It's time..Driveshaft selection/purchase is imminent. 3 or 3½" ?? Strange says 3" good to 7,500

[biomedtechguy]

So a few driveshaft manufacturer reps as well as individuals who make driveshafts, all reputable and referenced, have said for me to be assured that the critical RPM requirements won't be exceeded, my appx 50¼" chromoly steel driveshaft has to be 3½" diameter. Strange Engineering rep said 3" according to his chart would be good to 7,600 RPM. A rep for Mark Williams, a company that seems to be at the pinnacle of driveshaft tech and manufacturing, actually sent me a quote. $712 which is in the range that most have been in, from a low of $600 up to $750.
He said 3" has a critical speed of 6,875 and a 3½" of 7,982. Others have said that at or above 7k RPM requires 3½".
So it seems like Strange is alone in their claim of 3" good to >7k RPM.
The easiest way to stay below 7k RPM is to get a taller diameter tire. Mine are on the smaller side of 17" wheel tires at 25.8"
but their sidewalls aren't the best for traction anyway at 315/35/17. The next pair of tires I'm planning on trying for fitment, to be used as my cruising driving tire and track tire if they work are 345/40/17. They are a full 2" larger in diameter and that makes a big difference. Of course there's a myriad of other choices, like one I'm looking hard at for track only duty, the M/T ET Street R Bias Ply. It's 28x11.50-17LT and 28½" in diameter. Basically all of those but the ones I've been using for the last few years have me under 7k RPM at 139 MPH.
The other consideration is I have to get my exhaust pipes reworked some if I go over my current 3¼" diameter.
Comments/Suggestions please.
It's go time and I'm taking measurements tomorrow, so Monday is the day I'm ordering the driveshaft.
Thanks

 

[Jerry Hall]

Just my opinion here. I guess bigger is probably better in most cases. In the really distant past I have twisted a driveshaft or two. It was always with the initial launch. I have also spread yokes. After the first one I installed a driveshaft loop. My personal opinion is that with a properly balanced driveshaft that rpms shouldn't make a difference. Your driveline is only as strong as it's weakest link. I would worry more about U joints and yokes than the actual driveshaft. My experience from the late 60's and early 70's were as follows in order:
1. Getting a clutch to hold
2. Then quality U joints
3. Switching to the large yoke
4. Driveshaft.
If you are going into the high 600 to 700 HP range with your proposed stroker engine do as it seems to me you are doing on everything. The biggest and the best. I'm just personally glad you are enjoying your car. Keep the hobby fun.

 

[biomedtechguy]

Whether I get a 3" or 3½" diameter driveshaft, I already have the Strange chromoly 1350 pinion yoke and reluctantly sprang for the matching caps. The slip yoke is going to be a 1350 U-Joint as well.
Really the only remaining question is 3" or 3½", and if I can reliably spin the 3" as fast as neccessary, I'd prefer that so I don't have to redo part of the exhaust pipes.
It looks like if I had a 29" diameter tire, I'd be fine with the 3" shaft, but I have some more calculations to do.

 

[DeltaV]

How much exhaust pipe clearance do you have with the current 3¼" diameter?

 

[biomedtechguy]

I eyeballed it up close, and I think another ¼ may rub. It wouldn't be a big deal except it's right by the X pipe, and I'd rather not have to have that cut out and replaced.

 

[CHARGERSTEVEN]

Why dont you check on getting one made with modern composite material,lite weight n the strenght is better than steel

 

[biomedtechguy]

Why dont you check on getting one made with modern composite material,lite weight n the strenght is better than steel

Those tend to be a larger diameter.
Aluminum for example would definitely have to be 3½" and if I can go 3¼ or smaller, no exhaust work required.
I used the Wallace calculator and I keep getting 7,600 or so and that's with mild steel where I'm using chromoly.
By the time I order on Monday, I should have the info I need.
My 5th gear overdrive doesn't really figure into the equation because I'm not looking for top speed, just 1320 speed, at around 139-149 MPH which I may see the 139 mph end of that (roughly high 9's)

 

[MoparLeo]

How about what type of driving are we talking about ? Straight line quarter mile with acceleration loading only, or road racing with constant up and down shifting. Makes a big difference on the drive-line stresses.

