Driveshaft 101

[ss dave]

Got the car on the highway for the first time since the extreme makeover. At 72-74/75 mph had a fine vibration that we diagnosed as the driveshaft.
Had local company, shorten/balance the steel shaft: but unfortunately they can only spin it at 500 rpm.
I can send it away to get it high speed balanced or just chinch up and get a new one made, probably for not much more of a difference.
My dilemma is what material to use, steel or aluminum. I did the search and found that aluminum saves weight, increases driveline efficiency therefore putting more hp/tq to the wheels at lower rpm, but at a cost. How about Chro-moly? I would like to hear more opinions-got one?, sound off. Thanks

[chicane67]

Increases driveline efficiency ? I take that you mean that from an inertia and prarasitic perspective ?

I think one needs to answer a few questions before the selection of material. First of all... how much room do you have ? How close is the exhaust system to the shaft location ? What is your requirement for maximum shaft (critical speed) RPM ? What are your performance goals for your chassis ? And lastly... How much do you really want to spend ?

I see your first problem is that your shaft is only spun to 500 RPM for balance... and that is a problem. At a minimum, any OEM or performance driveshaft should be balanced to 8k... and of course, even higher if the critical speed dictates otherwise.

4130 shafts are going to be the strongest and depending on their overall diameter, will also yield the lowest critical speed. 3.00", 3.50" and 4.00" are the common shaft diameters for CM.

6061-T6 would be the entry level aluminum. But remember, not all 6061-T6 shafts are created equal... and that goes for any material, not just aluminum. 3.50" and 4.00" are common shaft diameters for aluminum.

MMC tube with 7075-T6 yokes would be the next step. Its much stronger than a 6061 shaft... with a cost increase, but with a much higher critical speed rating over the CM and 6061 shafts. 3.50" to 4.00" here as well.

An just for information... the next step would be a carbon fiber shaft with 7075-T6 yokes. Its claim to fame is it's considerable increase in critical speed, it can be used safely in longer shafts without issue, it's built in safety in consideration at point of shaft failure and its weight. Typically these shafts are 3.75" in diameter... and cost a little bit more.

If you provide me your drive shaft length, from center of u-jount to center of u-joint, and your rear end gear ratio, tire diameter and transmission ratios (or just the transmission would do)... I can do the math for you. This way we can narrow it down to a specific diameter to meet your critical speed needs and possibly save you a little money in doing so. I mean... it would be really easy for a performance drive shaft supplier to tell you that you need a 4.00" diameter shaft... when you may only need a 3.50" shaft. That is a difference of nearly $50 to $100 (depending on material) between the two.

If you send your existing shaft out for balancing... I would recommend Mark Williams myself. They balance their shafts up to 10k and provide you with a data sheet.

[ss dave]

Thank you. 500rwhp, 3.73 gear, 4l60e, 26" tire, 3900lb car, 7200 rev limiter, I can get shaft size on Monday 5th. Efficiency is important along with performance.