Suspension trailing link compliance 1

[modesign]

The Porsche Boxter rear suspension is a strut with an alloy upright (the same upright is used at the front (diagonal)). The toe is controled by a link at the rear of the upright(steering arm on the front). The bottom link has a trailing arm attached at the front by a uniball type joint to the chassis. At the rear it is attached to the bottom link with a rubber bush on the link. The interesting point is that the bush is compliant in the front to rear direction. A trailing arm is normally used to control front rear movement or have I missed something?

 

[GregLocock]

Yes, the trailing link controls fore aft motion.

It is usual to have some compliance there, since otherwise impact harshness, and more generally coarse road noise, usually use this as a major path into the body. On the front axle of refined cars it is not unusual to see longitudinal recession rates so low as to see 25 mm of rearward wheel motion under 1g braking. On the rear I'd expect to see less than half that. Typical rate would be say 500 N/mm at the rear

So long as your 'steering' system is designed to accomodate this it is no big deal.

You can go too soft and start to get the recession mode coupling into wheel hop, which is typically at 12 Hz. Then you get a lot more brake shudder, wheel balance sensitivity and steering shimmy than you would otherwise.

Cheers

Greg Locock

 

[neutralsteer]

Greg,

I was looking at some published results for some Peugeot & BMW models which claim a longitudinal stiffness value of 10 to 15 mm/kN. This translates to a stiffness of 100N/mm to 67N/mm. I would assume the unsprung corner mass to be say..40-50 kg. that leads to a freq. of around 6.8 Hz. This is sure to couple with the wheel imbalance speed! How do they manage?

BEst Regards
NS

 

[GregLocock]

That is an amazingly low rate. They probably use a hydrobush to give a lot of damping at resonance, and may well have snubbers in to give a higher rate under braking.

7 Hz would also couple with typical powertrain rigid body modes, giving potential secondary ride problems. On top of that it is also the frequency that humans are most sensitive to!

I agree, 40-50 kg is a good estimate for unsprung mass.

Can I ask which BMW?




Cheers

Greg Locock

 

[neutralsteer]

Its the BMW 528. Driven it?

 

[GregLocock]

Oh, yes, well, its bigger engined brother anyway.

That figure is very substantially wrong for a 2000 MY 528 at least, unless that is measured at the tyre contact patch, via the tyre. Use my earlier figure for a much better guess!



Cheers

Greg Locock

 

[SusTestEng]

Greg's First reply is exactly what I would have posted as well.

We use a very similar configuration to what you are describing in the rear of my current project. We looked at rates of 500N/mm up to 640 N/mm. I think I finally was satisfied with about 570 N/mm in the longitudinal direction(but I was pleased to see about 580 from the production part). Longitudinal stiffness at the wheel center is going to be around 1000-1100 N/mm (just a rough guess, I haven't seen the data off the K&C machine yet). In my case it was just as Greg explained, if the trailing arm bushing is too soft, you get brake shudder(especially with rear drums), and poor rear response. But if you get too stiff you end up with coarse road noise and the rear response is too quick and you reduce lateral grip. These are all static spring rates I am talking about, because dynamic spring rates can also be adjusted depending on the material, but similar trade-offs exist with noise transmition and grip/response.

The rate off 10-15 mm/kN would seem to me like a typical longitudinal Acceleration Compliance test at the tire contact patch. Although we usually see this value in cm/kN (around 1 cm/kN).

 

[modesign]

Thanks to all.
The Boxster seems to be a very good compromise between handling and comfort.
Greg's first reply has the words 'steering system'. are you refering to controled toe changes and/or true rear wheel steering. Both are used on road and race cars. They could also require/use compliance.

Cheers, Sandy

 

[GregLocock]

By steering system I meant that any suspension has some means of controlling the direction the wheel is pointing, either actively, as in the case of a steering rack, or passively, as in the case of a toe link in a multilink suspension.

The important parameter in this context is toe in recession (and for a rear wheel drive car precession), which is used to stabilise the car in response to throttle and brake in turns. I think it is normal to have toe-in in recession, ie as you push the wheel back the front of the wheel points inwards. This is easy to accomplish with twist beam and multilink suspensions, but rather tricky (I think going on impossible) with conventional beam axles.

I haven't had to tune this, so I'm not at all sure of the pitfalls and advantages.


Cheers

Greg Locock