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How to determine spring rate for torsion bars?

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Herbie
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How to determine spring rate for torsion bars?

Post by Herbie » Mon Oct 19, 2020 4:53 pm

Does anybody know how to get the factory spec for the spring rate on the torsion bars? I'm hoping it's buried in an FSM or something somewhere...

I'd really like a slightly firmer spring rate, so I'm trying to shop the GM parts bin to figure out if maybe the bars from an Express or Colorado would fit and have a higher rate. (Especially since there are separate torsion bar #s listed for ZR2 Colorado, etc.)

Length and end-shape are also obviously variables, but one step at a time! :D
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Re: How to determine spring rate for torsion bars?

Post by Captn. Crunch » Mon Oct 19, 2020 11:45 pm

Hey herbie
I found this. Hope it helps

http://astrosafarivans.org/bb2/viewtopi ... ght#p67266
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Re: How to determine spring rate for torsion bars?

Post by Herbie » Wed Oct 21, 2020 4:58 pm

Well, this escalated quickly. I have located a manufacturer of aftermarket torsion bars for various off-road and racing applications. They already make uprated bars for other GM applications, and are willing to work with me to create an uprated part for the Astro/Safari platform as well.

(I need to get them an OEM set to make drawings from, but that's a minor detail).

I thought I should ask around - IF I am able to get them to manufacture some front torsion bars with a higher rate, who all might be interested in a pair?

I think these would be a good option for those of us running with more weight than normal on the front-end. I know I've been hesitant to add a winch and aftermarket bumper, specifically because I wasn't sure adding another 80+ lbs to the front would be a good idea. Whether with stock or re-indexed keys, I feel like increasing only the pre-load without having the option of increasing the spring rate is sub-optimal.

Would anybody else want a set?
"My minivan is cooler than your bro-truck"
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Re: How to determine spring rate for torsion bars?

Post by Captn. Crunch » Wed Oct 21, 2020 10:41 pm

That is something I’d look at.
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"I don't beat my rig-I use it to it's maximum potential"
1994 Safari conversion (sold) and miss'n it!
1999 Safari SLE AWD junked
2003 GMC Safari AWD SLT
cast iron Torsen equipped front diff
S-10 leafs-G80 rear w/3.73’s
2” body lift w/Falken Wildpeak AT’s

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Re: How to determine spring rate for torsion bars?

Post by Astrophysics » Sun Nov 01, 2020 4:34 am

Hi Herbie,

That is a great idea.
I would be interested.
The Astro front suspension is so cramped for space. It would have been nice to be able to install coil over shocks but apparently shock absorber mounts are not too strong. Wow, maybe a beefed up shock absorber mount or a copy of the old British sports car idea of lever actuated shock.

I remember the Car and Driver mag road test of the H3 Hummer and how impressed they were with the optimized front torsion bar suspension. Torsion. Bars were very popular for 4x4 trucks since they are compact and allowed some ride height adjustment.
I have heavy front bumper and Warn winch on my 2003 Astro with 2 inch lift. I installed Bilstein shocks up front but front spring rate is kinda soft. I got the synthetic line on winch to save weight.

AP

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Re: How to determine spring rate for torsion bars?

Post by Herbie » Mon Nov 02, 2020 7:10 pm

I agree that this should work out well for a lot of people. No major fab work, no change to existing geometry, just overall changing how much things compress for a given load.

I have specifically avoided front-end weight because I didn't like the idea of further "compressing" front suspension and then trying to compensate with more preload.

I have purchased a pair of OEM bars from a wrecker and they've been delivered to the spring manufacturer. Hopefully they'll get to putting them into CAD soon, which is the first step to fabbing up-rated bars.
"My minivan is cooler than your bro-truck"
2003 Astro AWD Astrolander/ZMB - GTRV Top Transplant, 4" OLV Lift, NP233 T-case, evolving interior
1995 Safari GTRV Organ donor - gutted and gone.
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Re: How to determine spring rate for torsion bars?

Post by Astrophysics » Sun Nov 08, 2020 6:53 am

Hi Herbie,

That is great. Thank you for your excellent work to investigate the torsion bar update.

Still wish there was room up front to strengthen the shock mounts to use longer shock and maybe coil over shock.

AP

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Re: How to determine spring rate for torsion bars?

Post by Snowgeek » Thu Nov 12, 2020 4:01 am

Herbie, I am glad I logged on. I am going to replace mine when I replace all my suspension bushings and what not.

I am curious..... would the stronger torsion bars reduce the preload on the torsion bar keys? IE would i be stepping back to stock keys from my overland "re-indexed keys"? Seems that if it is stiffer than the stock bar than it would lift with less preload.

I would like to know what you find out regarding cost and I would need them shipped to Boise, ID.

I am not on here a ton so I will pm you with my phone number if you need a decision and I am not responding here.

Thanks,

Ryan
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Re: How to determine spring rate for torsion bars?

Post by Herbie » Thu Nov 12, 2020 4:59 pm

Hey Ryan,
Yes, I would expect that for the same weight on the front-end, one would need less preload on the bar-end to see the same static ride height.

In theory, this might dictate that the OEM keys may be retained. I have "lift" keys in a box in my garage, but am currently running maxed-out OEM keys, so I figure I'll be the test case for this.

We've got three (ish) variables: Spring rate, preload, and sprung weight.

Up until this point, Spring rate was fixed.
For most people, the sprung-weight is static-ish.
So the only real variable was the preload. We achieved higher static ride height by preloading the bar for a given weight.

Theoretically, once could compensate for changes in sprung weight by dialing in a little more preload.

