OK, so you know how the koni yellow sport shocks have two 19mm nuts on the top of the shaft that basically keeps the strut assembly together? So if you tighten these all the way down, they bottom out by hitting the metal collar that runs thru the tophat. This collar is the little cylinder that is about 1.5" long by 1/2" wide, and runs thru the middle of the two rubber pieces that press into the top hat.

Anyway, if these nuts were loosened a little bit, then wouldn't that give the shock more compliance, in the sense that the shock shaft would move up and down a little bit relative to the top hat. Would this make the car softer on small bumps?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by beanbag &raquo;</TD></TR><TR><TD CLASS="quote">Anyway, if these nuts were loosened a little bit, then wouldn't that give the shock more compliance, in the sense that the shock shaft would move up and down a little bit relative to the top hat. Would this make the car softer on small bumps?</TD></TR></TABLE>
No, it would just make a lot of noise.

 

If you want softer then back the adjusters off. The retaining nut(s) should be as tight as possible without risking damage to the damper shaft.

As Targa250R said, introducing free play at the crush tube will create a clattering noise as the upper and lower bush retainers bang up and down on the crush tube. However, while this would be annoying, the spring and chassis motion would also become undamped in the range of motion associated with whatever degree of clearance you created, which would have at least some affect on chassis dynamics (and it wouldn't be a good affect).

The stock rubber bushes are too soft in any case, if anything they need stiffening up, not softening up. Note that the two upper rubber bushes are not in any way significantly pre-loaded, when you install all the bits and pieces you'll see that when the bushes and crush tube are in place (but before you fit the retaining washer) that the top of the crush tube and the top of the upper bush are either level with each other or very nearly so. When all is installed the only static force acting on the bush rubbers is that of the damper gas pressure, which in the scheme of things is negligable.

As the damper shaft moves up in bump motion it meets very little initial resistance from the lower of the two mounting rubbers, the lower bush initially compressing very easily but then getting rapidly but progressively stiffer. As the shaft rises and compresses the lower bush a slight clearance will appear at the upper bush, the size of which is dependant on the degree to which the lower bush compresses (which is dependant on the softness of the rubber, the damper rate, the velocity of the damper shaft input, and the size of the input). The same thing goes in reverse for a rebound motion, i.e. as the shaft lowers it meets very little initial resistance from the upper bush etc...

You can easily see just how much motion is involved at the damper shaft / bushing by abruptly bouncing the fender down while you observe the shaft in the damper mount, it moves quite a bit (which isn't good for anything except for a cushier ride quality).

The rubber bushes act in effect like two very short stroke progressive rate springs (one acting in bump and the other in rebound) connecting the damper to the chassis. These 'springs' are effectively undamped, so with soft bushings with zero pre-load the damper cannot act as a damper at all in very short stroke bump / rebound inputs. This does no good things for chassis responsivenes.

Stiffening the bump rubbers by either replacing them with harder bushes (e.g. poly), or 'pre-loading' them by compressing the rubber with a spacer (i.e. a washer about 5mm thick with an OD about the same size as the retaining washer and an ID that's just a bit bigger then the OD of the crush tube, that fits between the retaining washer and the top of the upper rubber bush, thus compressing /pre-loading the bush rubbers when the upper bolt is tightened) gives the damper more 'authority' over the springs, the unsprung mass and the sprung mass, thus making a significant improvement to chassis response and to how well the dampers can keep the contact patches in stable contact with the road surface.

This is what 'pillow ball mounts are all about, i.e. eliminating compliance in the upper damper mounting to improve handling response etc.



Modified by johnlear at 3:53 AM 3/21/2008

 

Thanks for the detailed response.

On my setup, the rubber bushings in the top hat are preloaded by about 5mm or so when the retaining nuts are tightened all the way down. When I push on the car, I can't see the shaft move at all. If I wanted the bushings to be looser, I could easily add a small ring, like 14mm OD 12mm ID 3 mm tall and there wouldn't be any banging sound as the collar/cylinder is still preloaded all the way.

