Experience With "Balanced Cams?"

Post by **SchmidtMotorWorks** » Wed Mar 01, 2006 11:27 am

Seems like if we're prepared to fixate on balancing our crank assemblies to ½ or ¼ of a gram instead of say 5 or 10, we should pay at least some attention to cam balance... Think

Crasnkshafts are balanced with bob weights to compensate for the attached moving parts.

Balancing a cam without compensating for the motion of the lifters, pushrods rockers etc is pointless.

MadBill · Post by **MadBill** » Wed Mar 01, 2006 1:39 pm

But inline cranks are balanced without bob weights and since lifters are not attached to the cam they can't exert upwards force...
Also, since the cam is not infinitely stiff, and the bearings don't have zero clearance, seems like if an imbalance yanks some lobes upwards at particular points in their lift events and others downwards, there could be significant effects on valve motion, given that cam designers sweat the details re same all the way up to perhaps the sixth or seventh derivative of displacement.
Not saying that cam balancing is a must for every engine, just that it shouldn't be dismissed out of hand...

panic · Post by **panic** » Wed Mar 01, 2006 4:01 pm

Post by **SchmidtMotorWorks** » Wed Mar 01, 2006 5:20 pm

Also, since the cam is not infinitely stiff, and the bearings don't have zero clearance, seems like if an imbalance yanks some lobes upwards at particular points in their lift events and others downwards,

Another reason that balancing a camshaft by making the gear out of balance is pointless.

The amount that a cam would flex due to imbalance is trivial (to non existant) compared to the distortion is will have due to forces from moving the lifters.

Further, correcting the dyanmic balance of a shaft that long and flexible by making changes near the ends has zero benefit to any imbalance at the middle of the cam, in fact it is likely to make bearing loading worse on the ends while providing no improvment to the middle.

I think there is a big misunderstanding about dynamic balancing, it does effect vibration but only effects bearing loading when the balancing changes make the body more symetric.

Think of it this way, the bearing loading that occurs on a cam due to any imbalance caused by the mass of the lobe is opposite of the load caused by moving the lifter. On a cam with only one lobe, balancing it would add more load to the bearings not less. Add 15 more lobes and a flexible shaft and try to make the adjustments in the timing gear, I don't see any way that balancing a cam can improve anything.

Dave Flanders · Post by **Dave Flanders** » Wed Mar 01, 2006 5:38 pm

For what it's worth.... I had a balance problem with 3 different slider (non-roller) cams for a 2.3 Ford. One was so bad it destroyed the cam bearings in 25 street miles. The problem turned out to be the fact that they were turned off a cast core, if you would spin the cam in a lathe between centers you could see the cast portion runout about .060".

I had a couple heated arguments with the "tech" at Lunati (where 2 of them came from) and he told me any runout or imbalance doesn't matter. Surprisingly (or not) when I offered to send him the cam to try in one of their customer development vehicles he said "I'm not going to give them a cam that will blow up their motor!".

Half Fast Racing

MadBill · Post by **MadBill** » Wed Mar 01, 2006 6:07 pm

[quote="SchmidtMotorWorks...Another reason that balancing a camshaft by making the gear out of balance is pointless....Further, correcting the dyanmic balance of a shaft that long and flexible by making changes near the ends has zero benefit to any imbalance at the middle of the cam, in fact it is likely to make bearing loading worse on the ends while providing no improvment to the middle....[/quote]

Unlike an externally balanced long stroke crank with little or no journal overlap to stiffen it up...

Post by **SchmidtMotorWorks** » Wed Mar 01, 2006 6:13 pm

For what it's worth.... I had a balance problem with 3 different slider (non-roller) cams for a 2.3 Ford. One was so bad it destroyed the cam bearings in 25 street miles. The problem turned out to be the fact that they were turned off a cast core, if you would spin the cam in a lathe between centers you could see the cast portion runout about .060".

.060 on a radius of less than an inch would cause trivial imbalance. Certainly not enough to destroy bearings in 25 miles.

Consider this, the first and second counterweights on a V8 crankshaft are 100s of grams different because of the space allowed for them in the block. The load on the counterweight side of the 1st main and the load on the opposite side of the 2nd main are 100s if not thousands of times higher than could be caused by a shaft about 1.0 rotating at half the speed. Considering the size of the bearings are within about 50% of eachother it seems unlikely that such a small load would hurt the bearings.

