dynamic roll resistance

[dualdj1]

In our J-Bar/roll center discussions, I started thinking more towards the dynamic roll center, or more accurately dynamic change in resistance to roll. The purpose of this thread is to identify dynamic changes occuring as you roll through the corner, and what chassis changes can increase or decrease that resistance to roll. I'd like to preface this by saying, that these are just my interpretations, and that I'd like to hear from others on if they believe anything stated is correct or incorrect.

1) Spring rates vs axle travel: We know that changing the angle of the spring mounting between the axle and chassis changes the spring rate (more inward angle = less spring rate). As the axle travels, supposing a mid to short panhard bar such as j-bar or left side straight bar, Does spring angle change? And... if say an angled in right side spring loses angle, becoming more straight up, does spring rate dynamically increase?

I'm trying to do some simulation drawings, just to see if this holds weight. My gut is that it will be a minimal angle change, making any spring rate changes negligible. Is that the case? But if you got any dynamic increase or decrease in spring rate based on axle travel and roll, it would effect the end amount of roll, as well as planting of the RR tire.

More to come... work getting in the way.

[Matt49]

Mark Bush did an article in DLM Magazine several months back about how spring angle affected wheel rate (I believe he called it bump rate) and roll rate. A more upright spring on the right rear corner provided a higher wheel rate but decreased the roll rate.
I'll have to dig up that article (I've got DLM Magazines in literally every room of the house) and get more specifics from that.

[dualdj1]

Mark Bush did an article in DLM Magazine several months back about how spring angle affected wheel rate (I believe he called it bump rate) and roll rate. A more upright spring on the right rear corner provided a higher wheel rate but decreased the roll rate.
I'll have to dig up that article (I've got DLM Magazines in literally every room of the house) and get more specifics from that.

Correct, and that is pretty much common knowledge at this point. Where I'm not sure, however, is if that angle changes during chassis roll and axle movement, and if/when it does, if it is enough to change effective spring rate, which in turn would increase or decrease roll resistance as the car rolls (ie keeping your roll rate smooth to maximize loading of the tires, verse a "jerky" roll or hitting a hard stopping point). Again it may be negligible and not even make enough difference to matter...

[sj valley dave]

It seems to me that if the rearend has a tendancy to move to the "left" when the body rolls over that will straighten up the RR shock angle, thus stiffening what the RR "sees" for spring rate...

[dualdj1]

It seems to me that if the rearend has a tendancy to move to the "left" when the body rolls over that will straighten up the RR shock angle, thus stiffening what the RR "sees" for spring rate...

That was my thought as well.

As a corollary... We ran a short straight bar with our swingarm car. to begin with, we had RR on the swing, straight up. We were still getting that movement to the left, which would actually cause the spring/shock to angle out. It wasn't terrible, but I think we were losing a lot more spring rate. Once we went to birdcage mounted, angled in, it looked much better.

Has anyone ever messed with springs angled out? Would there be any time, such as left rear with a short bar, that it would be advantageous to run the spring straight up or angled out a bit?

[joedoozer]

Standing it up so it lays down more the further it travels. Wouldn't that be similar to a decreasing spring rate like a torsion bar?

[MasterSbilt_Racer]

Your chassis rolls over the rear axle. The rear axle moves left. The rr shock angle definitely changes. I have played with shock angles quite a bit. Don't forget to look at how angle changes effect the chassis' tendency to roll when thinking about what will happen.

[dualdj1]

Your chassis rolls over the rear axle. The rear axle moves left. The rr shock angle definitely changes. I have played with shock angles quite a bit. Don't forget to look at how angle changes effect the chassis' tendency to roll when thinking about what will happen.

So, if you've got a straight up RR, and a decent amount of base axle travel, as the car rolls over and the spring leans outboard, you're really just "shoving" the axle more left, aren't you? Because instead of the force being applied angled towards the RR or straight down, it's actually being applied angular towards the center of the axle. Not sure that it's totally detrimental, but does seem counter-productive. On the left side, I could see some merit to being straight up or slightly outboard (other than the at-rest decrease in stability).