Rear suspension - combating upwards bounce

[ShaunTheSheep]

I drive a FWD estate with an IRS setup (trailing arms with separate shocks and springs). As i go over a speed bump, perhaps with too much speed, the rear of the car feels like it rises too much, it springs up at the crest of the bump. I don't like the momentary feeling of reduced steering control this gives.

The springs are very soft at the rear, however i feel this might be more of a damping problem. Before i go bolting on some "upgrade" shocks having only blind faith in the effectiveness of the shock manufacturer's marketing department to guide me, what is this effect called?

I've heard the term jounce but reading the definition on wikipedia i don't think that matches this?

[Sam_68]

Damper compression and extension are generally termed 'bump' and 'rebound' respectively, so what you're talking about would be called 'inadequate rebound damping'.

Manufacturers tend to go to a lot of trouble to get damping right, however, so the chances are what you're really talking about is rear dampers that are just plain shagged.

[ShaunTheSheep]

Rebound, i should have remembered that, i'd heard that before and it makes sense.

The dampers are only a couple of months old (3500 miles on the car). However the previous car (same type) felt noticably woolier by 10k miles.

Might go for one of the adjustable jobs. Wonder what the trade offs are for setting stronger rebound control.

[Sam_68]

ShaunTheSheep said:

Might go for one of the adjustable jobs. Wonder what the trade offs are for setting stronger rebound control.

You'd need dampers with separately adjustable bump and rebound (ie. 2-way adjustables); the usual 1-way adjustables will increase both bump and rebound damping at the same time (with consequent effect on both ride quality and transitional handling).

If you get 2-way adjustables, then too much rebound damping will see the car literally jack itself into the road on rough surfaces, as the springs won't be strong enough to extend the dampers again before hitting the next bump.

...and even 2 way adjustables won't let you adjust high and low speed rebound damping separately, so the changes you make to ride (high speed) damping will knock on to roll (low speed) damping, which alters the transitional understeer/oversteer balance of the car (they 'prop up' or 'peg down' one corner of the car to some extent as you turn into/out of a corner).

You could go to 4 way adjustable dampers, or have something custom-valved to suit the car (Penske and others can build you a damper with pistons and shim stacks that will pretty much do whatever you want, at a price, and if you have the development resources), but we're off in fantasy land now... it simply isn't worth the cost and effort for a shopping car and the potential for screwing up the handling altogether with multi-adjustable dampers is enormous, unless you have a tame Lotus ride-and-handling engineer locked in your garden shed.

[ShaunTheSheep]

Thanks for that Sam. I don't think I want too much of a change, just a tweak so based on your above info I'll hang fire on any changes, at least until I have my anti roll bars upgraded. See what I think then.

P.s. What's the best bait for catching a garden variety lotus handling engineer? Do they like cat food? :-)

[Sam_68]

ShaunTheSheep said:

P.s. What's the best bait for catching a garden variety lotus handling engineer? Do they like cat food? :-)

Just lurk around Hethel and throw a net over anyone who doesn't have an extra thumb, whispy albino hair or a squint: they're unlikely to be Norfolk locals, so you're bound to get a Lotus engineer sooner or later.

[PhillipM]

ShaunTheSheep said:

P.s. What's the best bait for catching a garden variety lotus handling engineer? Do they like cat food? :-)

Jaffa cakes.

[davepoth]

One of the things about estates is that the handling has to be compromised a bit to make it work properly with a very wide variety of weight in the back of the car. I think this is done with an increased ride height at the back in non-levelling cars - and this extra travel could be causing the problem. If you aren't intending to move welsh dressers, take a look at the suspension in the hatch/saloon version and see where the differences are.