Drive line critical speed.

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belgiquebasterd
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Drive line critical speed.

Post by belgiquebasterd » Thu Oct 23, 2014 5:31 am

Doing a T56 manual gearbox install in a quadcab pickup and am worried about drive shaft critical speed.

OEM driveline is a 4,5" aluminium one piece driveshaft, tube length 175cm/69" long. The manual gearbox is a bit shorter so the new driveline tube needs to be 71,5"/182cm long. The peak rpm of my engine will also be raised from 5600rpm to 7200 rpm.

Have looked around on google and no one offers a driveshaft this 72" long (only tube length, yokes add on top of that) due to critical speed, my question is what are my options, I would like to avoid a 2 piece driveshaft.

Thanks in advance.

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Re: Drive line critical speed.

Post by clshore » Thu Oct 23, 2014 6:37 pm

Steel, larger wall thickness.
The critical speed has to do with bending strength.
Thicker wall increases bending stiffness.

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Re: Drive line critical speed.

Post by mike hohnstein » Fri Oct 24, 2014 1:23 pm

Rule of thumb is 55" is safe, for most apps. Sounds like a two piece is in your future to me.

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Re: Drive line critical speed.

Post by Dan Timberlake » Sat Oct 25, 2014 1:42 pm

Critical speed for a metallic drive shaft of a given length is mainly a function of tubing diameter. Aluminum's lesser stiffness essentially cancels out it's weight advantage over steel. Adding wall thickness only lowers critical speed (slightly).

http://www.wallaceracing.com/driveshaftspeed.php
Online calculators and Even the SAE driveshaft manual's critical speed formula and tables assume too much stiffness of the transmission and differential mounts.

What type of u-joints are you using? Cardan type joints create a secondary couple under load that can excite the shaft's bending critical while running at half the actual critical rpm!

Critical speed of a driveshaft depends directly on the driveshaft weight and stiffness.
With a constant diameter and length going from from steel to aluminum of same wall the weight does goes down to 1/3, but the "modulus of elasticity" also drops nearly the same, so in the end the critical speed is virtually the same as steel.

Doubling wall thickness increases stiffness by about 2X, but doubles the weight, so again critical speed remains about the same.
Unfortunately Adding wall thickness does increase torque capacity as expected.

Tube OD needs to increase as length or rpm increases.

http://www.eng-tips.com/viewthread.cfm?qid=362928

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Re: Drive line critical speed.

Post by belgiquebasterd » Sat Oct 25, 2014 2:33 pm

Thanks for the info guys!

Current setup has 1350 style ujoints, slip yoke into the T56 transmission and flange yoke at the pinion.

Image
On my old setup (stock 545RFE automatic trans) I had real bad vibrations when going over 100mph, now after reading up on drivelines and their critical speed I'm pretty sure that I exceeded driveline critical speed and that caused the vibrations. I had a lot lower than stock wheels which added rpm's to have the same speed as before, also the 546RFE 5th gear is someting like 0.60/1 overdrive. Putting my 65" long OEM driveshaft rpm at around 5500 rpm when doing 100mph.

Some companies offer big aluminium 5" diameter driveshafts in the length I need, any experiences with one of those?

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Re: Drive line critical speed.

Post by Supershafts » Sun Nov 02, 2014 11:11 am

Steel, larger wall thickness.
The critical speed has to do with bending strength.
Thicker wall increases bending stiffness.

Increasing stl to heavier wall is not the answer to spinning it faster. That's how you make a shaft that will cut your vehicle in half do it faster.

The opposite is what you do, however when you decrease the wall for rpm you lose torque ability.

This is why you increase diameter.

Your 7200 for a 1 pc will be in CF
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Re: Drive line critical speed.

Post by ScottKnight » Tue Nov 04, 2014 9:24 am

I clicked on this thread since it is one of the areas where I have a small amount of domain knowledge, having done a MMC propshaft project back in the 90's. The mid-90's Impala SS/Caprice had a woefully under-engineered propshaft that vibrated pretty bad at speed with the stock 3.08 gears. Putting in 3.73 gears just made it that much worse. Dynotech worked up a drop in replacement in Metal-Matrix Composite (a fancy aluminum alloy). Vibration was never a problem after that as long as the u-joint angles were correct.
Dan Timberlake wrote:http://www.wallaceracing.com/driveshaftspeed.php
Online calculators and Even the SAE driveshaft manual's critical speed formula and tables assume too much stiffness of the transmission and differential mounts.
The things we learn by accident. I have poked around on the wallaceracing site for things like the DCR calculator before, but apparently never came across this one. It looks familiar to me because it is actually mine from back in about 1997:
http://scottknight.com/calculators/js_driveshaft.html

That calculator was a direct result of getting Dynotech's sizing spreadsheet back in the day. It's possible that they have updated their engineering process a little since then and possibly even their sizing criteria.

