I was thinking the other day about the rear suspension, and i thought since the rear suspension beam is shaped like a big wide horse shoe, does it compress when we take hard turns
My MS painting skills....

if it does, do you guys think a brace could be added in there to tie the two beams together, to make it stiffer?

 

I believe the progress bar already does this?!

 

It doesn't compress (much). It twists... hence the "torsion" in torsion beam... Any RSB will serve to minimise this.

 

oh i totally get what a torsion beam is now! thnx man!! Wow i really need a RSB.

 

EXACTLY RIGHT!!

 

I don't know if the rear axle assembly allows for any significant lateral movement of the wheel hubs, the "H" form that Honda used is stronger than a simple "U". I think that the rear suspension arms are also wider than thick and that would reduce the lateral flex. Altough a bar would fix the distance and prevent inward flex while flat, a fixed length bar would actually pull the arms together as the wheels moved independantly with the designed torsion action. The Progress bar does incedentally beef this area up without the detrimental effect of a hub to hub cross bar. A remote mounted, linked sway bar would not help with this lateral flex.

I would think that a bigger problem would be if the wheel forces the arm to twist, changing the camber in the wrong direction under hard cornering. The heavily loaded outside tire will try to tuck the bottom of the rim under the car adding positive camber when you really don't want it. The only thing besides the suspension arm itself that resists this force is the bottom mount of the shock absorber which is a bolt through a rubber bushing, not much help. The Progress bar would help some, because of how it is mounted, a regular linked rear sway bar would not prevent this twist.

Good thread about real suspension problems and solutions.

Eric

 

More thoughts about bracing against the lateral flex. A set of fixed bars welded from the hub area to the middle of the cross bar of the "H" would create a brace that would still allow some independent movement... hmmm, that may focus the independent flex on just the center of the torsion axle, could be detrimental.

Eric

 

ewdysar, i think you are the only one that really understands what im talking about, yeah the Progress bar would strengthen it but not in the way that i want. what i was thinking is making a straight bar that connects the two arms together(#2) because it would be easiest to make but then you're right on how the bar would pull the two arms together when it moves independently and i have thought about it. so i came up with different design to triangulate the arms independently.(#1)place in heim joints so it can move freely.
like this...

 

Yep the angled bars are what I was trying to describe in my second post, though welded, not jointed like your suggestion. The angled bars might create a point loading on the middle of cross beam, instead of letting it bend smoothly across the whole assembly. The next question is whether the side to side movement of the wheel assembly is bad. I would think the primary concerns would be camber and toe-in. If the arm was bending inward the toe-in would go towards negative, which would add oversteer. Since the car has understeer issues already, perhaps some movement like this is OK.

But if the trailing arm part of the torsion beam gets twisted along it's axis, you would get the camber going towards positive, which will roll the tire contact patch off of the tread and onto the outer sidewall. That can't be good.

Just more food for thought...

Eric

 

hahaha very true, who am i kidding the honda engineers know what they are doing, im sure it flexes but not enough to feel or make a difference. Crap i think we just gave the big companies an idea for a new brace so they can make money off of people. oh well, it's kinda fun thinking about this kind of stuff

 

I haven't looked at it on my Fit. My GTI had a torsion beam as well, different design, but same basic idea. VW made the beam out of "C" chanel steel. I had 1/4'' steel plate welded across the open side of the "C"... so it was boxed in. Made the beam MUCH more resistant to twisting.
Keep in mind that as you make the torsion beam more rigid, you are making a (slightly) independent rear suspension into a completely dependent suspension... basically like a solid rear axle RWD car.

 

I hear you. I don't want to stiffen the torsion action of the cross bar (well, not more than the Progress bar that sitting in my garage) We're talking about how to keep the arms of the u from being forced together, affecting toe-in, and then how to keep those arms from twisting torsionally, affecting the camber settings. The focus is on what is happening to each wheel on it's own, not relative to the other rear wheel. The base of the "U" should remain flexible enough to retain the "independent" nature of the suspension.

Eric

 

If you can, look at an 88-91 integra's rear bear suspension, its simplistic genius. It works, look at what Realtime used to race

 

I know this may be counterintuitive, but there may be some overall track time improvement to be had by intentionally loosening up the rear to correct for the tendency of our cars to plow. Unless the front suspension can be improved to the point where the car naturally oversteers, beefing up the back may actually make things worse.

 

you may be right but usually when you tighten up the rear suspension of most cars they tend to oversteer. in fact, J's racing tried their rear sway bar on the fit and they said the car just oversteers too much because the sway bar was too stiff.

 

Errr, woops, my brain was in reverso-land for a sec there. I need to remember that little known Johnny Cochrane quote, "stiffer rear for oversteer!"

 

A step backwards perhaps? The Fit is the first FWD car I have owned that did not have IRS. My father did have a Chrysler Voyager (Gen 1, 1985) with a solid rear axel, albiet on leaf springs not trailing like the Fit. I can remember the fun of how it handled tho, especially in snow or rain. I spun it out twice.

The minimal amount of flex in turns will be beneficial IMO.

 

How much lateral flex to impact toe do you think you are seeing? In regular driving, I think this would be minimal?

 

I don't think that anyone has observed any lateral flex, this has been a mostly theoretical discussion, trying to figure out where our suspensions might need reinforcement, because the strut bars sold for the Fit seem to fix problems that don't exist. So it may be that the lateral flex that would effect toe-in and the torsional flex that would effect camber may not even happen during racing or AX. And I believe that under normal driving conditions, the rear suspension is stable enough, Honda did a fine job.

Eric

 

Eric, correct, we have talked about this on the C-post bar thread. I think you hit the nail on the head, the best thing that bar would do for me is a place to hang my sportcoats (dating myself here.....)

I can personally see NO way that a rear strut or C-post bar would help this car in any capacity. If the unit body car had IRS and rear struts/springs (not torsion bar, swing, rear subframe, etc) then sure (especially some of the rot buckets I have owned, back when cars actually got rusty). But on a solid rear trailing axel, I do not see it.

I stuck my head back under there today and again see little that re-enforcing the rear axel itself would do for the car, it looks very substantial the way it is.

EDIT: Eric, I see you have a Model T, any pictures posted anywhere?