Rear shock rebound adjustment with pillion?

[Fallout]

Ok, another pillion based question. I am pretty clued up on the theory of suspension now. Been doing a fair bit of reading for a while. I understand rebound control the speed the shock extends. Wind it in, close the valve, shock extends more slowly. I also understand the conditions in which you would change this. So if you brake and bike dives too much, wind up front compression, wind up rear rebound. If the bike front comes up a lot accelerating, if you had compression on the rear, wind up rear compression.

That's all fine. I get the theory. Since I only have rebound and preload on the Tiger shock, I've maxed out the preload and will only be able to make rebound adjustments now, but I'm kind of at a loss for what to expect. I would assume with a pillion, the nose will climb more under acceleration, but without a compression settings, there's nothing I can do about that. I doubt the bike would dive (and I've not noticed it really) under braking, as the pillion weight is on the rear, so I can't see the need to wind rebound in. I suppose the shock would extend slower due to added weight in normal low speed bump scenarios, so I suppose I want maximum rebound flow, and therefore want to open the valve (wind it out) more?

I know trial and error and experimentation is the key (and really I probably want an uprated shock), but it helps to understand the theory of what is most likely to happen with more rear weight, so I know what problem I am really trying to solve by adjusting the rebound. Any thoughts?

[yorkie_chris]

Personally I leave rebound alone when adding pillion, just add some preload and comp.

[Luckypants]

Originally Posted by Fallout

I suppose the shock would extend slower due to added weight in normal low speed bump scenarios, so I suppose I want maximum rebound flow, and therefore want to open the valve (wind it out) more?

I believe this is not correct. It is certainly what I used to think before the problems I encountered in France (see my thread) with the tyre chewing up.

As I understand it, when riding upright what you say above is true for the spring recovering from being compressed. However, when the wheel is unloaded (eg falling into a pothole) suddenly the loaded spring will extend with much greater speed and force. This leads to a feeling of being kicked up more than you would expect. The problem is made worse when cranked over, as the extra force of the spring extending can make the tyre skip and spin - which leads to it getting chewed. You need to ADD rebound damping to compensate for extra pre-load, to control the speed at which the spring extends when unloaded. The extra rebound has little impact on recovery from compression, because as you note the weight of the pillion will slow rebound down 'naturally', rebound damping only has an effect (mostly) to prevent the spring extending too fast.

I hope that makes sense and is certainly my understanding since my problems last year. If I'm wrong / not quite right, then I hope a suspension guru will correct me.

[Fallout]

It's definitely a dark art. There is no one set compression/rebound scenario, so it's hard to apply scientific and consistent logic!

I think I see your argument MrPants. So basically there is probably a desirable rate/speed at which the shock extends from compressed to "equilibrium" after a bump. With added weight and preload, the distance it is going to extend is much less, and the added stress on the spring will cause it to extend over this shorter distance much more quickly (during that split second after the bump when the weight is effectively less). So the trick is to ensure this extension of the shock happens at this desirable consistent rate to achieve a smooth ride, and therefore you need to wind in the rebound. Is that basically what you're getting at mate?

@YC - No compression setting on the Tiger rear shock. Think I need to add a good upgrade to my xmas list.

[Luckypants]

Yes that is how I understand it, add rebound to control the speed of the spring extension to near the 'ideal' speed.

You have the same adjustments as the VFR on the rear and I found that my rebound had been reduced by a previous owner (should have checked it!) and with the extra pre-load for pillion the tyre got chewed. Adding re-bound has resulted in a better ride and better wear of tyre when two-up.

[Luckypants]

Originally Posted by Fallout

Think I need to add a good upgrade to my xmas list.

Certainly on my list when I have the cash. I had Maxton suspendies on my previous VFR which really made a difference to ride and handling.

[yorkie_chris]

Originally Posted by Fallout

@YC - No compression setting on the Tiger rear shock. Think I need to add a good upgrade to my xmas list.

Penske, you know it makes sense

[Bibio]

adding preload to a spring does not make the spring act different all you are doing in increasing the static hight, unless you top the shock/forks out while adding preload, in this case you need a stiffer spring. there should be no nee to add rebound if carrying a heavier weight it makes no difference to rebound.

Change pre-load on the rear if the spring height allows imo. You can get in to spring rates & compression curves etc etc but changing pre-load to compensate for extra lbs in an effort to re-balance weight distribution of the bike, in simple terms, is what you're trying to acheive. It seems to me that the biggest issue is how much preload you have to play with. I'd have also said a touch more front preload but you mention this is maxed. Also remeber that changing preload at the rear won't have exactly the same effect as on the front (the trial and error part).