Keep in mind that your engine speed has nothing to do with your propshaft speed unless you have changed the rear gears to suit. The propshaft speed is only dependant on the size of the tire, the gear ratio and the vehicle speed. I have an old calculator that will help with that as well (there is probably a similar one on wallaceracing):
http://scottknight.com/calculators/js_tire.html

Plug your tires and gear ratio into that to get a general idea of how fast the shaft will be spinning at whatever top speed you plan to take it. Fiddle with diameter in the driveshaft calculator to get the critical speed well above that (30%? 50%?). Once you have a general idea of the required sizes for the different materials, you can start calling around to the custom shops. I only know of Dynotech, Denny's and Inland, but I am sure there are others. They are going to do their own analysis, but at least you have a sanity check up front.

If you can't get something in MMC, aluminum or steel that will work, carbon fiber is always an (expensive) option. Guaranteed you can get one in CF that will work.

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Re: Drive line critical speed.

Post by Supershafts » Tue Nov 04, 2014 8:44 pm

Youre mistaken about engine speed, Engine speed in 1.1 is the speed the shaft is at at that engine speed. Engine speed in od is the engine speed / .69 or what that ratio is for OD.

So say youre at 3500 rpm in OD the shaft is at 5070
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Re: Drive line critical speed.

Post by belgiquebasterd » Sat Nov 08, 2014 5:57 am

ScottKnight wrote:I clicked on this thread since it is one of the areas where I have a small amount of domain knowledge, having done a MMC propshaft project back in the 90's. The mid-90's Impala SS/Caprice had a woefully under-engineered propshaft that vibrated pretty bad at speed with the stock 3.08 gears. Putting in 3.73 gears just made it that much worse. Dynotech worked up a drop in replacement in Metal-Matrix Composite (a fancy aluminum alloy). Vibration was never a problem after that as long as the u-joint angles were correct.
Dan Timberlake wrote:http://www.wallaceracing.com/driveshaftspeed.php
Online calculators and Even the SAE driveshaft manual's critical speed formula and tables assume too much stiffness of the transmission and differential mounts.
The things we learn by accident. I have poked around on the wallaceracing site for things like the DCR calculator before, but apparently never came across this one. It looks familiar to me because it is actually mine from back in about 1997:
http://scottknight.com/calculators/js_driveshaft.html

That calculator was a direct result of getting Dynotech's sizing spreadsheet back in the day. It's possible that they have updated their engineering process a little since then and possibly even their sizing criteria.

Keep in mind that your engine speed has nothing to do with your propshaft speed unless you have changed the rear gears to suit. The propshaft speed is only dependant on the size of the tire, the gear ratio and the vehicle speed. I have an old calculator that will help with that as well (there is probably a similar one on wallaceracing):
http://scottknight.com/calculators/js_tire.html

Plug your tires and gear ratio into that to get a general idea of how fast the shaft will be spinning at whatever top speed you plan to take it. Fiddle with diameter in the driveshaft calculator to get the critical speed well above that (30%? 50%?). Once you have a general idea of the required sizes for the different materials, you can start calling around to the custom shops. I only know of Dynotech, Denny's and Inland, but I am sure there are others. They are going to do their own analysis, but at least you have a sanity check up front.

If you can't get something in MMC, aluminum or steel that will work, carbon fiber is always an (expensive) option. Guaranteed you can get one in CF that will work.
Thanks for the info and the calculator links!

Contacted several shops and all replies were the same, either a CF shaft or a 2 piece shaft for that length/rpm. Carbon is about 500 more expensive than a new 2 piece shaft, wondering if it's worth it?
This is a daily driven car, will the carbon fiber hold up to road salt, debris, rocks....?

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Re: Drive line critical speed.

Post by Supershafts » Sun Nov 09, 2014 1:38 pm

CF is more than $500 more for a 2pc, unless you're doing a 2pc in two materials, and even then it should be more than only $500 less for a CF shaft.

If this is a DD then you can do it in a 1pc shaft.

In a 1 pc shaft you'll need to set a governor for 135 mph.

You have to be specific when you talk to driveline shops, do not throw out 7200 rpm ceilings if you aren't going there in 6th gear.

When i tell you my motors rpm is 8300 in my race car, it is GOING to 8300 in top gear as well as 1st thru 3rd w/4.56 and a 28" tire.

My DD truck has a redline of 6600, it is NEVER going to 6600 in 5th gear, so i don't need to build a shaft to 6600 rpm in 5th which should be around 220mph, i only need to know i'll probably, maybe see 110 mph and shaft speed is around 4200.


