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Best T-bar match for XHD leafs? #2682047
07/28/19 03:02 PM
07/28/19 03:02 PM
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Swapping out some drag-race ft bars on a basically stock '68 383 Road Runner coupe, no options.
The car has new XHD rear leafs from Espo, which should be 125# rate.
Stock T-bar spec is either .88 (FSM) or .90 (most other info sources).
Looking around at the various sites (PST, Firm Feel, etc), I'm thinking either .920 or .960 bars
.920 = 115#
.960 = 135#
Other info:
FirmFeel offers a .940 bar, 142# rate, which seems like it should be 125#
1.00 bar = 160#
Clocking varies:
Info on OEM says either 20 or 30 degrees depending on source.
One of the aftermarket companies say their 15 degrees is correct.
I don't know why the discrepancies in info and specs, kinda muddies things up.
As for the car, it's basically stock, #s & codes, has 235/60R15 & 275/60R15 on 7" & 8" rims for street use. Not gonna be a corner carver, just want a neutral setup using the stock ft sway bar & no rear bar.
Throw all that + the typical nose-weight bias in the blender, and I guess the .960s come closest - am I right?

Re: Best T-bar match for XHD leafs? [Re: topside] #2682060
07/28/19 03:45 PM
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Thicker bars need less clocking as they flex/torsion less.

I have 1" bars on my A-body for daily driving.
Hellwig front swaybar.
7-leaf springpacks in the rear to support 2 steel propane tanks in the trunk.
1-way adjustable shocks (Hotchkis @ front, QA1 at the rear wheels) can fine tune the setup greatly.

It handles way better, safer and more like a modern car.

I would upgrade my Dart to 1.06" or 1.10" without hesitation if it weren't for the costs of shipping 2 bars of steel overhere.
I'm off the opinion that 1" bars should have come stock on these cars.
B-body's, at the very minimum, too.

Re: Best T-bar match for XHD leafs? [Re: BigBlockMopar] #2682066
07/28/19 03:57 PM
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Sniper Offline
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B bodies have longer bars so same diameter as an A body would result in a softer rate on a B.

1" a body bar has a 195 lbs rate whereas it's 175 lbs on a 68 B.

I'd run 1.06" on a 68 RR myself. Maybe even more.

Re: Best T-bar match for XHD leafs? [Re: Sniper] #2682352
07/29/19 01:15 PM
07/29/19 01:15 PM
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I run 1.12" bars on my E-body with MP XHD springs. Works well with the Bilstein shocks. 7/8" front and no rear bar, 235/60-15's on 7" steelies all the way around. It is not at all punishing with the big sidewall tires. It has a nice, neutral balance. Shocks are key. I should add that I have sub frame connectors and torque boxes.

I almost hate to mess with it, but plan on a larger front and adding a rear bar as well as 276/60s and having a local spring re-arch my springs and reduce the rate. The back end roll is my only complaint with the current, under-tired setup.

Re: Best T-bar match for XHD leafs? [Re: BcudaChris] #2682370
07/29/19 02:13 PM
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The car does have Calvert adjustable shocks all the way around, and subframe ties & torque boxes were added.
Back in the day I remember these things would understeer when pushing one in the canyons.
The 235/275 tires should actually increase understeer, so I'm thinking too much front rate wouldn't be wise.
Reference: I had a '70 Satellite wagon 20 years ago, added a larger ft bar and a rear bar - either 3/4 or 7/8 - and it was pretty neutral; obviously it had more rear weight than a coupe.
Everything else was stock, (factory HD susp & 11" drum brakes, 383 with A/C).
The cars I've used as canyon-carvers weren't Mopars - 911s, GM F-bodies, Mustangs - very different, and don't translate directly.
I could be over-thinking this, since this car isn't really going to be pounding around corners.

Re: Best T-bar match for XHD leafs? [Re: topside] #2682400
07/29/19 03:32 PM
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Others more knowledgeable than I will chime in and let me know if I'm off base, but I'd say over thinking a bit. I don't think you'll make the understeer materially worse with fatter bars, esp with XHD springs. My shorter wheel base e-body with no rear sway bar got better if anything when I went from stock 318 bars to the 1.12s, though I added the shocks at the same time.

