# If anyone knows?

Let’s assume tires were balanced on a properly calibrated spin balancer . Now , let’s say these tires are put on an accurate bubble balancer . Will or should the bubble be centered ? I was reading about the pros & cons of different ways of balancing tires on another forum & just got to wondering about this . At least one poster swore he had balanced tires on a bubble balancer & then put them on a spin balancer & they were good .
I don’t really want to get into whether or not he was being truthful , I just want to know if both ways would center the bubble .

“At least one poster swore he had balanced tires on a bubble balancer & then put them on a spin balancer & they were good .”

Done well, bubble balancing should work well most of the time, so I’m not surprised by that comment.

Would they both center the bubble? I would think so, in most cases.

It’s entirely possible to have a wheel that’s balanced correctly on both the bubble balancer and spin balancer, which is what your “poster” claimed.

I think whoever made the claim got lucky. A spin-balanced wheel will also come up as balanced on a bubble machine, but not necessarily the other way around. Bubble balancers cannot detect imbalance from the front of the wheel to the back. When I used to balance tires a million or so years ago, our service station only had a bubble balancer. We always used two weights of equal size at each spot on the rim to establish balance, then installed one on the front and one on the back. I don’t recall getting complaints, but this was long before the ultra-wide tire fad started. Very few people had them except on pony cars.

If I understand correctly a bubble balance you split the weights on the inside and outside of a rim, and a spin balance will calculate the weights for the inside and outside.

Yes, the difference is the bubble balance method only compensates for unequal weight distribution around the circumference of both rims together, while the spin (dynamic) balance method does that, but also compensates for weight imbalances between the inner rim and the outer rim.

Here’s a thought experiment. If you put a perfectly balanced rim perfectly centered on an axle and spun it, with the axle suspended by something that could measure any forces on the axle (both magnitude and direction), once the rpm stabilized there’d be no force measured on the axle. However if one angular segment of the rim weighed more than the others, but equally distributed front to back, it wouldn’t properly bubble balance. When it got up to speed the axle would get pulled in that heavy segment direction as the rim spun around. So the axle would get pulled round-and-round as the rim spun, with the force always corresponding to the heavy segment & directed at 90 degrees to the axle’s direction. If the axle could move a little w/the force, if you looked at it end-on, it would move in a small circle at the same rpm as the disc was spinning. Since the force was oriented at 90 degrees you’d know to put the weights on both sides of the rim.

If instead the weight imbalance on the segment was the same as above, so it still didn’t bubble balance, but all the extra weight was on one side of the rim, when spinning the force on the axle wouldn’t be at 90 degrees. That would create some twisting force on the axle, which could presumably be detected and you’d know to put weights on just the light weight side only, rather than both sides.

If the rim perfectly bubble balance but had extra weight on one side all the way around, hmmm … not sure if that creates a force on the axle or not. It seems like the forces cancel and that couldn’t be detected by spinning the rim.

Well, the bubble never, ever lies or is mistaken.
The digital readout on the spin balance could very well be to some extent; or a lot of extent.

Not exactly. Ever seen a “level” that’s out of calibration? I have. If the part holding the tube with the main part of the level isn’t oriented properly, the level will be out. This can happen to levels that are dropped or abused. I always double check before using a level by checking the bubble location first one way and then 180 degrees out.

The same principle holds true for a bubble balancer. If the mechanical structures that orient the wheel to the bubble are off, the balancer will be off.

Beyond that, even a perfect bubble balancer doesn’t check as much as a spin balancer. A bubble balancer will not detect out of round conditions, dinged rims, and imbalances in the two opposing masses on the inside and outside of the wheel’s center of gravity. Such problems create dynamic problems that static balancing cannot prevent. That, by the way, answers George’s question. Static balancing cannot prevent dynamic balance problems. “Road Force Balancing” is even better, because it adds a simulation of the forces of the wheel rolling down the highway.

Bubble balancers properly used are/were great for low speed vehicles and in the old days before superhighways. I personally think today’s superhighways need better balanced wheels. Can I prove it? Nope.

I used a “bubble balancer” for many years with no problems. The spin balancer came came along after I left my dad’s shop. It made the job easier and faster but not necessarily better.

@NYBo understands this. A bubble balancer ensures that the center of mass is in the center of the tire. In addition to this, a spin balancer ensures, using physics language, that the moment of inertia tensor is diagonal. Thus a spin balancer is better. However, I used a bubble balancer for years, and it was pretty good. But then highway speeds were lower. Anyway, both ways should center the bubble.

moment of inertia tensor

Whoa! I haven’t thought about tensors since grad school. Well played @melott!

A spin balanced wheel should always zero-out on the bubble-balancer but a bubble balanced wheel may not zero out a spin balancer.

Thanks guys , I appreciate your input . It was my guess that a properly spin balanced tire should center the bubble on a bubble balancer but when one gets to considering harmonic , static & a tire that needs more weight on one side than the other , out of round wheels & tires & all the variables I certainly wasn’t sure .