Scuttle Shake

[Keoke]

We're talking about harmonics here, just draw the sine waves for, by way of example the fourth harmonic, and you'll see how the shake frequency could evolve.

Well generally the power contained in a harmonic decreases as the harmonic frequency increases so i do not believe a fourth harmonic is going to contain much power.

 

[RAC68]

Eliminating the Source of Harmonic Dissidents
It is clear from the discussion that a vibrating source is needed at a resonance frequency tuned to the weakness in our substructure design for Scuttle Shake to exist. However, as there are many potential sources of vibration, how do we identify the specific one that is tuned to a 50-65 MPH exposure?

Rear Drum Imbalance
The Healey uses massive 11” x 2.25” cast iron rear drums which must have had little or no quality control during their manufacturer. As has been pointed out, these drums are not necessarily centered when machined and drilled. To add to the imbalance, even the casting density varies to accentuate the problem.

Given these manufacturing flaws, these drums represent a rotating mass that can cause substantial vibration and could be a definite harmonic resonance stimulant by itself. Now extend the rotating mass with the addition of a perfectly balanced wheel and tire and the diameter of this combined unit will extend this rotating mass and multiply the vibration produced by the imbalanced drum. The result, more force transmitted through the frame and appearing as Scuttle Shake.

Now, assuming the opposite drum/wheel/tire combination are placed into the equation and assume its drum is equally imbalanced and wheel and tire perfectly balanced. If both imbalanced wheel units synchronize, then their vibration forces will, to some extent, combine and exacerbate Scuttle Shake. If the fall out of synchronization due to the effect of the differential during a turn, then opposite wheel unit imbalances will could counter each other and reduce the effect of Scuttle Shake.

Again, I have indicated an assumption of wheel and tires being in perfect balance, however, any imbalance in this combination would also contribute or diminish drum imbalance dependent upon how they line up. Also, the combination or counter of opposite wheel units also will vary due to other forces in the rotating mix and the fact that each will transmit their force through different paths.

Although this sounds relatively understandable, since each Drum/wheel/tire combination transmits its vibration through different substructure paths, force contributions or counters are not that clear cut and will even be affected by the rotational direction of the drive shaft.

After all these discussions the only semi-conclusion is to address drum imbalance in an attempt to eliminate a major potential stimulant.

Ray (64BJ8P1)

 

[drambuie]

I don't think we have to beat our selfs up here! As long as the wheels/tires, alignment, driveshaft/U joints, suspension/shocks and frame are in good order....Our Healeys will drive reasonably smooth with no problem. I do accept the fact that Austin Healeys are more susceptible to so called scuttle shake when one or more of these components fail due to the inherit weak structural design and archaic suspension system. Most cars of better/smarter structural and suspension design, new and old, we would not even notice if the wheels/tires where out of balance except of poor and uneven tire tread. Did you ever hit a bump while taking a turn in a Healey?? The whole rear of the car leaves the ground as if it has a ridged rear suspension.

Let's face it, Donald Healey designed these cars on a shoe string budget with limited funds and engineering skills....unlike some of the major manufactures of the day that had budgets to hire a team of professional designer/engineers. I remember watching the golden age of british sports cars, Sterling Moss's sisters co driver who raced Austin Healeys in rally's because they both lost the Triumph factory sponsor said this in a interview...quote....Healeys handle like Pigs! Maybe HealeyRick can dig that video/audio clip up for us!

 

[Keith_M]

Ray,
Your analysis makes sense to me, and it explains why the shake (at least in my car) seems worse sometimes. Presumably, this is because the rear drums are changing position relative to one another during turns and either countering or enhancing each other.

Still waiting for the weather to break so I can get mine balanced.

 

[RAC68]

Thinking about how we can improve substructure rigidity,there were a few that came up mind during this and other discussions including:

2. Replacing the full substructure with one of thicker gauge metal and continuous welds.
3. Encase the longer runs of the present substructure with welded-on thicker box sections (the method Randy presented in another thread).
4. Adding frame gussets and reinforcing brackets to key areas of the frame.
5. Fill major frame sections and sills with expanding small-cell quick cure polyurethane foam (at least 2Lb psi) that will strongly adhere to the inside of these structures.
6. Other.