Re: Best T-bar match for XHD leafs? [Re: topside] #2682465
07/29/19 05:57 PM
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Since you are keeping the original sway bar up front, I'd suggest 1.0" t-bars.

My 67 Satellite ran .96 t-bars, 1.125 sway bar and a small block with XHD leafs. It was pretty well balanced. With the B engine, smaller sway bar, and heavier overall vehicle, you might want to make the t-bars work harder in the combination.

FWIW, I've seen XHD springs called out at around 140#. 120# was typically the rate of the four leaf, albeit thicker leaf, oval track springs.

Re: Best T-bar match for XHD leafs? [Re: topside] #2685438
08/07/19 06:01 PM
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Mopar Mitch Offline
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Any TB larger than the factory size will give you a notable better handling car... but its all a "package". In brief, from what you're suggesting, and having a BB, I'd suggest about a 1.10-1.12. And, get some better shocks... double adjustables are the way today... you can fine tune the ride. Also, work with your tire pressures to further fine tune the ride. Try to level-balance the four-corners (front sides, rear sides) and eliminate any large body rake.

I love my 1.24 TBS and 225/inch mono-fiberglass leafs, 1.25 front sway bar, 1.00 rear sway bar, Red Koni adjustable shocks (being replaced by double-adjust QA1).


Mopar Mitch "Road racers and autocrossers go in deeper and come out harder!"... and rain never stops us from having fun with our cars... in fact, it makes us better drivers! Check out MOPAR ACTION MAGAZINE, August 2006 issue for feature article and specs on my autocross T/A!
Re: Best T-bar match for XHD leafs? [Re: Sniper] #2685452
08/07/19 06:51 PM
08/07/19 06:51 PM
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JF_Moparts Offline
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Originally Posted by Sniper
B bodies have longer bars so same diameter as an A body would result in a softer rate on a B.

1" a body bar has a 195 lbs rate whereas it's 175 lbs on a 68 B.

I'd run 1.06" on a 68 RR myself. Maybe even more.




Totally agreed. For a big block car I'd go with at least 225 lb/inch bar. 1.06 is probably somewhere just above 225.

Here's a chart:
Diam. Rate
0.96 148
1.03 199
1.08 240
1.14 298
1.20 366
1.24 417

Re: Best T-bar match for XHD leafs? [Re: JF_Moparts] #2685468
08/07/19 07:29 PM
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What's goofy about the rates for torsion bars is that literally every source of info shows different rates for a given bar...often by a bunch.
Some even overlap the ratings a size smaller or larger, and no, I'm not comparing A & B bodies.
While I appreciate that most of you have some quick autocrossers & track-day cars, I have another car for that activity.
All this thing needs to be is a nice driver with predictability if there's a sudden need. It'll never see a road course.
On the roads around here, the XHD leafs @ 140# rate are pretty stiff, though in fairness I haven't looked at the rear shock settings yet.
Been "de-drag-racing" the thing (gears, some bolt-on stuff), and given the tiny T-bars on the thing now, it probably has 90-10 front shocks.
So, I don't have the usual benchmark.

Re: Best T-bar match for XHD leafs? [Re: topside] #2685610
08/08/19 09:11 AM
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Wheel offset will also be factor in the effective rate. I don't believe the TB's are much different in the same diameters, just they way they a rated/measured.

Doesn't matter much anyway IMO, everybody always chooses too small, except for Mitch. laugh2 up

Edit Clarification, yes length does matter, but in same length, little else does matter, other my above comment.

Last edited by jcc; 08/09/19 10:10 PM.

Reality check, that half the population is smarter then 50% of the people and it's a constantly contested fact.
Re: Best T-bar match for XHD leafs? [Re: jcc] #2686152
08/09/19 09:56 PM
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I thought that comparing spring rates on the A VS B was skewed because although they share different length bars, the A actually uses a slightly longer lower control arm?

