Driveshaft question. Need some math help.
03-22-2003, 05:51 AM
#1
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Thread Starter
Join Date: Jan 2002
Location: Bay Area, CA
Posts: 1,697
Driveshaft question. Need some math help.
You can see what i got in the sig. Stock driveshaft right now. I want to go to a carbon fiber shaft soon. (safer, stronger, and lighter). The carbon fiber shaft is about 9-15 lbs lighter that the stocker. Less lbs means less rotational mass. Less rotational mass means more efficient power transfer.
So if the A4 looses about 17% in the driveline, how much does the driveshaft account for (ie, 4%? 9%?)
So the main question is, how much improvement would i be looking at in the percentage of power lost? I was kinda hoping for it to drop to 13-14% driveline loss. What do you guys think?
So if the A4 looses about 17% in the driveline, how much does the driveshaft account for (ie, 4%? 9%?)
So the main question is, how much improvement would i be looking at in the percentage of power lost? I was kinda hoping for it to drop to 13-14% driveline loss. What do you guys think?
03-22-2003, 11:54 AM
#2
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Join Date: Oct 2002
Location: Upstate NY
Posts: 2,931
Some thoughts:
1) 9-15 lbs lighter? That seems like a lot. A 4 inch diameter steel tube 5 feet long with .083 wall thickness weighs about 17.3 lbs. It's the steel yoke ends and u-joints (which the cabon shaft probably still needs) that contribute to the total prop shaft weight.
2) A lighter shaft will certainly have less rotational inertia, but not a lot, because it's small in diameter. Less inertia helps acceleration, but the prop shaft only gets up to engine rpm in 3rd and only faster in 4th, where the acceleration (mph per second) isn't really quick. You won't feel much difference in the street or on the dyno, IMO.
3) Running less massive tires would probably make a lot more difference in acceleration. So would a smaller diameter torque converter with stock stall speed.
4) The efficiency of a drive shaft depends almost entirely on the u-joints and their angles; more angle = more losses. You won't really change driveline losses with a carbon shaft.
5) I'm unsure of the cost of such a prop shaft, but I expect it's not cheap. The cost/benefit ratio might not look too good. This is my math help answer.
6) I suggest that you might see more RWHP increase from underdrive pulleys than a carbon shaft.
My $.02.
1) 9-15 lbs lighter? That seems like a lot. A 4 inch diameter steel tube 5 feet long with .083 wall thickness weighs about 17.3 lbs. It's the steel yoke ends and u-joints (which the cabon shaft probably still needs) that contribute to the total prop shaft weight.
2) A lighter shaft will certainly have less rotational inertia, but not a lot, because it's small in diameter. Less inertia helps acceleration, but the prop shaft only gets up to engine rpm in 3rd and only faster in 4th, where the acceleration (mph per second) isn't really quick. You won't feel much difference in the street or on the dyno, IMO.
3) Running less massive tires would probably make a lot more difference in acceleration. So would a smaller diameter torque converter with stock stall speed.
4) The efficiency of a drive shaft depends almost entirely on the u-joints and their angles; more angle = more losses. You won't really change driveline losses with a carbon shaft.
5) I'm unsure of the cost of such a prop shaft, but I expect it's not cheap. The cost/benefit ratio might not look too good. This is my math help answer.
6) I suggest that you might see more RWHP increase from underdrive pulleys than a carbon shaft.
My $.02.
Last edited by OldSStroker; 03-22-2003 at 12:17 PM.
03-22-2003, 01:14 PM
#3
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Join Date: Nov 1998
Location: Hell was full so they sent me to NJ
Posts: 70,662
1 - You can't save 9 to 15#. The stock steel DS weighs about 17# - that's what mine weighed. The writeup on the T. Byrne Motorsports website for the ACPT DS shows:
steel - 17#
aluminum - 12# (5# savings)
ACPT CF - 9.6# (7.4# savings)
2 - There isn't much HP to be gained. Keith at ws6.com dyno tested the stock steel vs. 1LE aluminum shaft and gained 1.1HP and 5ft-lb. Although automatics can have drivetrain losses of 16-22% of flywheel HP, and the T56 can cost you 12-15%, most of that is lost in the "variable" portion of the drivetrain.... gear contact as a function of torque transmitted, bearing friction, hydraulic pumping losses, etc. I have data for several different HP levels, and it appears the "fixed" portion of drivetrain loss, the rotational inertia part, is only in the range of 18HP total. I believe Keith also did a dyno coast down test to compare HP losses with conventional and synthetic lubes, and found numbers in the range of 16-18HP for the coast down loss. Those losses are primarily rotational inertia, because there is little in the way of frictional losses in the "coast down".
