the 250-300lb range then what you might find is after preloading the springs to hit the desired rider sag numbers, when you step off the bike the suspension will extend fully with little or no free sag, and that's not good, since the wheel needs to be able to move up and down while riding, even with a rider aboard.

In other words, the rider sag alone isn't enough for the forks to extend when the wheel needs to drop down, so you need some headroom to allow that to happen without topping out. The free sag tells you how much headroom you have available.

MajorTom said:

Maybe. Sorry, but as you didn't quote anything I don't know which post you're referring to. Not being snarky, just trying to help.

Click to expand...

First dozen or so posts in the thread. Talk of adjustable fork caps.

Whooshka said:

First dozen or so posts in the thread. Talk of adjustable fork caps.

Click to expand...

Gotcha. From the thread title I didn't realize this started out as a thread about air-adjustable forks.

Having read the posts under discussion, the fork caps being discussed have Schrader valves in them to add air pressure to the forks. Someone else can explain why this is or isn't a good idea as my head is still spinning from dduelin and dwalby's discussion on progressive springs. All I know is it's a bugger to try and balance the two pressures if you're adding a handful of psi to the fork legs individually. I had air valves on my Seca XJ750 forks and I gave up trying to balance the air pressures, as the act of measuring changes the pressure significantly.

MajorTom said:

Gotcha. From the thread title I didn't realize this started out as a thread about air-adjustable forks.

Having read the posts under discussion, the fork caps being discussed have Schrader valves in them to add air pressure to the forks. Someone else can explain why this is or isn't a good idea as my head is still spinning from dduelin and dwalby's discussion on progressive springs. All I know is it's a bugger to try and balance the two pressures if you're adding a handful of psi to the fork legs individually. I had air valves on my Seca XJ750 forks and I gave up trying to balance the air pressures, as the act of measuring changes the pressure significantly.

Click to expand...

There was mention of caps off of a CBR 600 or something like that. I would think you need adjustable internals too, not just the caps.

Whooshka said:

There was mention of caps off of a CBR 600 or something like that. I would think you need adjustable internals too, not just the caps.

Click to expand...

If the idea is to change the damping characteristics of the fork from the cap, then yes, you'd need the necessary internals connected to the caps. I think all that was being suggested here was to add air valves to the forks so you could increase the pressure and adjust the preload without having to replace the spacer on the top of the spring.