I'm really tempted to purchase a set of Hotchkis front and rear adjustable sway bars for our car because of their hollow construction and 3 hole adjustability, yet, I'm a little worried that they are a bit much in stiffness. I'm looking for Whiteline/Tanabe levels of stiffness, but the former is solid and the latter is nonadjustable.

After speaking with Hotchkis, they recommended using an MX-5 set.

RX-8 Application:

33mm Front / 21mm Rear

Features:
Front Sway Bar:
Lightweight, durable and adjustable 1-5/16” (33mm) hollow

110% stiffer than the stock sway bar with a rate of 970 lbs/in
150% stiffer than the stock sway bar with a rate of 1150 lbs/in
195% stiffer than the stock sway bar with a rate of 1370 lbs/in

Rear Sway Bar:
390% stiffer than the stock sway bar with a rate of 195 lbs/in
470% stiffer than the stock sway bar with a rate of 250lbs/in
580% stiffer than the stock sway bar with a rate of 340 lbs/in
Easy to lubricate polyurethane bushings and heavy duty brackets with zerk fittings included.



MX-5 Application:

27mm Front/ 21mm Rear

It seems like the rear is the same for both cars (at least in O.D.).

http://www.hotchkis.net/c-831-06-07-miata-mx-5.aspx



What do you guys think? Our RX8 OEM bar is 27mm Front as well, but the adjustability of the 27mm Hotchkis can let me dial in some stiffness.

 

The 3 settings on the (MX-5) front bar give the following stiffnesses: 520 lbs/in, 615 lbs/in, and 735 lbs/in. It makes me wonder where the OE bar falls relative to that range. I may have to snag one of those bars and try it for BS.

EDIT: See my next post - derived OE rate to 460 lbs/in from above info.

 

they're tubular bars, you won't know how they compare without also knowing the wall thickness

SF, you may want to compare their price to Progress Technology bars

 

If we look at Hotchkis's info we can derive the OE rate. 150% stiffer means OE bar rate times 2.5 equals 1150 lbs/in.

1150 lbs/in = 2.5 x OE bar rate => OE bar rate = 460 lbs/in

That means the range on the MX-5 bar (520 lbs/in, 615 lbs/in, and 735 lbs/in) could be perfect for the RX8 in BS.

 

PT bars are great.
I'm just a little shy about using a 32mm solid bar, even at its softest setting.

 

Hotchkis did mention that a few guys were running this setup for BS.


I'm looking for something not too aggressive for my road course setup.

 

The PT front bar is not solid, it's the same OD and wall thickness as the bar Speedsource supplies to Mazdaspeed

the rear PT bar is solid though

I would prefer to know what the MX5 bar wall thickness is because those iterpolated results don't add up. It would have to be just short of solid to have those rates unless the physical material specs are substantially different

 

Ah. Thanks for clearing this up.


This is what threw me off:

 

Going back to my original concern and criteria however- wouldn't the PT bars (even with a hollow front bar) be very stiff as well?

 

yes, but the Progress bars cost less and have great resale potential

I ran them with street tires and didn't have an issue until running road race spring rates (800#/400#) on super slippery asphalt, R-tires won't be an issue at all, street tires only with super high spring rates. Plenty other people here run them without issue. I used the OE endlinks without issue.

I dealt direct with PT, we even discussed them making me some custom bars. It's 1.25" OD x 0.25" wall. The ends aren't flattened. They weld in a machined bar stock end that slides into the tube center and matches the OD, very nice piece. I probably have a pic posted somewhere. Three hole adjustment on the front bar.

The PT rear bar is 3/4" solid with two adjustment holes.

There's a huge thread on the PT bars if you search. I think the pair is $289 shipped from ptuning.com

 

If what is advertised is true, what happens to the front to rear balance with a rate that's so much stiffer on the rear bars?

 

As a comparo....


Using 462lbs/in as a stock rate (front), and some old DPE posted Whiteline percentages of 54% to 64% (from soft to hard)......

The Whiteline Adjustable front clocks in at 712 lbs/in, 758 lbs/in (from soft to hard), .
The Whiteline Non-Adjustable front clocks in at 59% (735 lbs/in) = MX-5 Bar at stiffest setting.
Bonus given to the weight reduction of the hollow Hotchkis vs the Whiteline solid (which weighs in at roughly 12.6 lbs.)


.

 

Now for the rear.

My pig math estimates a roughly 70 lb/in rear stock bar (* I could be wrong on this, which would upset my experiment).

