Suspension: how do you know where it technically falls short

[Jeroen1000]

Hi all,

Google led me here a lot when searching on technical queries. Most had nothing to do with F1 (don't worry, I'm a fan ) so I think my question is allowed (please do tell if it is not).

Eliminating tire size and pressure as possible causes, so no large rims and no "ecological high pressure", and taking into account below scenario:

- The car hits a depression (not a bump) spanning the entire width of the lane. Both front wheels hit it simultaneously and the rear wheels too but of course a bit later.
- Now this depression is U-shaped and the rest of the road is reasonably good.

Mostly, the rear suspension is a bit stiffer and therefore provides less comfort. Anyway, when hitting such depression the rear occupant gets a good jolt up his/her rear when traveling around 30-36 miles per hour, which is a normal speed for that type of road. The jolt will be less harsh when traveling at a lower speed (but that is kind of stating the obvious I guess). The feeling was described as the car slamming into the road creating a "stomach-in-throat feeling.

But was could be the cause and how can you identify it?

- Springs too stiff.
- Too much extension (rebound) damping so the rear wheels are not being pushed down fast enough when the road drops down below them.
- Too little extension (rebound) damping so the wheels are being pushed to the road too fast
- Or something else I have not considered (perhaps compression but somehow that seems less likely to me)

Many thanks for your thoughts and opinions.
Jeroen

[Tommy Cookers]

assuming that your (unnamed) vehicle is as designed
you need more rebound travel (with rebound damping suitable for this travel to develop)
and then more bump travel
a 4x4 (or other off-road vehicle) has this

however, no suspension can prevent the floating feeling if the depression is big enough

[Jeroen1000]

I kind of deliberately left it out as I wanted to generalize, as I've been noticing this development with more and more cars. It came to my attention during a Volvo V40 test drive. I then did a second test drive, this time insisting on a more reasonable tire pressure (2.35 opposed to the 2.75 bar up checking). Although lowering it surely helped, it did not really suffice.

How do you know it is the rebound and bump travel and not the springs that are too stiff? I can feel other cars sink into the depression further (hence, the are more comfortable). I can't say whether this is due to the springs or the damping though.

[Tim.Wright]

A lot of the comfort response depends on the wheel longitudinal movement due to kinematics and compliance. From the kinematic point of view, rear axle are usually better than front axles in this regard because in bump travel, the wheel centre moves backwards,

So to understand your system better, hat kind of axle is it? How is the wheel centre longitudinal movement in bump travel? How is the longitudinal wheel centre stiffness?

[Tim.Wright]

assuming that your (unnamed) vehicle is as designed
you need more rebound travel (with rebound damping suitable for this travel to develop)
and then more bump travel

If you don't know if the vehicle is hitting the bump/rebound stops, how are you so sure that increasing the travel will fix the problem?

[Tommy Cookers]

How do you know it is the rebound and bump travel and not the springs that are too stiff? I can feel other cars sink into the depression further (hence, the are more comfortable). I can't say whether this is due to the springs or the damping though.

FWIW I seems to me that cars are manufactured with suspension travel, stiffness (natural frequency) and damper forces all matched
eg reducing rebound damping might help the drop, but should be matched with reduced bump damping (to avoid other problems)
then you'd need more travel
etc

off-roaders are ready made with lots of travel
they might be worse at following the drop via their (lower) natural frequency, but the greater wheel dia smooths/reduces the drop
my guess is that rebound travel is proportionately greater in vehicles with a 'soft' ride

[olefud]

An intrinsic problem with front engine/rear drive cars is the mass ratio of the unsprung differential assembly to the relatively low- mass rear portion of the vehicle. Relatively strong springs/dampers are needed to control the differential et al assembly while the light weight rear body would prefer a wimpy spring assembly. The compromise gets dicier with the front and rear springs needing differing frequencies so they don’t are not in sync so as not to reinforce the whoop-de-do.

[Jeroen1000]

Suspension talk gets complicated soon:) but it is incredibly interesting how much science is behind it. Well the V40 is on the Ford C1 platform known from the previous generation Focus (MKII or MK2).

So there is no dummy proof way to find out what is happening at the rear? But I can certainly relate to what you are saying Olefud. Volvo themselves indicate they tuned the dampers to be 30% stiffer versus the smaller C30. I would have like 0% stiffer instead lol It is just hard to connect thing you read to things experienced.

[Greg Locock]

If it is a twistbeam it'll actually have precession in bounce and rebound, that is as the wheel moves vertically it will actually try and move forward, making the impact worse.

Subjectively it is hard to tell whether an 'imapct' is caused by the damper suddenly getting its act together, or the suspension striking through the jounce bumper.

The simplest solution is often a more progressive, but stiffer, jounce bumper. Trouble is that screws your ride up in lesser events.

[Jeroen1000]

Not twist beam I think please take a looksy here http://www.carbibles.com/suspension_bible_pg2.html
Scroll down to Ford Control Blade™ Suspension part. Thank you all for answering my questions. I will definitely learn more from reading around on these forums.

[Tim.Wright]

This type of suspension (often) has the same problem Greg mentioned since the longitudinal movements are similar to a twist beam.

I believe one advantage of this axle over the twist beam though is you can obtain a softer longitudinal wheel centre stiffness (by softening the trailing arm bushing) without having massive problems in toe compliance. This will improve impact comfort.

[riff_raff]

Not twist beam I think please take a looksy here http://www.carbibles.com/suspension_bible_pg2.html
Scroll down to Ford Control Blade™ Suspension part.

After looking the pictures of the "blade" suspension in the link, I think the jolt your rear seat passengers feel when the rear tires pass over the dip in the road is due to the limited amount of movement occurring at the spring/damper. It can be very difficult to get acceptable hydraulic performance in a damper that has very limited stroke, and thus limited fluid flow past the valving. The short damper stroke would make it more difficult to valve for a smooth progressive compression damping rate. The limited travel in the coil springs would also necessitate a much higher spring rate, which in turn would add to the difficulty in achieving precise damper valving.