large torsion bars on a convertible

I don't remember if I asked this or not. I know there are a lot of torsion bar posts but nothing I found said anything specific about convertibles. I'm looking at Firm Feel 1.06 and 1.12 bars. The wheel rates are a little higher on the A-body bars compared to the same size B and E body bars for obvious reasons.

I'm not concerned about the ride being too harsh with the larger bars. I'll be using some good quality shocks. My concern is in regard to the stiffer springs and the lack of rigidity in the convertible. Are the larger bars going to basically tear the car up?(or is a convertible stronger than I'm giving it credit for)

I'll be running larger disc brakes and 18" wheels up front. I'm definitely going to tie the frame together with some type of frame connector but I'm not sure what style at this point. The car does have factory torque boxes. I had the K-member fully welded and steering box mounts reinforced. I also added the factory fender to cowl braces that the 74+ cars used.(thought it couldn't hurt)

With good sub-frame connectors you should be fine. We haven't run big bars in a convertible, yet. And, we're not likely to. My son's Dart has .89 factory big block bars. I have the same for my Barracuda convertible. Both cars have frame connectors. Both cars have 440s.

I have 1.14 MP bars in my 68 Barracuda coupe with welded subframe connectors. 245/50/15 sidewall tires. Bilstein shocks

I've run .99" T-bars before that. 245/50/15 sidewall tires. And 225/60/15 before that. KYB shocks. Still softer and less harsh that 1.14" w/bilsteins

I also have a 68 Dart GT conv. So Cal car no frame or trunk rust whatsoever. No subframe connectors. It has .87" Tbars. KYBs. All rubber bushings. 245/60/15 sidewall tires.

I can certainly tell the difference in frame regidiity and cowl rattle/flex on the conv.


Tires sidewall is a big part of the harshness equation. What tire size are you running?

I would not run bigger than 1" in a convertible. I'd want to run subframe connectors too.. And I'd run rubber bushings. Paint tends to crack between trunk corner and convertible rear corner molding. Ebody verts seem even worse.

I would certainly not run 1.12" Tbars on a conv. Even with the subframe connectors. That's like putting stiff ends on a wet noodle.

That was exactly my concern. I thought a really large bar would tear up the car. The top of the quarters are not cracked...and I don't want them to. I'm planning on running a 225-40 tire up front. So you think the 1.06 is even too big... I'll look at some more options. I know there is a 1.03 out there too.

I want the car to feel tight and not spongy. I just put a caliper on the stock bars and they measure .88". So how big of a difference is there between .88 and 1"?

Are we talking about an A-body convertible here?

Mopars (any Mopar) needs frame connectors. Convertibles need all the body-strength they can get.

Larger T-bars can prevent bottoming out/too far on dips and speedbumps, especially if you like a lower stance on the car.

Shocks can make or break a car's "pleasure to drive".
Cheap shocks usually can cause a rough/hard ride with upgraded springs.
Your short sidewall 18" tires won't smooth out the car's ride either.

Because shocks need to match and work with the torsion bars, I wouldn't go too big on the bars for it being mostly a cruiser car.

I've got Just Suspension 1" bars on my dailydriven '73 Dart, but they don't reproduce these anymore.
I recall PST has the 1.03" bars and FirmFeel carries the 1.06" bars.

Yes, this is on a 68 Dart convertible but I was curious about everyone's thoughts on convertibles in general as well. Mine is going to be LOW. So low that full length TTI headers sit too low to the ground. I have to run shorties. I plan on running expensive shocks. I'm not concerned about the ride being harsh. I just need a torsion bar to be stiff enough so I don't bottom out but not be so stiff that it puts wear and tear on the body.

I'm trying to match the stance of this car. It has a complete RMS suspension on it though. If I found out the front spring rates of that suspension, would that translate over to torsion bars?

When I went from .88 (.87) stock a/c-340 bars to .99" there was a very noticeable reduction of roll, brake dive, responsiveness. Its more than double the spring rate. Of course there is increase road feel harshness.

When I jumped to 1.14" there is a huge difference than stock. With really low profile tires not only will you notice driving over coins on the road, but you'll be able to make out the dimes from the nickels!

That 225/40 series tire will feedback lots of road imperfections. That's a VERY low/narrow profile. That will magnify everything. With the .99", Ive ran 225/60, then 225/50 (got a good deal) and back to 225/60. Noticeable difference in road feel.

I would:
-1 1/8 front and solid rear sway bars. That will reduce the roll and help responsiveness.
-Stock rubber LCA bushings, rubber offset uppers and strut bushings. And rubber leaf bushings
-bilstein RCD shocks
-max 1.03" t-bars. MP years ago made a .92" bar, BTW
-And keep your .87 bars. You can swap T-bars w/o re-alignment. Just measure ride height, unbolt one shock end, back t-bar adj all the way out, then slide T-bar out.

Do you have a measurement from ground to top of fender for that car?

That would really help get an idea of clearances.

Also the RMS crossbar sits lower than stock Kmember. So you'll have more clearance than that car.

