LM: Driveshaft Critical Speed

Trying to get a plan pulled together for the T56 swap into my GMT400 4 door 2WD Tahoe. The truck has a 4L60 which means the stock driveshaft should physically bolt up.

However, the gearing in the T56 means I have to regear the axle from 3.42 to 4.56, and the driveshaft speed jumps dramatically. I'm worried that my 64" driveshaft is going to turn into a noodle when oversped like that, especially since I already have issues with insufficient U joint roller motion that causes the joints to notch and subsequently vibrate every 15k-20k miles.

There are a few critical speed calculators online and they seem to give me dramatically different results. It seems like they mostly fall between 3800 and 4800 rpm, with one outlier in the 6000+rpm range. Considering it's currently the stock setup and a 9C1/Z56/Limited Tahoe that came stock with no speed limiter is good for probably 115 mph, I'm inclined to believe it is at least 4800 rpm. 

With 3.42: 3800 rpm is around 95 mph, 4800 rpm is around 125 mph

With 4.56: 3800 rpm is around 70 mpt and 4800 rpm is around 90 mph

While this is a DD and not a race truck, 90 mph isn't acceptable if that's where the driveshaft becomes a jump rope.

Has anyone run into this problem before with a good solution? Am I right to be worried, or is there a way to make a 1 piece driveshaft work without going to a $$$ carbon fiber setup? There isn't room in the transmission tunnel to go to a big aluminum shaft. While I am conscious of cost, preventing the driveshaft from turning into a missile seems like a good place to spend money if I need to.

The local drive shaft place (who can modify shafts) basically just said "it will be fine" and told me not to worry when I called them, but I don't have enough experience with them to know how much to trust that advice. Not super confidence inspiring that they thought about it for all of 5 seconds.

I'm going to question your gear ratio choice.  What is the first gear ratio in the trans?  I know from personal experience that the 3.06 first gear in a 700R4  combined with 4:11 rear had me shifting out of first before I make it past the Christmas tree, and that is particularly useless.  3.73 is lovely.  3.54 would also work just fine.

Unless its an aftermarket unit with rational ratios, trying to turn sixth into a useful gear is probably not really worthwhile, since you have probably made first useless.

To your original question, you need a very carefully balanced shaft to run anything much over 4000 rpm or so.  Traditional balancers can't spin them much over 2,000 rpm, but my last experience there is from quite a few years ago.

Thanks for the gut check on speeds...sounds like concern is warranted at least.

Gear ratios are:

I think 4.56 seems steep because normally the T56 is in cars with much smaller tire diameters. 4.56 would be ridiculous with the 4L60, but it seems to get close to matching ratios between the automatic and manual.

Not sure if this will show up properly or not but here is the ratio spread comparing 3.42 4L60 and 4.56 T56, all else being equal. The 4L60 currently feels pretty good with 3.42s so I'm not trying to shift anything up or down all that much. I know, lazy chart with no axis titles lol.

I do tow some with this truck, so hoping to avoid destroying the clutch by keeping first gear lower to compensate for not having torque converter slip anymore.

Isn't there a shorter gear set T56 available?  If so, that seems like it would be a better choice here. 

Didn't the GRM magazine face this same issue back in something like early 2017 when they rebuild that ramp truck they got out of Connecticut and tried to go to a 1-peice driveshaft and it got all blowed up?  They they went to a 2-peice driveshaft to fix the issue.  That's how I remember it going at the time anyway; somebody that remembers better can check me on this.

The 4.56 gear ratio with what I believe the stock tire size of the 9C1 'ho and the T56 ratios you've quoted all seem about right for a good daily as far as engine speed vs. road speed in each gear via a quick check on Apex Garage's online gear/speed calculator.

I think making 5th on the t56 line up with 4th on the 4l60 will be a good compromise.  6th on a t56 is supposed to be an interplanetary travel gear that has the engine loafing along on the highway. 

rslifkin said:

Isn't there a shorter gear set T56 available?  If so, that seems like it would be a better choice here. 

At the risk of seriously dropping the torque capacity, yes.  Side loads on the case get pretty intense.  It's also pretty cost-prohibitive.

Tons of Caprice/Impala folks do this swap.  Impalas came with 3.08 rears, and in order to keep the same gear spread when you swap a T56 you need 4.10s.  His gearing math is sound.

