DIY rear ARB...

[johnl]

Colin,
if the dampers start off cold then they perform quite OK for the duration of my commute (half hour on rural roads). If they start off warm then they are pretty ordinary from go to whoa. I'm assuming the colder ambient temp manages to keep them cooler, and that they aren't getting as much heat into them from usage as might be expected (though I haven't put my hand on them to feel the temp, but I will be checking). I wonder if a colder damper has less clearance between the working damper tube and the piston? This might mean less leakage past the piston??

I had to drive into town today (to pick up a dead cat from the vets), and the suspension performance was fairly poor, ambient temperature was 24°C. Last time I drove the car (a couple of days ago early morning, pre-dawn) the ambient was 3°C, and the suspension performance was very good. It's a night and day difference (no pun intended), and no way is it some sort of placebo effect. Even in cold temps I think the dampers would be better if just a little stiffer, but as is they are far from unacceptably soft, which isn't the case once the temperature rises.

I assume the TRW dampers are at best marginally stiff, good enough when cooler, but cross a tipping point as the temp rises, becoming worse than marginal. I don't know if this is just the TRW 'stock replacement' dampers, or whether the OE fitment (Boge?, Sachs?) are much the same. Is it a problem with any 'stock replacement' brand damper that attempts to emulate the OE Alfa damping stiffness (e.g. Monroe or the probably much better made Bilstein B4), or is the TRW damper just a poor quality damper and the problem specific to these? I don't really know, and it's confusing that the rear TRWs work much (MUCH) better than do the front ones, but then the rear dampers don't have to control nearly as much unsprung weight, with a much smaller brake, no substantially heavy 'upright' forging, a much lighter hub assembly, and no driveshaft.

At some point this is going to just shit me too much and I'll have to do something about it. Do I punt on the 'stock rate'  twin-tube Bilstein B4 or fork out a lot more $ for the (known to be significantly stiffer) mono-tube B6 (thus severely denting my kart racing budget...)? The B4 may be just what I need(?), but I really don't want to risk wasting even more money on another set of front dampers that don't perform as I expect them too (I consider the $ spent on the front TRW dampers to be a sheer waste of money, they are hardly any better than the old ones they replaced).

Regards,
John.

PS, I don't think 'sticktion' is the problem. If it were then the damper would be initially stiffer and ride more harshly on small bumps / coarse but not 'rugged' surfaces, but it doesn't. If anything I think the problem lies in the dampers response to 'short stroke' inputs where it is too soft, more than the response to 'long stroke' inputs, which they seem to deal with at least somewhat better...

[Colin Edwards]

Hi John,

Maybe cavitation?  Cooler damper fluid will probably have a higher vapour pressure - less likely to cavitate.  However if the existing fluid is as sensitive to temperature change as you believe then maybe the TRW's should be replaced. 

Maybe the cooler (thicker?) damper oil is providing better hydrodynamic lubrication of the pistons, guides and seals?

Right at the top of my "Must Have" list when it comes to dampers is as low stiction as possible.  Dampers wont damp unless their is a flow of damper fluid.  No fluid flow will take place while the damper is resisting rod / piston movement due to excessive stick slip.

[johnl]

Colin,
I doubt cavitation is the problem. The dampers are not getting more than somewhat warmer than the ambient temperature in use, even though my normal use is up to 100 kmh (or so...) speeds on badly to fairly bumpy rural roads. The dampers must be working at least reasonably hard, but not getting 'hot', which I would expect would have to happen before cavitation reared it's ugly head. The front dampers never lose the ability (when the ambient temp is 'warm', it may change with much hotter weather) to control the suspension reasonably adequately when larger bumps are encountered, just lose the plot a bit on coarse surfaces where there are lots of relatively small irregularities.

'Stiction' would result (?) in an initial resistance for the piston to move in the tube bore, and then to 'free up' once it starts to move (somewhat similar to when you 'crack' a tight nut), so wouldn't 'stiction' create an initially stiffer damper? Once the piston starts to move the fluid will flow through the valves, a 'stictiony' damper would be at least somewhat erratic in it's action (and so a highly undesirable problem), but overall I can't see that it would ever be 'softened' in any part of the stroke by the 'stiction', rather, in it's movement it would initially be too stiff and then suddenly be 'as designed'...? The problem I have here (when the damper is warmer) appears to be excessive ease of short stroke piston motion due to some lack of resistance in the initial part of the piston movement...

I'm not sure what is going on, just stabbing in the semi darkness. In the end I think the dampers will annoy me so much that I replace them with something better, if I can justify the expense. If it is indeed the dampers then I can't do much short of replacing them with something better...

Regards,
John.