Re: Best T-bar match for XHD leafs? [Re: Kern Dog] #2686163
08/09/19 10:36 PM
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Yes, the arms are different, but I see little logic in comparing A's and B's in the first place, different in so many ways (WB, Track, arm length, TB length, F/R weight, aero, etc, etc) its almost pointless. I thought the premise I was responding to was how one could compare TB's pf same diameter from different manufacturers, and to repeat my original point, there should be little difference in the effective rating of any bars of similar diameter on the same car, even if the manufacturers each use different measuring metrics, unless the wheel offset is changed . twocents


Reality check, that half the population is smarter then 50% of the people and it's a constantly contested fact.
Re: Best T-bar match for XHD leafs? [Re: jcc] #2686230
08/10/19 09:21 AM
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Spring rate is determined by two things on a torsion bar, length and diameter. Nothing else applies.

Wheel rate, and this is what I think you mean, is the same as the spring rate in ort Tbar equipped rides because the torsion bar works on the pivot.

On coil sprung and some T bar system I have seen where the spring attaches to the control arm and some point between the inner and out pivot then you have to do some math to calculate wheel rate.

https://www.hypercoils.com/spring-rate-calculator/

In that above calculation on out T bar suspension dimension A and dimension B are identical.

Re: Best T-bar match for XHD leafs? [Re: Sniper] #2686262
08/10/19 12:02 PM
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Originally Posted by Sniper
Spring rate is determined by two things on a torsion bar, length and diameter. Nothing else applies.

Wheel rate, and this is what I think you mean, is the same as the spring rate in ort Tbar equipped rides because the torsion bar works on the pivot.

On coil sprung and some T bar system I have seen where the spring attaches to the control arm and some point between the inner and out pivot then you have to do some math to calculate wheel rate.

https://www.hypercoils.com/spring-rate-calculator/

In that above calculation on out T bar suspension dimension A and dimension B are identical.


A LONGER control arm will have an effect on the stiffness of the torsion bar. Basic sense tells us that. A longer lever makes work easier. Call it wheel rate, spring rate, whatever.

Re: Best T-bar match for XHD leafs? [Re: Kern Dog] #2686275
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I didn't create the terms, those are what they are called. A longer lever has no effect on wheel rate or spring rate in our torsion bar suspensions. If you had a Camaro then yeah, that would matter because the coil spring acts in the middle of the arm and not at the inner pivot so spring rate will always be higher than the wheel rate. Basic suspension geometry which you need to understand if you are serious about handling.

Re: Best T-bar match for XHD leafs? [Re: Sniper] #2686452
08/11/19 11:17 AM
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"I didn't create the terms, those are what they are called. A longer lever has no effect on wheel rate or spring rate in our torsion bar suspensions. If you had a Camaro then yeah, that would matter because the coil spring acts in the middle of the arm and not at the inner pivot so spring rate will always be higher than the wheel rate. Basic suspension geometry which you need to understand if you are serious about handling."


We disagree.


Reality check, that half the population is smarter then 50% of the people and it's a constantly contested fact.
Re: Best T-bar match for XHD leafs? [Re: jcc] #2686480
08/11/19 12:31 PM
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That would be because you don't understand the terms wheel rate and spring rate.

Learn them, it is not a matter of opinion it is a matter of science.

Re: Best T-bar match for XHD leafs? [Re: Sniper] #2686483
08/11/19 12:40 PM
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Originally Posted by Sniper
I didn't create the terms, those are what they are called. A longer lever has no effect on wheel rate or spring rate in our torsion bar suspensions. If you had a Camaro then yeah, that would matter because the coil spring acts in the middle of the arm and not at the inner pivot so spring rate will always be higher than the wheel rate. Basic suspension geometry which you need to understand if you are serious about handling.


Really the only thing unique about Torsion bar suspension is that Wheel rate = Spring rate. So its a little easier to pick a "spring" since you don't have to calc the motion ratio but that's it. The rest of the Wheel rate calc is the same as a coil.

So you're saying the length of control arm makes no difference to the wheel rate?
But isn't a sway bar a torsion bar spring too? I'm sure you'd agree the swaybar's "stiffness" and its contribution to the wheel rate is 100% effected by the length of its lever arm, right? but not a Torsion bar?

Well, either way, you best tell SwayAway to get that "ARM LENGTH" off their Torsion Bar Rate Calculator. They got it all wrong.
https://swayaway.com/tech-room/torsion-bar-wheel-rate-calculator/

Last edited by myduster360; 08/11/19 03:46 PM.

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Re: Best T-bar match for XHD leafs? [Re: myduster360] #2686503
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