I had trouble finding a shop that could balance a driveshaft above 3000 rpms.  You might have better luck.

What year is your truck?  Early GMT400s had steel driveshafts.  Later ones had monster 3.5" aluminum foam-filled tubes.  The later ones are nearly impossible to modify because of the foam, but you could try bolting in one of those if you can find the right length.

I ran into this with the Datsun swap. Critical speed depends on joint selection, shaft type, tube type and thickness, and length.  

I used Spicer's calculator. I went from a 2 1/2" shaft with a CS of 4000 rpms to a 3.5" shaft with a CS of 10000 rpms. That gave me a max safe speed of about 5000 rpms or about 130 mph. 

The Solstice driveshaft was about 2 1/4" in diameter and should have failed at 3500 rpms. But it was apparently some really good tube and it was also foam filled. I wasn't willing to chance it. The OEMs have access to engineering teams I don't. 

How about thinking of this from a different perspective.  Do you know of a manual transmission car with a similar drive shaft setup?  In an old 4 speed, 4th gear is 1:1, input to output.  Did those engines run 5,500-6,000 RPM, so did their driveshaft.

@pres589 - yes, I did find the ramp truck article. It's actually that article that originally tickled my memory into checking the driveshaft speed. I think the ramp truck ended up with a 3 piece shaft made of Toyota parts. A multi-piece driveshaft is an interesting idea....

@sonic - lining 5th up with 4th on the 4L60 basically means keeping the 3.42 gears. The problem with my excel graphs (and the online comparisons) is it doesn't account for converter stall/slipping. A good example in first gear is the chart shows shift point of 45 mph @ 5500 rpm. In reality, the 1-2 WOT shift is 35 mph @ ~5400 rpm. That's a huge difference, and makes the 4L60 gearing look higher than it actually feels.

Can you guys see the graph below? this is both transmissions with 3.42s.

@curtis73 - My truck is a 2000 Tahoe Limited with a steel driveshaft. I don't think the aluminum driveshaft would clear the center and rear crossmembers on the frame with the suspension compressed. The steel one only has just enough room for comfort as it is. I had an '89 C1500 5 speed with the aluminum shaft; those things are beefy.

@Toyman01 - definitely an interesting datapoint, but I think the Tahoe's long driveshaft is what gets me in trouble compared to an F body or a B body swap. The Datsun has a pretty stubby shaft. Using foam to kill resonance is an interesting idea, but like you said, it would probably take an OEM level of know-how to do that with any confidence.

@Scott_H - I had thought about it from the perspective of cars with a T56, but not the old non-OD boxes. The F body got away with it because of the 47" driveshaft vs. my 64" shaft. Interestingly, the manuals also got 2 piece driveshafts where the automatics got one piece.

I took a look at some of the old Chevy trucks and it looks like at least some had a 2 piece driveshaft with a really crappy center support bearing. It almost looks like instead of a telescoping shaft, the bearing carrier just has a bunch of play that allows it to flex axially. 

All of this taken together seems to point towards some kind of 2 piece driveshaft as the 'economical' solution, it seems.

You will hate the 4.56 gear, and will wish you had your money back.

But that's just my opinion, I could be wrong.

In reply to Streetwiseguy :

What would you suggest he move to assuming he stays with the T56?  Here's what I worked up with 2500 RPM being a hypothetical engine speed to upshift at. 
 

The T56 might be better with one less ratio that's between the existing 5th and 6th gear ratios... but that's not what this thread is really about.

Streetwiseguy said:

You will hate the 4.56 gear, and will wish you had your money back.

But that's just my opinion, I could be wrong.

Why do you say that?  It's just math.  The T56 has way higher ratios than the 4L60E.  4.56s will put his first gear and 6th gear the closest to his current ratios.  In fact, he'll have a lower cruise RPM with the 4.56s/T56 than his current setup.

You know, the other way to do this is, keep the 3.42's and never go into 6th.  This would solve the driveshaft speed issue as well.   

In reply to pres589 (djronnebaum) :

Right, but with 30" tires and a final drive of 9:1 in first, good luck getting moving without burning the clutch or stalling.

