Boulevard Jerk

[SusTestEng]

Boulevard Jerk: 2-6Hz occilation of the vehicle on the tire stiffness while the suspension is basically locked.

I hope none of you ever have the task of fixing Boulevard Jerk by means that are truely insane! One of our platforms has a very bad boulevard jerk issue in the rear suspension. I noticed it right away when trying different spring rates and damping forces. Almost every report I did included a line requesting the investigation of the parasitic friction of the system, and requesting a fix for the the boulevard jerk. Nothing was done, and my task was to find a fix for by tuning the dampers. I'll be the first to admit that I have masked some pretty big problems with damping force tuning, but this was tough. I did the best I could, but a very small amount still remains. So my question to others in my line of work: Do you get requested to do work that you know will not have the best outcome? Isn't the obvious solution to investigate why this is happening and most likely focus on the damper rod diameter to limit the friction between the piston and cylinder, and at the oil seal? I'm in a real tough position, because I have masked so many mistakes by the chassis design people, that it is expected that I can do it again. It is nice to have a reputation to be able to help the company, but I really wish it didn't mean that other would just continue to make mistake and not fix them the proper way.

Reposted:Accidently posted in the steering forum

 

[GregLocock]

It's an interesting problem to work on. Of course, every development engineer gets asked to cover fundamental errors by the design guys, that's a fact of life.

Anyway, get on the design team for the shocks and get them working on the seal and piston ring stiction. Benchmark other shocks to demonstrate whether the problem is due to your unique components.

However, in the past I've seen it solved (or substantially improved) a different way. Incidentally the fact you've managed to improve it by changing the shock tuning indicates that the suspension wasn't locked.



Cheers

Greg Locock

 

[SusTestEng]

A larger diameter rod helps. Found that out after production drawings were issued! Nice timing!

The shock tuning involved messing around with the "blow off point" of the bleed damping force. Changing the "knee" in the damping force curve and getting the response curve just right, so that it would be weak enough to allow the suspension to move easier, but then play some catch up so it wouldn't feel so weak. Extremly time consuming and difficult to compromise with the initial steering feel that is created by the low speed damping force. Most of it included changing the slit/notch area and balancing the bleed between the bottom case valve and the piston. Opening up the bleed on the piston and reduceing some on the base valve allowed the damper to compress a little easier, while the base valve is build damping force. It all looks the same on the graphs, but it feels so different on the car.

How have you seen this problem solved/helped in the past?

 

[GregLocock]

Yes, we had a microshake/boulevard ride concern. Those are different manifestations of the locked shock absorber problem. In addition to shock tuning, we tuned the shock absorber rubbers ,made them harder, and put some damping in them. That way it was easier to get the shock to start moving, and if it didn't, at least there was some damping in the system.

Personally I doubt the damping was the important part, since the bushes were so stiff that no energy could have been absorbed.

Incidentally Merc use a ball joint, not a rubber bush, at the end of the shock absorber, in some of their suspensions. I'd like to do that, and as an experiment I'd like to do it at both ends of the shock, and work back to an equivalent rubber part, tracking noise and ride quality with each change.



Cheers

Greg Locock