3 - Spending $700 for a CF DS, to save less than maybe 3HP isn't a very cost effective way to build rwHP. It gets even worse when the $700 ACPT CF DS fails.... and mine (actually, the 3.9" heavy duty unit, with billet yokes) broke BEFORE the car even made it to the chassis dyno. The front input yoke broke loose from the CF tube under the load if driving the car with a "limp home" tune from the engine shop to the body shop... a distance of less than 2 miles. ACPT was adamant that it was all my fault, and it took months for them to finally agree to refund my money for the POS CF DS. I bought a Mark Williams 3" chrome moly shaft (19#) and really don't care about the small HP losses.... its more comfortable knowing that the DS isn't going to break for no reason at all....
.
Might want to reconsider......
steel - 17#
aluminum - 12# (5# savings)
ACPT CF - 9.6# (7.4# savings)
2 - There isn't much HP to be gained. Keith at ws6.com dyno tested the stock steel vs. 1LE aluminum shaft and gained 1.1HP and 5ft-lb. Although automatics can have drivetrain losses of 16-22% of flywheel HP, and the T56 can cost you 12-15%, most of that is lost in the "variable" portion of the drivetrain.... gear contact as a function of torque transmitted, bearing friction, hydraulic pumping losses, etc. I have data for several different HP levels, and it appears the "fixed" portion of drivetrain loss, the rotational inertia part, is only in the range of 18HP total. I believe Keith also did a dyno coast down test to compare HP losses with conventional and synthetic lubes, and found numbers in the range of 16-18HP for the coast down loss. Those losses are primarily rotational inertia, because there is little in the way of frictional losses in the "coast down".
3 - Spending $700 for a CF DS, to save less than maybe 3HP isn't a very cost effective way to build rwHP. It gets even worse when the $700 ACPT CF DS fails.... and mine (actually, the 3.9" heavy duty unit, with billet yokes) broke BEFORE the car even made it to the chassis dyno. The front input yoke broke loose from the CF tube under the load if driving the car with a "limp home" tune from the engine shop to the body shop... a distance of less than 2 miles. ACPT was adamant that it was all my fault, and it took months for them to finally agree to refund my money for the POS CF DS. I bought a Mark Williams 3" chrome moly shaft (19#) and really don't care about the small HP losses.... its more comfortable knowing that the DS isn't going to break for no reason at all....
.Might want to reconsider......
03-24-2003, 11:07 PM
#6
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Join Date: Jul 2000
Location: MD
Posts: 520
You know, it’s funny how many of otherwise sensible, fairly knowledgable people manage to buy something like this (I know that I managed to buy a MMC 3.5” one at one point, and obviously Injuneer, sounds like somehow you got suckered for at least 3 bills more then me 
).
Yea, there is a little less weight, but then you move it out to 3.5” or so (stock 2.75” or so, depending on what you’re replacing) and the net gain is usually nothing or worse.
It did get rid of my 'vibeshaft' problem.
If I ever need another aftermarket driveshaft I doubt that I’ll ever come up with a reason to spend money on something ‘better’ the a decent steel/4130 DS…
).Yea, there is a little less weight, but then you move it out to 3.5” or so (stock 2.75” or so, depending on what you’re replacing) and the net gain is usually nothing or worse.
It did get rid of my 'vibeshaft' problem.
If I ever need another aftermarket driveshaft I doubt that I’ll ever come up with a reason to spend money on something ‘better’ the a decent steel/4130 DS…
03-25-2003, 03:48 PM
#7
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Join Date: Nov 1998
Location: Hell was full so they sent me to NJ
Posts: 70,662
You can actually run into situations where longer metal shafts can get into critical frequency problems... very long, very high rpm... and only CF will work. The shop that watched mine break swore to avoid them in the future, but when they went to Mark Williams for a DS for their sub 7-second Pro 5.0 car, MW told them they had to go to CF to get the critical frequency above their operating point..... 
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