What's the critical speed of a stock '91 V8 driveshaft with 3.23 gears? For those not familiar with the term, thats the speed where basically the driveshaft explodes(theres alot more to it, but you get the idea). For example, the 94-96 Impala SS was governed at 154 MPH not because of tires, but because the driveshaft critical speed is 156-158 MPH, and isn't safe over that speed. Thanks for the info.

 

Kinda sounds to me like someone fed you some bs. A properly balanced driveshaft should cause no problems at any speed youll see on the street.

 

If you google "driveshaft critical speed" you will find enough info to make your head spin.

Regards,
Gary Anderson

 

I did google "driveshaft critical speed" and it talks about the critical speed in terms of RPM of the shaft and not the speed of the car. Other factors include weight of the shaft, diameter of the shaft, and the material the shaft is made from.

Correct me if I’m wrong but, as I see it the speed of the car has less to do it than the RPMs of the shaft.

I guess if you take in to consideration the transmission highest gear and the gearing of the rearend there is a maximum speed the car might reach safely. But I bet it will be faster than the car will be able to go anyway, especially if the Transmission has OD.

 

Yes, it is the RPMs of the shaft that matter, not the speed of the car. But the RPM VS speed will change depending on what gears you have in your differential. For instance, the same Impala SS with 4.10 gears instead of the stock 3.08's drops the critical speed down to 110 or so MPH with the stock driveshaft. So, what I'm wondering is what the vehicle speed is when the driveshaft reaches it's critical RPM with a stock F-body shaft and 3.23 gears. Thanks.

 

thats something you should NEVER have to worry about

i mean if your just interested to see what it is, then thats cool....

but unless the shaft is defective, it will NEVER explode from being spun to fast at any speed your car will ever reach.

most quality aftermarket shafts are spun balanced to 10,000rpm

 

It doesnt matter what rear you put in the car, if you have 2.xx or 4.xx's. With the engine revving 6000 RPM your driveshaft will still be spinning 4200 RPM in OD.

With the 3.23s spinning the engine up to 6000RPM in 4th will get you 220MPH which the car will probably take flight long before you hit that.

 

Thanks for humoring me. I've got the info I need now. Although I know I'll never get close to 220 MPH, I might do some landspeed racing in the future. I know the critical speed is an issue in my buddys not so stock ex-cop Caprice, I just wanted to make sure I wouldn't have similar issues if I decide to follow in his footsteps. He tells me that the salt gets in your blood. BTW, did you know that on a '94 Cop Caprice the dual spotlights cost you 4-5 MPH of top speed?

 

I thought about it some, and I think your math is wrong. 3rd gear is 1:1, 4th is .7:1, meaning that the engine spins .7 times for every 1 revolution of the driveshaft. Thats what makes it an overdrive. So, you need to divide the engine RPMs by the gear ratio instead of multiply.

So, in 3rd, because it's 1:1, the engine spins 6000 RPM, the driveshaft spins 6000 RPM(obviously)

But in 4th, the engine spins 6000 RPM, the driveshaft would be spinning 8571 RPM(that's a whole lot for a stock steel shaft to take)

If the stock shaft is only good for 4200 RPM, that knocks down the top speed conciderably from 220 MPH into a much more attainable speed(not good).

 

It is not that your driveshaft will explode or have a mechanical failure but it will reach its natural frequency and become unstable. The critical speeds are really steered toward that mentality. Any shaft that becomes unbalanced at that speed can drastically upset the car and become dangerous.

 

ahh crap, yea i guess i did it bacwards

So @ 6000RPM the driveshaft will be doing 7,800 in OD. i think...

Now im all confused lol

 

Always rememeber, OverDrive increase driveshaft speed and decreases engine speed.

I know it was mentioned, but a lot of people to get cought off guard by that.

later
Jeremy

 

Everything appears to be correct and seemingly dangerous on paper, but we live in the real world. Hardly ever do the two parallels have the same results. I mean come on, have you ever seen or heard of a driveshaft exploding. Ive seen a couple twisted in two right off the line, but never one exploding at the top end of the track.

 

Like mentioned above, it's not so much that it "explodes", but more like the afore mentioned "becomes unstable" where instead of it spining in a circle, it starts to wobble, which can cause rapid failure of the u-joints or yoke welds, or upset the car enough to cause loss of control at speed(not good on the salt because it's like driving on ice when you get up that fast). It's not typically a problem on drag cars because they either don't go fast enough or have shorter driveshafts in them(like a funny car). They also don't run for extended periods at high speeds. Also, a larger diameter aluminum shaft like you find on many drag cars will have a higher critical speed than a small diameter steel shaft like the stocker will. Basically, the lighter the shaft, the higher the critical speed, and the larget the diameter the higher the critical speed.

 

umm even the basic of drag cars easily hit 125+ mph , ill take camaros for example for obvious reasons, alot of them use stock driveshafts and nope not once have i heard read or seen anywhere anything about driveshaft failure at the end, and for ****s n giggle, correct me if im wrong, dont they usually run pretty low gearing? 3.73+? (numerically)

 

the axle/driveshaft/tire rpm wouldnt change at the same speed regardless of gear the trans is in


tires still spinning same,. IE axle, IE drivesahft


at a given speed the driveshaft cant change rpm no matter what gear youre in


the engine would change RPM, not the driveshaft/axle/tires

 

Yeah i know...

if the driveshaft spins independantly of the axles theirs a bit of a problem lol


Im curious as to how you figure i said their changing while doing the same speed

 

And who is going to be turning 6000 RPM in top gear? They must have one ****FAST car...

 

Me, because i have 411s and a th350c...
6000RPM only works out to around 70mph


And like i put in there if it got up that fast with that gearing it would just fly away anyways lol