Although I expect all of these alternatives will increasesubstructure rigidity, I am beginning to believe that most are either too costlyor impractical to perform on a completed Healey or with the skill set and equipment many of us posess. However, there is one that holds some promise but has been previouslyrejected by many in past discussions, the use of closed small cell quicksetting urethane expanding foam produces substantial increases in substructure rigidity(as presented by other non-Healey examples), is DIY in one day with properpreparation, and at a very reasonable cost of around $100. I am not talking about the types of foamfound on the shelves of Home Depot but a 2-part chemically activated foam thatwill produce an expanding foam of 2 lb psi or better and will substantiallybond to the sheet metal walls of the inner frame and sills. By applying the foam to the a selection of framesections (to be determined) and outer sills, I anticipate, from the accounts ofsuccess posted, a substantial improvement is substructure rigidity as well asthe deadening of the transmission of much of the harmonic resonance and vibrationscaused by imbalances of brake drums and other components causing Scuttle Shake.

Some draw-backs are obvious and include the inability to completely eliminate installed foam and the elimination of future welding onfoam filled frame sections. I expect there are other positives and negatives but will need additional research to determine.

Ray (64BJ8P1)

 

[HealeyDave]

Some random musings just for fun: Since the scuttle shake seems to occur at 55-60 mph on most reported cars, this would seem to be a resonance condition. Since it seems to be tied to vehicle speed rather than engine rpm or gear position (driveline rpm), one might conclude that it is a resonance in the structure that is excited somehow by a rotating component in the wheel/tyre system that passes thru that particular frequency and lights off the scuttle shake.
I did a rough calculation on this. At 60 mph, I assumed the OD of the tyre is about 26 inches. A once per rev imbalance in the tyre or brake drum would give a forcing frequency of 13.5 Hz at 60 mph. Although I haven't measured the frequency of the scuttle shake, this 13 Hz frequency makes some general sense to me. Others may report that it feels like a lower frequency than that.
U-joints generate a 2 per rev excitation, and the 6 cyl engine generates a fundamental at 3 times per rev (firing frequency). The engine firing frequency would excite the 13 Hz resonance at around 13 = x rpm/60 * 3. So x would be 260 rpm. 2nd or 3rd harmonic of the engine firing could come into play, but that would still be low in rpm. 60 mph in a Healey is usually up above 2000 rpm.
U-joints would then be at 390 driveline rpm. I think ...........
There are 2 general approaches to resolving a structural resonance issue: 1) shift the mode up or down out of the operating range with mass and/or stiffness changes (bracing the cowl) or 2) reducing the forcing function (the hammer that is hitting the system), such as balance improvement. Actually, 3 things Increase damping in the system, but that is out of the scope of this discussion.
The resonant mode we are talking about here could be some combination of structural interaction. Frame and chassis. Bracing the cowl could just be the easiest, most effective solution found emperically back in the day.
This is why race cars want to have very light and stiff chassis - no unexpected inputs to the driver that aren't coming from the road.
my 2 cents.

 

[Guest]

Cars that hardly ever get driven get flat spotted tires too....

I understand the short comings mentioned above but sometimes it just the simple stuff that folks overlook while reinventing the wheel. Tire "hop" is much easier to spot too.

 

[Bob_Spidell]

FWIW, my dad was a factory rep for Ford when the Mustang was launched. He said they had numerous complaints of vibrations--i.e. a type of scuttle shake--especially in the convertibles. He said they tried everything--shaved tires, tire/wheel/drum balancing, etc.--but the one thing that seemed to give the most relief was what they called an 'export bar;' i.e. a brace from the front shock towers to the cowl. Even then, the cars still had unwanted vibrations. The best engineers at Ford couldn't solve the problem.

 

[Keoke]

Let's face it, Donald Healey designed these cars on a shoe string budget with limited funds and engineering skills....unlike some of the major manufactures of the day that had budgets to hire a team of professional designer/engineers,
And who had the Queens blessings to anything they needed to get their product across the pond.

Consequently DMH not having any track record as a car exporter was relegated to the scraps.

 

[Ed_K]

I found out by accidental luck that my brake drums ( BJ8 rear ) were way out of balance. This is something anyone can check.
I have only done this once and it is probably not an OSHA approved diagnostic technique.
I had the back end of my car jacked up and supported by jack stands under the axle as well as a floor jack in reserve.
I started the car and put it in 4th gear and then turned on the O/drive to see if it was functioning.
While the back wheels were moving and the speedo was showing 30 to 35 MPH, the entire back end of my car
was noticeably shaking. I could feel that the shake was much more on the drivers side than the passenger side.
Since I had no noticeable up and down or in and out play in my axle/wheels, It must have been the brake drums.
To make a long story short, I took my brake drums to Hendrix wire wheel and found out that the brake drum on the drivers
side was way out of balance. Alan told me that whenever this is discovered that the axle bearing is also damaged from the
out of balance vibration. He told me that it wears flat spots on all of the rollers in the bearing. I took my now balanced brake
drums home and spun the axle/hub and I did not really feel or notice any roughness that I would have expected to feel if
flat spots were worn onto the bearing rollers. I only had 67, 000 miles or so on the car so I thought perhaps Alan's advice
only applied to cars with many more miles than mine.
A few years later, I bought and installed a Lempert gear set for the rear end. When I pulled the axle shaft out of the drivers
side and THEN spun the hub, I could feel a roughness that I could not feel when everything was installed. I pulled the hub
and discovered that there were flat spots worn onto all the rollers of the bearing just as Alan said would be the case.
So in addition to installing the Lempert gear set I also replaced the rear axle shaft bearing on the drivers side. When I first drove
the car again after this it was as smooth as I have ever noticed it at any speed.
The moral of the story is to go ahead and also replace the rear axle bearing(s) on on any side that had a way out of balance brake
drum installed. In one of the above posts, someone stated that the balanced brake drums improved the shake by 1/2. Try replacing
the axle shaft bearing(s) on the side(s) that had way out of balance brake drums.
I only had to replace one as the passengers side brake drum was only slightly out of balance.
They apparently had absolutely NO quality control at all over the manufacture of these brake drums at the factory !

