Notice the location of the longitudinal cross beam on the Fit rear axle:


Vs. the CRZ:
[/quote]

 

I am having a hard time conceptualizing how the comparatively thin Progress rear sway bar will reduce flex in the massive rear trailing arm? Seems like a much more effective (and cheaper) alternative would be to weld in an additional cross member in the rear axle at the location of the rear springs.
Thoughts?


I am unable to find the part # for the Fit Sport RSB...and it does not show on the parts diagram (www.hondapartsdeals.com).

 

Excellent info here:
http://www.worldautosteel.org/upload...eam_Design.pdf


And here:
http://grassrootsmotorsports.com/for...s/23549/page1/


And here:
http://autospeed.com/cms/title_Rear-...9/article.html

 

moved...and i'll try to elaborate when i get home. i feel like i have to draw a picture.

 

This picture taken from the above pdf will help so that we all use the appropriate nomenclature:




And this is a more realistic depiction of the rear torsion beam (in the same orientation as above^) to help conceptualize:

 

well, you have the right idea right there. that's effectively what a RSB is. it's an additional cross member that stiffens up the rear end so that the inside rear wheel is picked up and doesn't drag with the outside rear wheel during cornering. this behavior is already present on a stock FWD car with a torsion beam but a RSB magnifies it.

and to add, the bushings where the front trailing arms are connected to the rest of the frame are made to compress and flex which allows the left and right wheel to move a bit independently from each other. a RSB minimizes this.

 

Color me skeptical...but I just have a hard time believing that a RSB the size of my finger is going to reduce flex in the massive rear trailing arm. I think we are on the same page in regards to the theory, but I just don't buy the hype of the Progress RSB.

Polyurethane or spherical bearing would also help stiffen up the rear. I wonder why no aftermarket company has gone that route? Maybe I will give Energy Suspension a call.

 

that's understandable. i was skeptical, too. but then i installed a RSB on my (previous) car. it was night and day. it made the car easier to toss around but also felt more dicey in the rain (when you really need that traction). a big reason why i probably won't be installing one on my fit.

your rear passengers will love you for that. ouch.

 

Maybe this is something I just have to see to believe.

I'm curious to know from those who auto-cross if the RSB introduces gradual oversteer or snap oversteer?

 

Yes it's comparatively thin, but that doesn't mean it isn't reducing twist.

Lots of cars have 19mm diameter or smaller sway bars - they all twist - but in doing so, they're also fighting a lot of force. It shouldn't matter that the Fit already has a rear torsion beam, that thing twists too. No matter what the type of suspension, a RSB (even if it's in addition to a torsion beam), will reduce body roll more than having no RSB at all. It's rigidly bolted to each side of the suspension and will take force in order to twist, force that would otherwise be exhibited as body roll.

You could weld an additional crossmember, but that'd probably make it too stiff.

 

The trailing arm isn't a solid chunk of steel. So, as mentioned, it allows for some flexing. Let's say it takes X amount of force to twist it a few degrees.

The RSB, as small as it is, is a solid rod, bent into shape. It ALSO requires some amount of force to twist it... let's call that N amount.

When you put those two together, you now need X+N amount of force to twist both of them.

X... is less than 2600 lbs, that's a given. The proof is going over a bump in the road on one side, the back wheels will differ in height because one wheel is over the bump. The car weighs about 2600 lb, spread over 4 wheels. So X could be less than 850 lbs or so. Actually, even less, since the rear is lighter than the front... Less than 750 lb? less than 700?

In any case, you can easy find that a 19 or even 17 mm rod made of the right alloys can take a few hundred lbs of force to twist. These I would assume are maybe what... 100 or so? I mean even as low as 25 lbs... 25 is still a large enough percentage to add to up to 900lbs of force. on the flipside, narrowing the gap to say 100 lbs for the RSB and 700 for the trailing arm, is a HUGE percentage.

Obviously, I don't know the actual specs for both... but this should give you an idea.

 

@Goobers and doctordoom

I understand what you both are saying. I'm just skeptical the results are as dramatic as people are stating. It would appear that most people who install the RSB have no data (autocross events, track times, etc) to back up their sensational claims. I think a little skepticism is not unwarranted here.

You can add X+N forces all you want, but if it's below the threshold of any measureable effect then the RSB is worthless.

I do appreciate the discussion though. I've learned alot about tosion beam suspensions from the Wiki page as well as the pdf above.

 

so I guess the next thing for you to do is try it out yourself. then again it might be tough to find a fit with the exact suspension, wheel and tire setup as you to compare back to back.