Post by **SchmidtMotorWorks** » Wed Mar 01, 2006 6:19 pm

Unlike an externally balanced long stroke crank with little or no journal overlap to stiffen it up...

I'm not sure what you mean hear but, externally balancing a long stroke cranks does nothing for bearing loads, it only reduces vibration.

You could reduce bearing loads by making the end CWs lighter and the middle ones heavier and more opposite the rod pins on 99.9% of V8 cranks.

This is rarely done because it makes them heavy and it is expensive to do. But if bearing loads are the issue this is the only way to improve them through balancing.

Post by **SchmidtMotorWorks** » Wed Mar 01, 2006 6:23 pm

For what it's worth.... I had a balance problem with 3 different slider (non-roller) cams for a 2.3 Ford. One was so bad it destroyed the cam bearings in 25 street miles.

If this small imbalance were the problem, then counter balance shafts would certainly destroy their bearing even faster....but....they don't

BRENT FAY · Post by **BRENT FAY** » Wed Mar 01, 2006 11:42 pm

Jon, In the article (Circle Track March 2006) the cam they tested was out 4.8 ounce/inch out of balance when they were done it was .4 ounce/inch. You have worked at custom crank manufacturers,wouldn't this help? I thought that any object that spun over a certain RPM had to be dynamically balanced instead of static balance, particually a shaft. I agree with you on FLANDERS deal though. I agree with MAD BILL's quote(BTW why are you so mad Bill?) and CHRIS U and PANIC has some merit also. Brent

Post by **SchmidtMotorWorks** » Thu Mar 02, 2006 1:43 am

Jon, In the article (Circle Track March 2006) the cam they tested was out 4.8 ounce/inch out of balance when they were done it was .4 ounce/inch. You have worked at custom crank manufacturers,wouldn't this help?

I don't believe anything I read in those magazines since I made some claims about a carb entry I made when I was about 24 and they printed it as fact without checking anything. Also when I worked at Honda there were lot of articles about the engine we had specifying many details, almost all of them were wrong, not even close.

Would the balance change help? Probably not, do you know that if you remove weight from a the end counterweights of a V8 crank and make it out of balance that the bearing loads will be reduced? This is because the weight on the end CWs is oversize and offcenter to compensate for the other CWs being too small. It is not possible to determine how much from the information returned from the balancer. But there is a way to know from measurements.

The point is that just because something in dynamic balance doesn't vibrate, it doesn't mean anything to bearing loads or distortion of the shaft.
You can easily make a crank that doesn't vibrate that will pound your bearings to pieces. For example you could cut all the CWs off a crank and balance it with pieces added to the flywheel and balancer. It would be just as smooth as any crank but it will be very hard on the bearings.

Besides, think about it, what does 4 oz mean to a thin shaft that takes thousands of pounds load? Nothing.

ChrisU · Post by **ChrisU** » Thu Mar 02, 2006 6:40 am

Can we separate the 2? A vibratory frequency and bearing loading? I would think so....

Dave Flanders · Post by **Dave Flanders** » Thu Mar 02, 2006 12:30 pm

SchmidtMotorWorks wrote:
For what it's worth.... I had a balance problem with 3 different slider (non-roller) cams for a 2.3 Ford. One was so bad it destroyed the cam bearings in 25 street miles.
If this small imbalance were the problem, then counter balance shafts would certainly destroy their bearing even faster....but....they don't

I didn't know what else could do it. Changing the cam made the problem disappear on each of the motors.

RW TECH · Post by **RW TECH** » Thu Mar 02, 2006 8:15 pm

Hey Dave Flanders,

Rob Wright here. You still running the turbo Pinto combination? Pretty cool piece.

Today I was talking to a guy who was involved with the Mazda road racing efforts (5 consecutive championships in IMSA) in the past who also raced a 2.3L Pinto engine.

It's ironic that I mentioned your deal today and now I'm seeing your post too.

Hope all's going well for you.

Dave Flanders · Post by **Dave Flanders** » Thu Mar 02, 2006 8:32 pm

Hey Rob, long time no hear! Send me some contact info, I lost your phone no.

One more thing on the cam deal as relates to 2.3's is they only have 4 cam towers. Could that help to cause camshaft "whipping"?

Half Fast Racing