Stock FC front sway bar spring rate?
 

I've searched and searched and can't find the answer. Does anyone know the spring rate of the stock FC front sway bar? Ideally, I'm looking for this rate at 5 degrees of twist.

When you look at aftermarket sway bar manufacturers, they all make statements like "A 20% increase over stock," but they never say what "stock" is.

Get ready to do some math:
http://farnorthracing.com/autocross_secrets5.html
That's the best I have, sorry it's not a canned answer.

Thanks for the cool link. I'm not sure if it'll do what I need it to do, though. Nowhere do I see where it would output the spring rate of the bar. I might have to make a rig one weekend and measure it -- I was hoping to not have to do that.

Well the short answer is: about 47 lb/degree. So 5 degrees would generate 235lb force.
Note this value is a simplified calculation, based on a straight bar (no kinks). If it matters, you should measure it directly (analysis only provides you an estimate). Presumably you are just comparing to another bar with known torsional stiffness.

A longer answer would start with a question - are you going to use this in a suspension calculation? If so then you're going to convert this number to wheel rate using the motion ratio Things only get more complicated from here since the bushing deflection should also be considered. At that point Google (and a set of scales) is your friend.

The problem I'm having is I bought a suspension setup for the FC road race car I'm building. The setup is a proven ITS club racing setup and comes with shocks, springs, and swaybar all dialed in. I'm having issues getting the swaybar to fit. The bar is a Speedway bar, and it won't work with the big (yet stock-style) radiator I have. I'm trying to figure out if it's worth going to the trouble of relocating the radiator and modifying the swaybar mounting points to get this swaybar to fit, or if I should replace the swaybar with another swaybar with the bends that accomodate the radiator. But to replace the bar, I need to find a swaybar that has a spring rate of about 510 lbs/inch at 5 degrees of twist with adjustments to make it even stiffer.

The issue with shopping for a replacement bar is no one lists the rates of their swaybars, they only reference stock...and nobody could tell me what "stock" was (until now -- thanks!). From the looks of it, I need someone selling a bar that's about 2.2 times stiffer than stock, with adjustment holes.

I do have another option, but it involves buying a $500-$600 radiator. That is my last resort.

A speedway style bar is as good as it gets (presumably a blade style adjuster is illegal in ITS). A pic would be helpful to see what is interfering, but I'd suggest making what you have work. If you have to relocate the rad and alter the bar mounting point height, so be it. Otherwise you might need a cu$tom swaybar.

For the record, I measured the stock sway bar OD at 0.91". Sound like you need a bar OD of around 1 1/8". Your build sounds good so far.

Speedway style bars are legal for ITS, but I'm actually not building for ITS -- I'm building for STU, a relatively new class within SCCA. STU allows you to basically build an ITS car that weighs 280# less and has a street port. You can also relocate the battery, replace the glass with lexan, and remove items like the washer bottle, etc...stuff you can't do in ITS.

A custom bar is certainly not in my budget. I'm about $9k-$10k into this build so far and running out of money quickly. This weekend I'm going to remove the radiator, fit the bar, and see if I can get the radiator to fit. Up until now, I've had the radiator in place and been trying to fit the bar. Maybe tackling it from the other direction will help. Hopefully.

Here's a pic from the passenger side of the car. You can see the mounting bolts for the swaybar, and it's pretty easy to see the straight swaybar will hit the radiator.

https://lh3.googleusercontent.com/-L...20radiator.jpg

That's a terrific (and revealing) shot. Whoa, not much room to work with, but forget trying to move the rad to fix it. Hard to tell but there might be enough room to weld a new tube onto the rad. It looks kind of sketchy, but might work (see pic).

P.S. I figured a speedway bar was OK in your class. I was asking about blade adjusters since you mentioned a bar with multiple holes, which is rather primitive compared to blades. There are other front bars that might work, but I don't know any with multiple holes for the front.

Yeah, my setup is using aluminum swaybar arms with adjustment holes, not the blade type. But blade type arms would be legal in IT. Swaybars are open.

So I mounted the swaybar and tried to fit the radiator. I ended up lowering the radiator 1.5" and angling it a tiny bit, and came up with this:

https://lh4.googleusercontent.com/-W...52520arm02.jpg

The swaybar doesn't seem to move enough to make contact with the radiator, and the radiator doesn't move enough to make contact with the sway bar. But I'm sure I'm not applying as much force on these items as racing will. This seems like a solution, but I'm not sure. And while the radiator did get lower to the ground, it's still significantly higher than the sub frame.

Wow that's close! I'm impressed.

That is very tight! I wonder if you could cut the tube off the bottom of the radiator, rotate it inboard, then reattach. Might buy you a coulple more millimeters. :)

Good luck with your build, it looks like a very interesting project.



.

Another great pic. Man that's close. I guess it could work...a little rubbing won't break anything critical, especially if the rad is rubber mounted. Hard to tell - what kind of bushings support the bar? It's not like you'll need to remove the rad often. I'd still mod the rad tube though.

Surprisingly, there are no bushings on the swaybar mounts. You grease the bar up and slide it in the mounts. Once everything is bolted in place, there is almost no slack on the bar yet it still moves free -- supposedly it works. I guess I'll find out.

I mounted this radiator using more rubber than stock. I took the rubber out of the stock radiator mounts and placed them between the car and the radiator, so there's actually twice the rubber when compared to stock. With this and through washers, I managed to move the radiator probably close to 1/2" back to make room for the bar.

Your idea of modding the radiator tube is probably the real answer to my problem. Honestly, I'm just being cheap by not wanting to spend more money on this...but I can see my allocated funds coming to an end, and with my front hub issue (I just ordered 2 hub/knuckle assemblies) I need to be careful spending "optional" money so I can get the car "finished."