However, if the sprung weight changes A BUNCH (like say adding a plate bumper and a winch), you run out of adjustment on preload. You're also running in a mode where you've ALWAYS got the springs statically compressed beyond their normal spec, which may also have the side-effect of compromising suspension action. The torsion bars aren't "infinite springs" - you can't just keep loading them up and compensating with preload - sooner or later they exceed their limits and either permanently deform or break.

So the goal with this little project is to tweak the spec of the bars a bit so that one could safely run a bit more front-end weight, (with a little bit higher static ride height), without having to add so much preload, thus generally putting the loading of the spring back into a "normal operating range".
"My minivan is cooler than your bro-truck"
2003 Astro AWD Astrolander/ZMB - GTRV Top Transplant, 4" OLV Lift, NP233 T-case, evolving interior
1995 Safari GTRV Organ donor - gutted and gone.
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Re: How to determine spring rate for torsion bars?

Post by Herbie » Thu Nov 12, 2020 8:38 pm

Just a follow up, folks. I'm working directly with the torsion spring designer now. The OEM bars have been input into CAD as a baseline, and we're talking now about next steps.

I have learned a few things that I want to gauge people's reaction to.

1) They can certainly make a thicker bar and keep the factory "clocking" arrangement for both hex-ends. However, this can sometimes get into problems where as the spring rate increases, we might need *less* pre-load on the keys, and it is possible to run out of adjustment range. This makes it difficult to offer a big range of spring rates, because they would need to tune the clocking of the bar-ends to each spring rate.

2) Their usual solution for this is to instead manufacture the new bars such that the aft-end is splined, rather than hex. The bars are sold with modified adjuster keys that have matching splines cut. This gives essentially infinite adjustment on the clocking and preload (and also makes things easier to install), and allows them to sell torsion bars at basically any spring rate desired. Obviously this increases the cost, though. They're currently selling similar kits (bars + splined keys) for other GM IFS vehicles for $1300 - $1400.

Given my plans for my van, I still think this is worthwhile (and especially so for anyone with an AWD-based Class-B conversion, Tiger's etc.), but I want to be able to give an honest assessment of how this might affect demand.

I will keep this thread updated as I learn more, but I'd appreciate comments from anyone with opinions. If this pushes you into "too rich for my blood" territory, that's good info too.
"My minivan is cooler than your bro-truck"
2003 Astro AWD Astrolander/ZMB - GTRV Top Transplant, 4" OLV Lift, NP233 T-case, evolving interior
1995 Safari GTRV Organ donor - gutted and gone.
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Re: How to determine spring rate for torsion bars?

Post by Astrophysics » Sat Nov 14, 2020 5:59 am

Hi Herbie,

That is good news. The price seems reasonable.
The spline idea sounds like a smart way to go.
My 2003 AWD Astro with 2" lift has heavy bumper and Warn winch on front so front end is heavy.
Wow, I should go to road scales or scale at the metal scrap yard,
and weigh the front end of my Astro .
Did you weigh front end of your Astro?


I live in San Jose, CA.
Is your fabricator in SoCal?

AP

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Re: How to determine spring rate for torsion bars?

Post by Herbie » Sat Nov 14, 2020 8:13 pm

AP, I had a chance to put my van onto the free roadside scales when I was up in Oregon for Descend-On-Bend in 2018. Front axle was at 2900lbs, with stock bumper, but the addition of OLV bumper brackets (beefy) and a trailer-hitch bolted to the front cross member. At that time, I also had my solar panel rack mounted to the front end of my pop-top roof. (Also the pop-top roof itself adds some weight both fore and aft.) This was with just me in the vehicle

I have since relocated the solar panel to the rear (and generally put the whole rig on a diet). I'd guess front-end is still over 2800lbs, especially if I add a front-seat passenger.

A winch and/or bumper and/or the planned skidplates will definitely put me very close to (or over) the factory GAWR on the front - hence my quest.

Yes, the spring maker is close-ish to me here in SoCal.
"My minivan is cooler than your bro-truck"
2003 Astro AWD Astrolander/ZMB - GTRV Top Transplant, 4" OLV Lift, NP233 T-case, evolving interior
1995 Safari GTRV Organ donor - gutted and gone.
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Re: How to determine spring rate for torsion bars?

Post by Astrophysics » Sun Nov 15, 2020 6:06 am

Hi Herbie,

Recently, I got a Suzuki 650 V Strom motorcycle for Adventure Touring on the somewhat less busy roads. I have been a bit distracted from my normal tinkering with my Astro.

But thanks to your great enthusiasm, I am getting ready to finally get my Astro 3.42 re geared to 4.10 and add a True Trac Torsen type rear locker.

Also, your research on the front torsion bar source could help my front heavy Chevy Astro which has a beefy heavy thick aluminum I beam style Romeo Rim safety bumper (80 lbs estimate) bolted on with heavy steel brackets Uni strut plate simpson strong ties etc. Also, there is a steel Trail Gear winch plate, (about 30 lbs) and a Warn Winch (12,000 pound pull) with synthetic rope. I still need to go to scales.

see photos.

AP
Attachments
IMG_3744.JPG
winch and Trail Gear steel winch plate 2003 AWD
IMG_3129.JPG
heavy alum front bumper and brackets 2003 AWD

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Re: How to determine spring rate for torsion bars?

Post by Astrophysics » Sun Nov 29, 2020 5:26 am

Herbie,

I went to trash transfer station to dump an old 40 gal water heater on Friday. They had a drive on scale.

The front wheels only on the scale read 2900 lbs on my 2003 modified Astro 4x4 with winch.

Then I drove the entire Astro onto scales.

Total weight 5680 lbs.

I forgot to weigh just the rear 2 tires.

Oh well. It would be nice to get some stronger torsion bars.

For the near term, I may adjust my torsion bars with the key.

AP

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