 

[Dibbons]

This may not help your decision, but I remember a broken down garbage truck across from my high school one afternoon-the driveshaft was in the street and looked like a cinnamon twist.

 

[biomedtechguy]

How about what type of driving are we talking about ? Straight line quarter mile with acceleration loading only, or road racing with constant up and down shifting. Makes a big difference on the drive-line stresses.

Cruising around and a couple of test and tune nights a month.

-the driveshaft was in the street and looked like a cinnamon twist.

 

[66Satellite47]

A 3" CM with the correct wall thickness should take well past 7000 RPM I would think. Check with the best vendors. Also the balance is really important. 3 1/2" is just that much better if it fits.

 

[biomedtechguy]

So I got back with Strange Engineering this morning. Unless the measurements I just took changes anything, they say the 3" shaft has 100% critical RPM at 7,500. They are at $544 for a seamless chromoly 3" diameter driveshaft, forged chromoly slip yoke, and 2x 1350 solid U-joints balanced and assembled.
Others are $100-$250 more!
My actual measurements are:
Back surface of Output shaft to yoke face/U-joint center 53⅜"
Trans case to yoke face/U-jount center 54½"
I had told them the existing driveshaft was 50¼" U-Joint to U-Joint, but it is closer to 50¾". Anyway, the actual length of the new driveshaft U-Joint to U-Joint should be about the same.

 

[biomedtechguy]

Here's a few pictures from today.
Assassin traction bars and Calvert split mono leaf springs:

I sat in the car so he could measure and mark the sliders to weld them on.
The sliding bolt is going to be close to the middle of the slot when finished.

I really like the Assassins! I see my spring perches are nice and flat too!

 

[DeltaV]

How about some pictures showing your 1/4" exhaust clearance?

 

[biomedtechguy]

How about some pictures showing your 1/4" exhaust clearance?

Driveshaft is already out. 3" exhaust pipes all the way..
I just reviewed the pictures I took at the shop, and none of them show the area by the X pipe where the driveshaft gets close.
Believe me...it was close...

 

[biomedtechguy]

Going through my pictures, this is when I had the Doug's 3" cutouts and X pipe installed. This is before the pipes were completed and hangers mounted and the system snugged up to the body.
The area that is tight is by the X pipe.

 

[DeltaV]

The obvious answer is you need more clearance (like an inch) no matter the shaft size you pick. That will be the perfect time to get rid of the x-pipe and have a proper H-pipe installed .

I saw your other thread. I would go with the larger diameter aluminum shaft and re-do the exhaust as needed to provide more/adequate clearance.

Nevertheless, use reputable suppliers and installers and discuss everything with them thoroughly. You seem to be in a rush. Don't rush.

 

[biomedtechguy]

That will be the perfect time to get rid of the x-pipe and have a proper H-pipe installed

I prefer the superior performance of the X pipe to the "correct" but outdated H pipe..
I have never had the driveshaft make contact with anything, ever, and I'm going ¼ smaller in diameter!
Thanks for looking out for mebut in this case, I think a case of 5.7 x 28mm rounds would be better.

Need the driveshaft! I've been researching for at least 2 weeks! Time to act!

 

[440rb]

I'm not looking for top speed, just 1320 speed, at around 139-149 MPH which I may see the 139 mph end of that (roughly high 9's)

Doing a few calcs for ya and I can't see you going past 7k trap speed in 4th with a 28+" tyre and 4.10's...assuming your@3700lbs all in (all ally motor) and say 700hp it'd be 6388rpm in the traps. At 800hp=6679rpm and@850hp at 9.60's/140mph=6815rpm. And say at 900hp you'd still only trap@6947.
All this rpm theory is of course with the hp no's being there from the hit to the line which won't be the case for most of them?....@149mph and 1000hp you'd be going 7195rpm and 9.00's.

 

[DeltaV]

Need the driveshaft! I've been researching for at least 2 weeks! Time to act!

I know, somebody said...When you have to shoot, shoot, don't talk!