What is the point of the CSB?

[Calypso Moor]

And why should I continue to replace that piece of shit, instead of just getting a solid drive shaft?

[M-technik-3]

The point was to give better ground clearance.

[george graves]

Hmmm. The line-of-sight from tranny to Diff is almost a straight line - no? Low point is exhaust.

Regardless. Universal joints actually are best when they have a tiny angle to them. It has to do with the joints being pre-loaded. Well, I could explain it better, but I’d need charts and graphs and an easel.

Also - the center joint is there to prevent binding and loads on the DS while the engine moves under braking and acceleration.

Take your hood off your car, and go for a spirited run. You'll be AMAZED at how much your motor will move about on stock rubber mounts. (I suspect poly mounts will still give a bit of play - but I've never run them with out a hood so no clue)

Also, you may not know this, but rotational slop in your drive train is suppose to be there. If you had a perfectly tight/rigid DS, it wouldn't dampen the un-even rotational acceleration from the motor(as each cylinder fires, it speeds up and slows down in a way your eye could never see). Slop in the power train, smooths it out a bit, delivers power better, and reduces stress on everything.

[LJ851]

And why should I continue to replace that piece of shit, instead of just getting a solid drive shaft?

A CSB is required on a 2 piece driveshaft like bmw uses. For a driveshaft of a given length, a 2 piece shaft can be much smaller in diameter than a 1 piece shaft which saves weight, space and materials.

A 1 piece steel driveshaft would probably have to be 3.5-4 inches in diameter to match the critical speed of the stock driveshaft.

Also, bmw CSBs are very reliable, you may have another problem wearing the CSB out. Worn ujoints, improperly aligned DS, shitty aftermarket CSB, out of balance DS can all shorten the life of the CSB.

[hyperE30]

Also, bmw CSBs are very reliable, you may have another problem wearing the CSB out. Worn ujoints, improperly aligned DS, shitty aftermarket CSB, out of balance DS can all shorten the life of the CSB.

This might be your issue, I installed my junkyard driveshaft on since 2 1/2 years ago and have had no issues with the csb and also did 9 passes @ the track.

[Calypso Moor]

Also, bmw CSBs are very reliable, you may have another problem wearing the CSB out. Worn ujoints, improperly aligned DS, shitty aftermarket CSB, out of balance DS can all shorten the life of the CSB.

I changed my csb last December, and it hasn't even lasted a whole year. my mom always had trouble with her csbs' in her old BMWs too. guess I should take it to certified mechanic. sigh.

[george graves]

A 1 piece steel driveshaft would probably have to be 3.5-4 inches in diameter to match the critical speed of the stock driveshaft.

Lol LJ851. What are you walking about? I'll assume that was a strange typo, or mis-worded - cause you totally lost me.

[LJ851]

Lol LJ851. What are you walking about? I'll assume that was a strange typo, or mis-worded - cause you totally lost me.

No typo. Any spinning shaft has a speed (rpm) that it becomes unstable at and will likely self destruct called the critical speed. The longer the shaft, the lower the critical speed. Increased diameter and lighter weight are the two ways to increase the critical speed of a given shaft length.

BMWs generally have higher rpm engines so a 2 piece shaft (effectively two short shafts) is good solution.

[frankenbeemer]

What does engine speed have to do with drive shaft speed?

[rwh11385]

GG: http://www.roymech.co.uk/Useful_Tabl...cal_Speed.html

http://www.wallaceracing.com/driveshaftspeed.php

Also, http://www.bimmerforums.com/forum/sh...ce-drive-shaft

Search the track section. Guys have tried it, the stock driveline with single piece driveshaft will develop nasty harmonics / vibration. The OE 2-piece shaft is actually really nice, the joints are designed in phase with each other.

I ran a 1pc in my E30 for a while.

Had all sorts of harmonic issues.

Length Vs Diameter said it should be fine, and dynamic ballancer also said so, but not in the real world.

Tried 2.75" & 3" shafts, thin & thick walled steel.

Ended up with a beefed up 2pc back in there.

Finally, you can get away with a stiff and light 1 piece, but CF is probably not what BMW wanted to put into an entry-level sedan for the yuppies. (Nor did the experience really exist back 30 years ago) So a 2-piece steel design is pretty keen. Now... the F8X Ms are different as far as worth it to make it 1-piece and CF: http://f80.bimmerpost.com/forums/sho...d.php?t=921523

Big picture comparing the new and old: http://f80.bimmerpost.com/goodiesfor...ays_dj1087.jpg

http://www.caranddriver.com/features/dissected-2015-bmw-m4-feature-every-bragging-point-counts-page-3

Every driveshaft has a “critical speed” where whip and vibration become destructive. In the case of the M3/M4, with the manual transmission in top gear, the shaft is overdriven, so it’s susceptible to whipping like a jump-rope at high rpm. The low weight and high stiffness provided by CFC construction move this driveshaft’s critical speed beyond the normal operating range.

[LJ851]

What does engine speed have to do with drive shaft speed?

Well, the engine is what spins the shaft. :p


Typically, a high rpm engine will run a numerically higher rear end ratio and see higher driveshaft speeds than a low rpm engine.

Another thing about 1 piece in a bmw is the layout of the drivetrain in the chassis. A 1 piece shaft likes to see equal and opposed U joint angles on either end of the shaft, usually on a vertical plane. The drivetrain in a bmw is basically straight all the way to the diff with a slight Ujoint angle there on the horizontal plane. You really would need to shift the location/angle of the engine/trans to create a friendlier environment for the 1 piece shaft.

[frankenbeemer]

Got it.