Smart choosing the TTI shorties!

Bottom line is something will become the bump stop:

1) LCA bump stop
2) bottom of Kmember
3) head pipe
4) H-pipe (probably will high center on tall speed bumps)
5) torsion bar via LCA pivot and rear T-bar anchor
6) shock

I've successfully, err unsuccessfully, used ALLLL those things as bump stops at some time. . Sometimes more than one at a time!

The 225-40s should have three inches of sidewall visible beyond the wheel lip. I had a Stratus that I dropped 2" and ran those sized tires. It was a stiffer ride but nothing excessive. I know comparing the two cars is apples verse oranges.

I don't have a fender to ground measurement. I was just going to eyeball the fender to wheel gap and then see what my clearance is to the top of the inner fender. Even with the RMS suspension, that car has stock inner fenders. He's working with the same clearance As I will be.

I knew caster/camber would be an issue so I bought SPC double adjustable UCAs. Those should work great to get the best alignment. I have rubber bushings on the car with a stock sway bar. I can upgrade that once the car is on the road. I've got the MP 0 arch road race springs on the back. They might be a little too low. It depends on how low the front can go.

I appreciate the input. It looks like I'll get some 1.03 bars and see how they work.

This is your conundrum, because any bottoming out now, is like having a 6?" diameter TB at that point, and the impact loads the chassis sees, and any TB of smaller diameter, softens the impact loads the chassis sees. The actual forces don't change, just the rate they get to the chassis does, if everything else stays the same. Seems like everyone has different bottoming out thresholds, so no one answer fits all IMO. Pick your poison.
Me, I go big.

But the bottom of the K-member should be what bottoms out first on your car. The other stuff can/has been manipulated more. Like, aren't you running dropped spindles?

The bottom of an RMS crossover will bottom out before a stock K-member because the RMS sits lower.


Do you know the rim diameter and tire size of that Red Dart Conv?

If you did, I think I could figure out your K-member ground clearance.

The larger diameter rim and less sidewall give an optical allusion of being low. We naturally see these cars with a lot of black tire between the rim edge and top fender lip.

Ride height being constant: as the rim diameter goes up with sidewall thinning, that rim edge to top fender lip gets smaller. There is less black between the rim and fender.

Yes, I asked him what he was running for wheel/tire sizes. He has 18x8s up front with 225-40s. I have 2" drop spindles. I quickly mocked up a 225-50 and put it where it looked good. I only had 1 5/8" from the top of tire to the bottom of inner fender. I thought that's what I would hit first...not K-member to ground. A 225/40 will have some more clearance then the one I tested.

I've got another picture of the car that is a closeup of the wheel lip to fender distance. It doesn't even look like there is an inch between them.

Until last year, on my '73 the A518 transmission had become the lowest part under the car and would scrape on anything over 4" on the road.
I tuned the car's height based on that, and a bit on the front tires hitting the fender lips on full steering lock.

Last year I installed full length Doug's headers and much to my dislike, they now have become the lowest point under the car with an extra inch.
I had to turn up the torsion bars a bit to make them clear the multitude of speedbumps I encounter every day.

The 1" bars prevent the LCA's from bottoming out, but I would upgrade to 1.06" mainly to be able to adjust the car lower.

Did it have the stock shallow 727 type pan or the deeper A518 type.

Didn't all 518s use the deep pan ?

Yes all A518-transmissions have a deep pan. The A500's too I think.


I mentioned 4" clearance earlier, but it's much less than that.
I'll measure what I currently have.

When you say "bottom of inner fender" is that the same thing I'm calling "top of fender lip" ??

I'm going to assume it is, but please correct me if I'm wrong.

So let's just make that 1 5/8 rounded to 1 1/2 for easy math.

The sidewall on a 225/40/18 is 3 9/16"...3 1/2"

That would make it 2" to the rim bead. Then 18" rim diameter bead to bead. Then assume tire compresses 1/2" with weight of car. So that would leave 3" from ground to tire bead.

2" + 18" + 3" = 23" from ground to top wheel lip is my guesstimate for the red dart convertible.

Now I'll go out and measure my car...

Your calculations are pretty close to mine. A 225-40 has an overall diameter of 25.1"(depending on tire brand) An 18" wheel is actually 19" lip to lip.

25-19=6÷2= 3" sidewall. If I leave 1" between wheel lip to fender opening, that leaves me with 19+1+3= 23". Then subtract your 1/2" for flex under weight = 22.5"

The 1 5/8 measurement was INSIDE the wheel opening, from the top of the tire to the bottom of the inner fender. Look at this picture. This was a 50 series tire though. A 225-50 is close to 27" tall. A 225-40 would gain me an extra 1" of suspension travel before hitting the inner fender.

Here is where I had the tire positioned. I tried to estimate a 1" wheel lip to fender lip distance. I was shocked at how close the tire was to hitting the bottom of the inner fender.

Never measured tire-inner fender distance but with the 225/50/17 tires I've never noticed any contact.

Just measured my Dart today which has 3" ground clearance, mainly because of the headers.