Driveshaft balance

Chris Wire

Chris Wire

New member
My local driveline specialist says he can balance my driveshaft. However, I asked him how fast they balance them and he said 500 rpm. Huh? He does do a lot of custom driveshafts for hotrods/cruisers/drag guys and says that he should have no problem balancing mine, but I am a tad skeptical.

Remember that at 8500 rpm, an RX7 shaft is turning in excess of 6400 rpm with the .756 5th gear.

1. How would extrapolate a balance condition at 500 rpm over to 6000 rpm?

2. Does someone out there know of a vendor (QuarterMaster, Winters, etc.) who will balance an OEM driveshaft at speeds more like those the shaft will actually see?

3. Is it even necessary to try to approximate the speeds the shaft will see?

The local guy said that there are machines which will balance up to 3000 rpm. I may try to find a vendor who can accomodate.

Any help or direction is appreciated. Thanks.

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Chris Wire
Team Wire Racing
ITS Mazda RX7 #35
[email protected]
 
In an overdrive transmission, doesn't the output shaft turn faster than the input shaft?

That would make your driveshaft RPM higher than your engine RPM, right?

MC

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Mark Coffin
#14 GP BSI Racing/Action Digital/Airborn Coatings/Krispy Kreme VW Scirocco
 
I think your right. With those figures, the DS should be turning about 10,574...

When you shift 'up', you get less engine speed per road speed...in 5th, the wheels and DS are spinning faster than the engine at anything 'below' 1.000.

No?

AB

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Andy Bettencourt
New England Region, R188967
www.flatout-motorsports.com
 
Originally posted by Andy Bettencourt:
I think your right. With those figures, the DS should be turning about 10,574...

When you shift 'up', you get less engine speed per road speed...in 5th, the wheels and DS are spinning faster than the engine at anything 'below' 1.000.

No?

AB
Click to expand...

Driveshaft yes, wheels no. (The diff reduces the wheel RPM about 5:1 on an RX7 ITS car).

MC
 
Is balance something fluid? Does it change with RPM's? I don't think it would.

The effect of out of balance is likely greater at higher rpms. What is a minimal impact at 2,000 rpm may be highly detrimental at 10,000 rpm. The question is "balanced" at what rpm would be sufficient for balance across the band.

Either way if the balance is improved at 500 rpm - it would be improved at 10,000 rpm by the same factor that out of balance is negative.

So if there is some incremental more balance condition when done at 3,000 rpm and the cost is the same, sure go for it. But if the 3,000 rpm balancing does not produce a more than insignificant improvement and it costs more (in terms of cash cost or costs associated with searching for vendors, shipping, transporting, time out of hand etc.) then why bother.

All else being equal I would go with the 3,000 rpm, but I think that any incremental difficulty getting a 3,000 rpm balance when a 500 rpm balance is readily available would make the decision pretty easy. As long as you have faith in the skill of the blancer.

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Ed.
240SX ITA
 
Originally posted by racer14itc:
In an overdrive transmission, doesn't the output shaft turn faster than the input shaft?

That would make your driveshaft RPM higher than your engine RPM, right?
Click to expand...

Mark,

You're right, my math is backasswards.

Output shaft speed is increased relative to engine speed.

TurboICE, I see your point about "how much is too much" in worrying about whether or not to go with the 500 rpm balance. I can't help thinking about the wheel imbalance that doesn't show itself at 30 mph, but becomes signigicant at 70 mph. Now I'm sure that their 500 rpm balancer is more sensitive than my 70 mph buttmeter, but the idea of a driveshaft failure at 8000 rpm on the banks of Daytona does not excite me.



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Chris Wire
Team Wire Racing
ITS Mazda RX7 #35
[email protected]
 
I can appreciate all of that. Believe me, my post was adding questions, not providing answers. My post is there to generate responses for my education on the subject.

I guess really I am looking for "Yeah balanced at 500 rpm will be sufficient." or "Are you crazy you need to balance to at least 5,000 rpm to be comfortable with the balance at 8,000 rpm."
 
Here's my take on it. Mazda driveshafts spin way fast. Faster than most. A little out of balance... VERY little... gets magnified as speeds increase, and the result is significant. An engineer caqn give yu the number, but I think it rises non linearly. (In other words, a 1 oz out of balance condition at 3k rpm is not a 2 oz condition at 6k rpm...the term exponential rings a bell)

All the guys who run Mazdas I know who know their stuff suggest the sure fire way to balance a known out of balance driveshaft is to call mazdaspeed and give them your credit card.

For over a year I chased a weird vibration that would build with speed, and get slightly better, it seemed at a certain RPM. Wheels, tires, bearings were all replaced, to no avail.

New driveshaft from Mazda in time for the ARRCs, where the shaft would spin faster than ever solved the problem completely. Smooth as glass.

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Jake Gulick
CarriageHouse Motorsports
ITA 57 RX-7
New England Region
[email protected]
 
I kind of hate to be a party pooper, but I don't remember any rule allowing balancing of the driveshaft. Can someone point me towards it?