Ed ( 65 BJ8 )

 

[drambuie]

ED K, The Healey brake drums where machined from a casting, imagine a large steel bin filled with a bunch of rough steel brake drum castings straight from the steel mill. Then they are sent out for machining, NOT a machinist, but a machine operator takes each rough cast brake drum out of the bin. First operation, the brake drums "rough" major O.D is clamped in 3 jaw chuck on a Lathe, the brake lining I.D. Is bored to spec using high speed tooling....they did not use carbide tooling back then, which means they had to machine the I.D. At low R.P.M ......which means T.I.R. "Total indicator runout" was not a concern while the brake drum was turning in the lathe. 2nd operation the brake drums where sent over to the vertical turret mill where the bolt hole face was fly cut and the holes where drilled... The other way that they could have been mass machined for high production was all operations could have been preformed in a large vertical mill as well! Point being... Concentricity was not a concern for the designers or the machining process. Which is so obvious to all of us!

I have extensive machining experience and thought about machining my own brake drums from better and lighter material holding a tight concentric tolerance, and even thought about mass producing them....however, I wouldn't think I would sell enough of them to make it worth my time and expense.

 

[Jeffsbj8]

I believe 90% comes from wheels and tires. With all my British car experience for the past 40 years that has been the problem.

 

[Wausau 911]

To eliminate the shake I simply added a 10" piece of foam weather proofing at the top center windshield area were the bonnet meets the cowl. Although not concours, I also installed the BJ8 bonnet sound deadening material. No shake sound and my BJ7 runs much more quietly. Just my 2 cents.

 

[KarlHB]

I've now read several discussions and threads on "scuttle shake" and have come to a conclusion that my '66 BJ8 seems to have the same problem. If the problem is caused by imbalanced brake drums, would the problem be rectified if I just bought and installed new drums? If the "standard" off the shelf-ones are prone to be just as out-of-balance as the ones I already have, would the aluminium ones be a better option? I can just forget to find a machine shop around here where I live in Norway to balance the ones I have, I'm afraid...

 

[John Turney]

Denis Welch sells vented rear drums, and for double the price will balance them. Since they usually supply pretty good stuff, my guess is that the off-the-shelf new ones are not balanced.

 

[Keith_M]

Here's a quick update on this thread. Things have finally warmed up enough for me to get out in the garage and pull my brake drums for balancing. The guys at my local machine shop were amused by the roughness of the castings and by the fact that there is no indication that anyone has tried to balance them in the past. Their comment was: "It's hard to imagine that these are anywhere near balanced." I should have them back on the car by Sunday and I'll report back then.

 

[RAC68]

Hi Keith,

I am looking for a local machine shop to do the job but, to date, don’t seem to have found anyone that was remotely interested or had the proper equipment (according to them).

What type of Machine Shop have you found or what I should be looking for?

All the best,
Ray (64BJ8P1)

 

[Lotuswins]

Ray,

I had mine done by a local speed shop, they seem to be very accomodating.

 

[Keith_M]

Hi Ray,
I took mine to the same shop that did the machine work on my engine. Very, very competent father and son team that pretty much all the mechanics in the area use. They said they could spin balance it for me.

Picking them up this afternoon. I'm eager to see how far out of balance they were.

 

[Keith_M]

Got the drums back. One of them wasn't too bad, but the other was really way out of balance. It was so bad it almost hopped off the machine when they first spun it, and they started running out of places to take off metal by they time they finally got it balanced. I've attached a couple of photos of the bad one and circled the drill holes where they removed metal. The guys who did were amazed and amused that it came from the factory that badly out of balance. I gotta believe this will make a difference in how smooth the car runs.