A quick search shows both the 2.66 1st and 2.97 1st T56s as being rated for the same 450 lb-ft.  The lower geared trans would have almost the same overall ratio in 1st with 4.10s as the tall geared trans with 4.56. Both are around 12.1 overall ratio, which is probably still on the tall side for 1st gear when towing (it'll be fine the rest of the time). 

(ninja edit) rslifkin -  I actually already have the transmission - a buddy sold me the one out of his 1994 F body for a deal I couldn't refuse. But yes, it would be nice to have the lower first gear. I have not looked into if it's practical to swap gearsets, but only the 1993 F body transmission had the shorter first gear.

I suppose I could install the transmission first, see how it goes, and then modify the driveshaft setup and regear if needed. My current plan would be to do the driveshaft first (since 4L60 and T56 are dimensionally interchangeable) then the transmission, then the rear end as that seems like it would minimize downtime and takes care of potentially the longest lead time item (custom driveshaft) first. 

I would be really, really surprised if I end up okay with the T56 and 3.42s, especially when towing stuff. This is a 5200 lb SUV to begin with.

It appears that some extended cab 2WD GMT400s came with a 2 piece driveshaft, but they are rare and hard to find. Most are in 2500 and 3500 trucks which would be set up for a 4L80 and a larger pinion yoke on a 9.5" or 10.5" rear end. I believe both use different universal joints, but still in the early stages of research on that.

GMT800s also had one that seems to be slightly more robust, and since I would have to build a center support bracket and shorten the driveshaft regardless, that may be an even better option. Still seems fairly hard to find, though.

Towing does change things.  What about one of the NV3500's instead of the T56?  Ratios in the T56 don't seem well set up for the vehicle and purpose.

Curtis73 (Forum Supporter) said:

In reply to pres589 (djronnebaum) :

Right, but with 30" tires and a final drive of 9:1 in first, good luck getting moving without burning the clutch or stalling.

Welcome to Ram SRT-10 life.

Curtis73 (Forum Supporter) said:

Streetwiseguy said:

You will hate the 4.56 gear, and will wish you had your money back.

But that's just my opinion, I could be wrong.

Why do you say that?  It's just math.  The T56 has way higher ratios than the 4L60E.  4.56s will put his first gear and 6th gear the closest to his current ratios.  In fact, he'll have a lower cruise RPM with the 4.56s/T56 than his current setup.

Mostly I say it because getting an aftermarket 4.56 set up so it doesn't howl like a bitch are long odds indeed.  

Almost as long as making a driveshaft not vibrate at that speed.

Wonder if somebody would straight trade T56 for an NV3500(or with a little adapting NV4500). I just feel like it's a better option for what you want to do with it.

They shift better too(don't shoot me)

I didn't realize I had to be concerned about gear whine with the shorter rear end gears, but I suppose it makes sense.

I had an NV3500 in my old C1500 TBI, and another in my old Caprice, with a 3.08 rear end in both I believe. First gear was a little awkwardly between a granny gear and a normal first; if I wasn't pulling stuff I did a lot of starting in second just driving around town.

I bet there are a lot of people who would trade an NV3500 for a T56; that's the direction most people go. I will have to think about that some more but given how thin on the ground known good 2wd NV3500s are, for now I'd say that's an unlikely direction. They seem to all have intermediate shaft bearing damage and the resulting gear whine/driveline lash by now, and the stock engine in the Tahoe is already at or over the torque rating.

For the NV3500, there are 2 different ratio sets available depending on what it came out of.  The wide ratio ones have a very big 1st to 2nd drop.  The close ratio ones aren't as bad, but with the 3.42 diff, you'll only have an 11.9:1 1st gear, which might be marginal for towing

For occasional towing, I'd be tempted to find an NV4500 and keep the 3.42s.  When not towing, drive it like a 4 speed (start in 2nd), as you'll have a 10.4:1 overall 2nd gear.  When towing, use the granny 1st, as it'll give you 19.2:1 overall, which will be very easy on the clutch for hill starts with a trailer.  5th is just a hair shorter than the 4L60E, so you'll be spinning less than 100 rpm faster on the highway than with the auto. 

I have an NV3500 in my 4.8 Silverado, 3.42 gears, and I have never once worried about first gear being too tall towing a trailer.