Determining ball joint rate

[nitan]

We are designing a suspension system for a small car. It is a double a-arm suspension, and we are trying to determine the spring rate, and one of the values that we need is the ball joint rate.

How do I find this value?
Any help would be appreciated.

 

[JayMaechtlen]

Ok, I'll bite-
What do you mean by the "ball joint rate"?

Jay Maechtlen

http://home.covad.net/~jmaechtlen/

 

[GregLocock]

Possibilities would include: stiction friction damping or stiffness, in axial radial or torsional terms,in all 3 axes.

You should ask the manufacturer of the balljoint. Or test them yourself.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[nitan]

It has also been called wheel rate.

Is it the result of the value of the motion ratio. I.E. starting spring rating*motion ratio=the spring rating that I buy.

 

[GregLocock]

No, we want to know which property of the bj you are interested in. The answer is going to be the same, if you are serious the manufacturer of the bj will tell you, otherwise you'll have to measure/calculate it yourself, or hope that someone here can help.

Now, that is not an especially helpful answer, but since I strongly suspect you are an FSAE student then that is fair guidance.

Real engineers check the sensitivity of their models to the factors in them. If you told me that the calculation of wheel rate showed a significant contribution from ball joint radial stiffness, I would laugh at you.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Fabrico]

Confusion may be coming from some of the spring suppliers, like the one linked below, which openly refer to ball joint rate. They also may or may not mention that any significant wheel offset needs to be factored in also.

http://www.proshocks.com/calcs/imotion.htm

 

[GregLocock]

Nitan, with that terminology the ball joint rate is the same as the wheel rate for all practical purposes, so your other thread is the answer. Slightly silly terminology, very confusing.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

By the way, good catch Fabrico, and I think that they've got it wrong on that page? Inclining the spring has an effect on MR doesn't it?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Fabrico]

As for the spring, short travel not much, long travel like crazy.

Or another way to look at it, for a given spring, it changes the ball joint rate.

The term "ball joint rate" is fairly new to me too.

 

[WolfHR]

And strictly speaking, IIANM, camber change would be a factor in difference between ball joint and wheel rate. I guess it's not that much- the suspension I'm currently fiddling with would have wheel rate between 1.065 (max rebound) and 1.081 (max bump) times bigger than ball joint rate (I just checked).

 

[Fabrico]

One of the off-road shock makers has a suspension calculator that includes MR correction for shock angle. Although probably sufficient most of the time, the ACF does assume travel that moves eqauadistance from level.

http://www.swayaway.com/Suspension Worksheet.htm#

 

[2SlowJoe]

The spring rate would be the wheel rate (or ball joint rate) times the linkage ratio squared.

The spring load would be the wheel load - unsprung times the linkage ratio.

-Joe