 

I will wait till autocross season starts up this spring and install it. If it's pretty straightforward, I could install it for the afternoon runs and run without it for the AM runs. This would allow pretty good back to back testing. Although, one could argue that any improvement of times is the result of becoming more familiar with the track as the day wears on.

 

in that case, you would need a LOT of laps. As you get familiar, you push harder and harder until you reach a limit.

But the problem with adding a RSB, is that it can be a "game changer." RSBs can change a normally under steering (or neutral) car into an over steering car. This means that you have to relearn what this new limit is... and it is possible, that due to the over steer, the limit is LOWER. That's if the over steer gets induced sooner with the RSB than the under steer used to without it.

If the RSB is to PROVE that it is benefiting you, a chart has to show that you essentially hit a limit without the RSB... and then managed to get past that limit with the RSB.

then again... just the fact it induces over steer when you would normally encounter under steer is proof that a small bent rod is doing something, even when mounted to a larger frame.

Of course, the preferred result is when the car can perform "harder" turns before under or over steer is induced.

PS. you gotta wonder why Honda would add a 17mm RSB to a Fit... in a location not easily seen, if it didn't prove beneficial (in some way).

 

^Agreed.






10 char will you ever learn

 

late to the party...


i didn't think the progress rsb was dramatic, but certainly noticeable... i also drive a ptgt which has a torsion beam rear but with a different design...

the front trailing arms (arm forward of the axle) are much longer than the fits... this would make the rear softer, but also allow for less alignment changes...

the rear trailing arms (arm rear of the axle) are non-existant... as opposed to the fit... the shock and spring mounts are on the axle itself, which would help to stiffen the rear, but also might allow for some squirely stuff to happen in transitions...

the ptgt also has a watts link on the rear axle to try and defeat lateral motion in high load corners... not an ideal solution, but pretty effective...

i put energy poly bushings at the rear trailing arm mounts, and for the watts link, and the effect was noticeable... more predictable/crisp turn-in, with more stability mid-corner...



the downside to all of this, especially when you see that the torsion beam is effectively a spring without a dampener, then add a sway bar, another dampen-less spring, and the springs themselves, the rear dampeners are asked to do A LOT of work keeping the rear in order... especially in quick transitions and when experiencing mid-corner bumps...


i still think the rsb and poly are a great idea... but in a rear torsion i think the shocks are going to be key...

 

I am very interested in this discussion and am following it.

Jack up your car in the rear and disconnect the shock on one side and press down on the trailing arm with your foot, it bends A LOT with not much force, I tried it when installing springs lol

the fact that the mounting point for the RSB is shaped like an L I think would do a decent job keeping the wheels more parallel in a turn by keeping that bending to a minimum. It's not like it's mounted with one bolt.

I don't think it does as good a job as if the bar was mounted to the chassis somehow but I do think it's better than nothing.

 

There are two ways to go about this. The easy, quick fix route - a RSB. Most owners opt for this solution since it is inexpensive. However, to truly combat the rear wheel lift issue you must eliminate the lengthwise twist/flex that is designed into the framework of the Fit.

I do not have a RSB. I have a front lower tie bar brace (control arm brace), a 4-point sub frame brace, and a 4-point mid-chassis ladder brace. I recently got back from a couple days of rat racing with a small group of mixed cars in the mountains of TN & NC. At each break, we realigned (faster/quicker cars move up the chain). Each time I asked the driver behind me if they noticed ANY wheel lift, and all of them said no. I got that feedback from 4 different drivers. I've been running my chassis setup since May and haven't been out with anyone to get the rear-end assessment. Now I know that my plan (to eliminate wheel lift) worked, but the braces do so much more than a RSB alone. The car tracks differently, the under-steer is night and day over stock, it's solid but not harsh, and the braces are the perfect compliment to my Swift springs.

Here's 2 pics where I'm damn near the adhesion limits of my tires. Never once did the rear-end get loose in two days of hard driving. If anything, I was pushing the front end in some of the turns I entered too fast, and the only way to regain control was to change the track by accelerating. FYI - I was beating the shit out of the car (I was TRYING to get squirrely to find the limits), and I scrubbed 1/3 of the tread off the front tires in only 225 miles of driving.

This particular turn allows you to build speed and set the line early to stick the tires/suspension and accelerate through the turn. I was running my tires at 42psi hot. Notice the tire roll - even on the rear tire - I was nearing the limits here.


Here's the apex of that same turn (same photo series). Again, notice that I'm rolling the right rear tire, yet the left rear is firmly planted. No RSB, and haulin' ass the whole way.

 

^Jim, off-topic but what tires are you running here if you don't mind me asking?