Anyway, the whole purpose of this would be to reduce the amount of high frequency bump transmitted to the chassis. It effectively makes the shock curve more digressive. In my old age, I've been starting to prefer a softer ride. Simply turning down the damping on the shocks would make the car underdamped.

Also, it seems like some of the high end shocks these days try to improve the "turn around response" (when the shock goes from rebound to compression) by letting a small amount of oil bleed thru. I was thinking that adding this kind of compliance to the shock shaft could hide some of the problems of a shock at this turn around point.

I see how having this compliance reduces the damper's ability to "control" the spring and chassis motion, but it's like 1/8" or less. Is it really that bad? I guess I could try tighter and looser to see what effect it has.

 

I can't seem to keep it non detailed!

Same with my Konis (though of course could done with any damper using this type of bush), but it's a mod I had to make it's not the stock bush arrangement (which is very soft). Pre-loading these bushes does make a noticeable improvement to handling response.

OK, the crush tube etc would be tight, but the bush itself will have no preload, as seems to be the case with the stock Honda set up (at least on the CB7), so it would be like the stock set up.

It would do this I think, as would lowering the tyre psi or softening damper rate. Personally I’m happy to live with a little high frequency harshness for the sake of a more stable and responsive chassis.

Part of the problem as I see it with a softer / looser damper mount is not so much the initial softness of the bump or rebound damping (depending on which direction the shaft is moving), but that the transition from say, ‘loaded’ in bump, then to ‘unloaded’, then to ‘loaded’ again but in rebound, will be less controlled and sloppy.

I suspect it will take away more handling response than you think it might (?), since these initial damper shaft motions have a significant affect on transient responsiveness.

Um, IMO, no. It effectively makes something that looks somewhat similar to the damper ‘force / shaft input speed curve’ more progressive. A digressive curve starts off stiffer then gets softer, or at least the force component rises less steeply with increased shaft input speed. At any rate the ‘curve’ you seem to be talking about isn’t a force / shaft input speed curve, it’s a force / shaft linear motion curve, which will start off softer then quickly get stiffer (i.e. crudely progressive).

What softening the bush really does is to partially disconnect the damper from the spring and chassis. The bush is like a short stroke undamped spring between the sprung mass and the unsprung mass, and the softer it is the less control there will be of high frequency oscillations associated not only with the bushes themselves, but also the coil springs and the tyres (which are very like springs with a fairly high oscillatory frequency).

The result will be relatively poor damping of contact patch loading over even mildly rough surfaces, and less than crisp steering and handling response due to ‘delayed’ transient weight transfers. Pre-loading the bushes makes the damper behaviour more linear, softening the bush has the opposite affect. IMO ‘linear’ is always better with suspension set ups, non linear behaviours can mean surprises for the driver.

I’m 49, but I still like to properly feel the road. I just hate vague suspensions!

I think it will just make the damper ‘lazy’ and less ‘authoritative’. I’m not sure what you mean by “by letting a small amount of oil bleed thru”, nor do I really understand the subtle tricks that may be used by some of the better damper manufacturers.

The only dampers I know of that deliberately allow a degree of ‘bleed’ past the valves are Monroe ‘Sensatrac’ dampers. A truly hideous damper, they have a series of shortish longitudinal grooves in the cylinder wall to allow fluid to bypass the valves in a certain part of the stroke (at and near static ride height). The idea is to make the damper soft at / near ride height but stiff nearer the extremes of suspension travel. This creates the crudest kind of ‘progressive’ force curve, and the dampers handle atrociously, feeling worn out even when new.

Mate, try it and see. We can theorise till the cows come home, you won’t really know if you’ll like it until you drive on it. It’s not like trying to think a problem through from A to Z in an attempt to avoid spending too much money on something that possibly ends up not working. If you don’t like it then it’s easy to put back the way it was, and you’re pocket won’t have even noticed!

 

I tried loosening them, and couldn't tell any difference.