VERY big difference.........this is also why many vehicles have speed governors on them, and why you don't just remove speed governors when you get a programmer that can remove them.....

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Re: Drive line critical speed.

Post by MadBill » Tue Nov 11, 2014 10:46 pm

Supershafts wrote:Youre mistaken about engine speed, Engine speed in 1.1 is the speed the shaft is at at that engine speed. Engine speed in od is the engine speed / .69 or what that ratio is for OD.

So say youre at 3500 rpm in OD the shaft is at 5070
I'm with Scott on this one. Engine RPM is the result of tire size, overall gearing and speed. Shaft speed is dependent only on tire diameter, axle ratio and vehicle speed. The majority of current vehicles have one or more overdrive gears and might well be capable of more speed in 4th or 5th with a 6 speed manual, but this is not relevant to DS speed. Your last post actually confirms this.
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Re: Drive line critical speed.

Post by Supershafts » Sun Nov 16, 2014 8:01 pm

You're missing what im saying here.

Engine speed in the gear of the trans that is 1.1 (3-spd usually 3rd and 4-spd usually 3rd), now say you are at 6000 rpm while in that trans gear that is 1.1, the shaft is also at the same 6000 rpm that the motor is, at that point everything from the front balancer to the pinion gear is at 6000 rpm, now you engage od and you are only slowing the front half of the trans and the motor.


This is what He said
Keep in mind that your engine speed has nothing to do with your propshaft speed unless you have changed the rear gears to suit. The propshaft speed is only dependant on the size of the tire, the gear ratio and the vehicle speed.

I don't care what you changed in the rear or what tire or what motor you're using, If your motor is turning 8000 rpm with a T-400 , T-350, 727, 4l80 or anything else and you are in 3rd gear in any of those transmissions which is 1 to 1 or 1.1 your shaft is at the same 8000 rpm.




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Re: Drive line critical speed.

Post by MadBill » Sun Nov 16, 2014 9:39 pm

You're confusing cause and effect. For most applications most of the time, one picks a vehicle speed and so the prop shaft RPM is a result of rear gearing, tire size and the chosen speed. Engine RPM then is a result of prop shaft speed and transmission gearing, which these days is typically not 1:1 in high gear.

You don't get your motor running, head out on the highway and say to yourself: "I'm going to cruise at 2,056 RPM 'cause I don't want to get a ticket."
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Re: Drive line critical speed.

Post by Supershafts » Tue Nov 18, 2014 1:04 am

MadBill wrote: You don't get your motor running, head out on the highway and say to yourself: "I'm going to cruise at 2,056 RPM 'cause I don't want to get a ticket."
I don't and most others don't do that, and you're right but, thats not what i am saying about shaft speed, but in some cases you MAY be limited to just such a issue.

However, they build a motor for a designed power area and use, you/they then build the rest of the vehicle to suit that motors ability, or you limit it at certain areas.

So when you say i have a 7500 rpm ceiling and you have a 545 or 4l80e/ect you take into account those ratios in those transmissions, of which you can then figure out what shaft the vehicle needs by its highest possible torque generated by those and it's highest possible shaft speed, which is now more than 7500 rpm.

Gears and tires play NO role in that design, because at 7000 rpm in 5th or 4th the shaft is at 10,000+ rpm, regardless of what tire size or rear gear you have at this point this is what that shaft is seeing with a 35" tire or a 12" tire, the difference between gears and tire size is speed, and that may be 230 mph or barely 90 mph while the motor is now out of ability to see more rpm and at the same time put the shaft speed any higher also, except possibly in the case of the 12" tire as 90 mph may not have enough aero drag on a severe hill to hold it at 90 mph.

So when you want to build or change a shaft design in a vehicle this is what it will be based on, and SHOULD MOST DEFINITELY be based on, then explained and then if no one is going to try and make a 5 mile all out run other options can be discussed for the shaft design.

This is why the factories have limiters or governors on many vehicles, not to keep you below 110 mph, but because the shaft is beyond it's ability.


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Re: Drive line critical speed.

Post by MadBill » Tue Nov 18, 2014 8:25 am

I think we're looking at the same issues from different directions. For example, if you have a 500 HP @ 7,000 RPM engine in your street/strip '69 Camaro and mostly run 1/4 mile, you may have a 4.33:1 gear and go through the lights in 4th, but if your transmission has a 0.6:1 5th gear and you decide to wring it out on the standing mile and a half ex-air force runway, you better know that pulling 7,000 RPM in 5th means a prop speed of almost 12,000 RPM. On the other hand, if the same engine is in your van-based motorhome, still with the 4.33 gears, even 500 HP wont push that brick at more than 125 MPH so the maximum possible prop speed is only ~ 6,300 RPM in 4th and less in 5th, since the power will be way down at ~ 3,800 RPM.
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