Drive shaft question.

[Breticus]

So my car is going to need a new drive shaft eventually. I was thinking about a one piece custom aluminum drive shaft. What are your thoughts on this?

[J!m]

You need the splines in the shaft to account for engine movement. A one-piece shaft does not have them.

US driveshafts have the splines at the transmission end so these can be one-piece; BMW does not do this.

Just get a replacement, or have yours re-built and re-balanced at a reputable shop. Also replace the carrier bearing and guibo while you're at it.:wink:

[Breticus]

They also have custom 2 piece aluminum driveshafts.

Edit: and steel

[J!m]

I don't know how much weight you could save with an aluminum shaft.. 10# maybe?

Unless the cost is similar, I would simply have it rebuilt and balanced.

[xxxJohnBoyxxx]

Here is the deal. Every pound of rotating mass you remove is the same as removing 15 pounds of car weight. So you remove 3 pounds of driveshaft and since it is a rotating part it is like removing 45 pounds of car weight (Rotational mass R x 15= lighter car without removing weight. Lots of small things add up and make a big difference

[Breticus]

The cost is similar it's like 700 to a 1000 for the aluminum shafts. :)

[J!m]

Here is the deal. Every pound of rotating mass you remove is the same as removing 15 pounds of car weight. So you remove 3 pounds of driveshaft and since it is a rotating part it is like removing 45 pounds of car weight (Rotational mass R x 15= lighter car without removing weight. Lots of small things add up and make a big difference

In the case of a flywheel, I believe that formula applies nicely. Similarly with tires and wheels.

However, in those cases the mass is quite far from the axis of rotation, and although you are absolutely correct in your statement that the acceleration will be slightly quicker, since the mass (in question) is so close to the axis of rotation, you would use a different formula.

On-axis it is precisely "1 times" (1X) of course, and as you move away from the axis of rotation, it increases (probably logarithmically). It takes less torque to get weight close to the axis of rotation moving that weight that is far from the axis of rotation. (a longer lever) This frees up torque to get the car moving.

Since the drive shaft is ~4" as opposed to ~12" (for flywheel) or ~18" for a tire/wheel you can envision how this would apply. Based on your formula, and assuming linear as opposed to log curve (15" fits well between flywheel and tire diameters), I would say it is more like ~4X, not 15X weight reduction. The down side is once it is moving (finished accelerating), it is still just dead weight. The formula applies to acceleration; deceleration is a strict 1:1 factor, unless this weight is contributing to the continued motion of the car (such as a flywheel [if left in gear] or heavy wheels) in which case it also affects deceleration. Overall weight (regardless of location) also applies to cornering, but like dynamic weight, higher weight has a greater impact than lower.

I therefore stick to my suggestion because unless you are watching every ounce in the car (rotating or not- which is not a bad idea mind you) you may be better served to spend money elsewhere, such as rotors, wheels or a flywheel to get the better return on investment. Or gut the interior for a greater weight loss at no $$ cost, and loose the weight higher up too.

I see the price is similar, and if it is in fact lighter, I say go for it! It can't hurt as long as the durability is comparable.:biggrin:

[Breticus]

Do you guys think this is an okay replacement for my completely shot driveshaft? http://www.wholesaleimportparts.com/BMW_318ti_Driveshaft_1995.php

or should I spend the extra money and get this?
http://www.turnermotorsport.com/p-5014-driveshaft-e36-318ti-manual-transmission.aspx

or are they about the same?

[Breticus]

Bump.

[J!m]

First one looks fine, asuming you cannot find a local drive shaft shop to do it cheaper for you...

[Breticus]

Well idk. It's got 180k miles on it and it makes awful AWFUL clunking sounds when I drive. Mostly starting, stopping, and shifting.

[J!m]

Did you check the guibo and center bearing? Those usually fail first anf are something you could do yourself (if you're creful).

It could also be broken engine and/or tranny mounts too.

check everything rather than just throwing parts at the problem...

[Breticus]

I'll check it out before I buy anything. Thanks. :)

[J!m]

Good man. Keep us posted...

[zoner]

Anyone ever tried these guys: Driveshaft Specialist of Texas (http://driveshaftspecialist.com/Import%20html/BMW.html?gclid=CJ_lpKm11rACFUQaQgodJDtm2Q)? $358 shipped sounds like a pretty good deal!

Also, what about Driveline Service of Portland (http://www.driveshafts.com/pages/bmw.html)?

I hear that Beyer Driveline is one of the best, but they only sell to shops.

I'm in need of a new driveshaft myself... I've been pushing my luck for far too long.