Using the Whiteline percentage range of 79% to 91% stiffer than stock....
Whiteline rear adjustable clocks in at 125 lbs/in, 134 lbs/in.
Whiteline rear fixed clocks in at 87%, or 131 lbs/in.

Rear Hotchkis MX-5 clocks in at 195/250/340 lbs/in. (Same as Hotchkis RX8 rear bar).

 

Using the Whiteline target rate which I'm searching for in terms of stiffness, I'm looking for:

Roughly 735 lbs/in (front), *130ish lbs/in (rear).

The Hotchkis MX-5 setup would be:

Roughly 735 lbs/in (front @ stiffest), 195 lb/in (rear @ softest).

 

I simply want a mild increase in rate to ease into a stiffer anti sway bar at the track, as I get used to the change in vehicle response. I've been working at suspension adjustments in baby steps over the past 14 months- resulting in a productive learning curve.
I guess a PT setup on full soft can make that intro.

 

if you want to play with the pt bars, send me a pm..

they are not on my car yet, and will not be for a while..


beers

 

I would venture to guess that an MX-5 front bar on full stiff and an MX-5/RX8 rear bar set on full soft would be an ideal setup. Only problem is that you're automatically limited to the extremes- not giving one much more adjustability in either direction if necessary.


And thanks Swoope, for the offer!

 

While the OD is the same for the 2 rear bars the rates look to be different according to Hotchkis' website. They probably have different wall thicknesses or different arm lengths.

RX8 rear bar
390% stiffer than the stock sway bar with a rate of 195 lbs/in
470% stiffer than the stock sway bar with a rate of 250lbs/in
580% stiffer than the stock sway bar with a rate of 340 lbs/in

MX5 rear bar
595% stiffer than the stock sway bar with a rate of 390 lbs/in
745% stiffer than the stock sway bar with a rate of 470 lbs/in
940% stiffer than the stock sway bar with a rate of 580 lbs/in

This would make the OE rear bar rates ~40lbs/in on the RX8 and ~56lbs/in on the MX5

 

^
Hey, thanks for bringing that up. Definately anoversight of mine.

 

I always knew the front bar was a direct replacement, but up until now I didn't know that the rear bars can also be swapped between the SE3P and the NC.

A question that needs to be answered is how much of a stock RX-8's roll stiffness is attributable to sway bars. This might address any balance concerns as if say the sways are only responsible for 25% front and 10% rear, then the comparatively large percentage increase in the rear isn't so bad. It'll still change the balance, but not by a huge amount. Of course if the sways are responsible for a majority of the roll stiffness, then this could be concerning.

 

I just spoke with Tanabe. Without me telling them that I was looking for a neutral set of bars (not biased to dial out under or oversteer), they described their bars as offering an evenly proportional increase in stiffness, front and rear. They couldn't give me a bar rate, but the increase in stiffness was described as not too aggressive. The hollow Tanabe could be the bar set I'm looking for then.

 

https://www.rx8club.com/series-i-wheels-tires-brakes-suspension-55/sway-bars-109096/

guess I have to show you how to rebuild brakes too

 

^
I'm familiar with the thread. Haven't read it in its entirety.

Only concern still is that the PT bar in its softest setting, is probably stiffer than bars like Whiteline and Tanabe, and comparable to the Racing Beat bar. I simply don't want that much bar. Is there anyhting in that really long thread which elaborates on this? That's my only concern. Otherwise, I'd be purchasing the PT bar set.

 

IMO there's no valid reason to take that position. You're making a determination without any valid experience for this particular application to back it up. It's typical for someone to throw a part on and talk about how great it is with any reference other than OE. However, I've played around with a number bar rates and IMO they're not too stiff with the exception I previously noted.

 

^

Well, all I can do is call Progress tomorrow then and ask them for their F/R bar rates.

I drove a forced induction RX8 on the track this weekend which had RB sways paired with Cusco coilovers. That combo was great. Then again, it was on coilovers. I would imagine that my car's H-Tech/Koni setup at 50%/25% is probably going to want a bar a couple ticks down in stiffness by comparison.

Also, you mention that the bars you've experimented with aren't too stiff- for what application? AutoX or Roadcourse? Again, I just want something a little stiffer than stock as an intro- it's how I like to set up my car- I'm not out to win a Time Trial championship. Mazdaspeed bars would be great, but they are pricey. If the PT bars at soft meet the criteria, then, yes, I'm all for them.

What I don't want happening is strange dynamic changes at the limit on Turn 1 at Sebring at 90mph with concrete walls flanking me, cursing at the big *** bar set that I've chosen while I'm putting two feet in to catch a spin.