Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Suspension trailing link compliance

Suspension trailing link compliance

Suspension trailing link compliance

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?

RE: Suspension trailing link compliance

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.


Greg Locock

RE: Suspension trailing link compliance


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

RE: Suspension trailing link compliance

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?


Greg Locock

RE: Suspension trailing link compliance

Its the BMW 528. Driven it?

RE: Suspension trailing link compliance

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!


Greg Locock

RE: Suspension trailing link compliance

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).

RE: Suspension trailing link compliance

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

RE: Suspension trailing link compliance

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.


Greg Locock

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close