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George Roffe
Houston, TX
84 944 ITS car under construction
92 ITS Sentra SE-R occasionally borrowed
http://www.nissport.com
 
IIRC, the shaft arrives from the factory with balance weights attached....

not that it is appropo of anything...
wink.gif


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Jake Gulick
CarriageHouse Motorsports
ITA 57 RX-7
New England Region
[email protected]

[This message has been edited by lateapex911 (edited May 09, 2005).]
 
Back to the difference of balancing at 500 vrs 300rpm - look at your wheels & tires. You think that machine the guy stands in front of turns more than about 25-30mph?

If it is out of balance, you can pick it up at any speed, provided you have sensitive enough equipment.

Yes it turns faster than engine speed, but only in an overdrive gear. Most transmissions have 4th gear at or about 1 to 1. So, anytime you are in a lower than 4th gear, the drive shaft is turning less than engine speed. Now most cars do run 5th gear from time to time, but think about how many seconds per lap you are actually doing that, and what RPM range you're in. Unless you're using insanely short gears on really long courses, you're not going to see that much time at high driveshaft speeds. (IE if you can get 8500RPM in 5th gear in an IT car, you let me know, I'll tip my hat)

Another item - the driveshaft is not terribly heavy, nor is it very big around. Both of these things are to your advantage. If it is out of balance, it doesn't have that much mass to work with, therefore it will have less effect on the whole vehicle. Second, the smaller diameter makes for slower radial speed, again making the out of balance effects less than on something like a tire that's 10 times the OD. This, coupled with the amount of time really spent at high speed would lead me to think that if it is done right, 500rpm is probably fine.

Now - so you find a guy who will balance it to 3000RPM. Great. You still didn't find that harmonic that happens at 4500 when coupled to the input flange on the diff.
 
Originally posted by MMiskoe:
Back to the difference of balancing at 500 vrs 300rpm - look at your wheels & tires. You think that machine the guy stands in front of turns more than about 25-30mph?

If it is out of balance, you can pick it up at any speed, provided you have sensitive enough equipment.

And that is the crux of my problem. How do I know that the local guy (who admits his equipment is not the most sensitive. His is solid mounted, the 3000 rpm model is rubber isolation mounted) is capable of detecting that mild imbalance at such low rpm.

Unless you're using insanely short gears on really long courses, you're not going to see that much time at high driveshaft speeds. (IE if you can get 8500RPM in 5th gear in an IT car, you let me know, I'll tip my hat)

Well, most of the RX7s in SeDiv use the 5.125 gear. And down here we are blessed with tracks like Daytona, Road Atlanta, and Charlotte, all of which have pretty long runs down the backstretch (w/ Daytona & Charlotte around to the front stretch). And while 8500 rpm in 5th might be a stretch, 8000 in 5th w/ the 5.12 and Hoosiers is around 140 mph, and that's not a stretch.

Another item - the driveshaft is not terribly heavy, nor is it very big around. Both of these things are to your advantage. If it is out of balance, it doesn't have that much mass to work with, therefore it will have less effect on the whole vehicle. Second, the smaller diameter makes for slower radial speed, again making the out of balance effects less than on something like a tire that's 10 times the OD. This, coupled with the amount of time really spent at high speed would lead me to think that if it is done right, 500rpm is probably fine.

All good points, and I may still use the 500 rpm guy. But on issues such as these, I like to run 'em through the brain trust (did I just call you guys the brain trust?) to see what anyone else has experienced. And I am grateful for the input. Keep it coming.

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Chris Wire
Team Wire Racing
ITS Mazda RX7 #35
[email protected]

[This message has been edited by Chris Wire (edited May 09, 2005).]
Click to expand...
 
WoW, I didn't think you would ever get that many turns going in 5th gear. But that is a pretty short set in the diff.

I will also ask a friend how fast cranks are spun in order to balance them.

You are right, you don't know that the guy w/ the 500rpm machine is sensative enough for what you want.

Don't call me the brain trust, I just stayed at the Holiday inn once.
 
For comparison sake, we turn turbo impellers between 2000 and 10000 to balance them and they see speeds in excess of 70000 at full boost. You should see what the gram/mm accuracy of the equipment is as rpm of use should not matter if the machine is used properly. The imbalance grows by the square and gets huge fast as rpm increases. As for the mazda shaft, throw it away and buy a new one. You will chase a ghost and find it is the non replaceable U joints that cause the problem. Get the 89 - 91 shaft as it is smaller and lighter. Check the tailshaft bushing in the trans for play as it will double the problem. Good luck!!
Steve Eckerich
Triple E Performance
ITS Speedsource RX7
 
I think one of the benefits you get by balancing it closer to its operating range, is that you see if you have any kind of secondary harmonics.

Note: I'm not a mechanical engineer, but I did stay at a Holiday Inn Express!!
biggrin.gif


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MARRS #25 ITB Rabbit GTI (sold) | MARRS #25 HProd Rabbit
SCCA 279608
 
Chris, buy a new one from Mazda like Steve say's. Believe us oh new to mazda one....... Stick with grasshopper that have been there done that.

FYI I saw almost 8500 in 5th this past weekend at Daytona. That mother is spinning fassssssttt!!!
 
FWIW I had this same conversation with Maike @ ISC two weeks ago.. Decision was made that a new Mazda oem driveshaft was the only way to go.


MM
 
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