Scuttle Shake

[RAC68]

I am amazed at the creativity in this discussion. From harmonic frequency to single piece dash have been though proposed and yet over the more than 50 years this condition has been present, no definitive solution has been accepted.

Is there a definitive definition of “Scuttle Shake”? If so, what is it?

As Rich indicates, he and many others have found that this condition has responded positively to rear drum balancing and some have indicated that dog house reinforcement has cured the problem for them. Yet, others have countered that performing these solutions has not solved the problem for them and presented additional ideas.

Steve Hall has indicated he is on a journey to a solution and I must agree. Over the years our Healeys have existed with this condition, many have modified, repaired, and replaced many structural components of their Healey's original 15-gauge construction and have often noted deterioration (rusting) from the inside of their frame's thin sheet metal box sections. Since these changes would change the original structural integrity of the Healey’s substructure and the level of rigidity they impart, it seems odd that the range of what we are identifying as scuttle shake can still be identified as happening within a defined speed rang (50 MPH and 65 MPH).

Since I don’t believe the lack of a single-piece dash has much to do with this condition and the harmonic frequency of the substructure would definitely have changed with the progression of rust or modification/repair, what are the reasonably potential causes of this condition and how do we validate them.

1. Rear Drum Imbalance (possibly complicated by improper wheel balance/tune) could be a cause but only if those who have implemented rear disks do not experience the same shake.
2. Vibration from imbalanced drive shaft or fly wheel could be the cause but only if not experienced by those who installed Toyota 5 speeds.
3. etc.

So, what can we definitively rule in with the least amount of speculation and how can we validate.

Ray 64BJ8P1)

 

[ttrotter]

Ray, I agree 100% with
"Steve Hall has indicated he is on a journey to a solution and I must agree. Over the years our Healeys have existed with this problem, many have modified, repaired, and replaced many structural components of their original 15-gauge construction and have often noted deterioration (rusting) from the inside in the thin box sections. Since these changes would change the original structural integrity of the Healey’s substructure and the level of rigidity they impart, it seems odd that the range of what we are identifying as scuttle shake can still be identified with shaking within a defined speed rang (50 MPH and 65 MPH)."

I certainly should have made that clear in my previous post which assumed that the structural integrity of the car was as it came from the factory (which my BJ8 is, btw). If not then all bets are off So, I think we could definitively rule in those items mentioned in my last post.
Regards,
Tom

 

[HealeyRick]

From the February 1965 Road & Track test of the MK III:

"Despite improvements to the rear suspension, the ride and handling are still in the classic sports car tradition of the early '50s before people like Chapman and Cooper had got in on the act. The ride is firm and there is some chassis flexing evident from the authentic cowl shake which occurs on any but the smoothest surfaces."

So it didn't take years of chassis rusting to make them shake, they did it from new.

 

[RAC68]

Rick, I can’t disagree that the issue was present from new but so was the imbalanced drums. Again, what is the definition of Scuttle Shake and, as quoted from R&T, was the shake from the rough road or was it apparent on any road at a specific speed range. I am beginning to suspect many have been associating any shake through the dash and/or steering wheel that becomes noticeable within the 50 mph to 65 mph speed range as scuttle shake.

So, what is the definitive definition of Scuttle Shake or is there one?

Ray (64BJ8P1

 

[John Turney]

I think we are talking about two situations:
- Shake from hitting a bump, which happens at any speed and would last for a second or so, or when the next bump was hit. This is what R&T is describing.
- Shake from the rotation of unbalanced tires, wheels or brake drums at specific speeds (the 50 mph to 65 mph shake). This is the harmonic shake where the frequency of the rotating imbalance matches the frequency of the chassis. This will continue as long as one remains in the critical speed range.

The second type can be eliminated by either balancing the rotating parts, driving faster than 65 MPH or both.
The first has to be solved by strengthening the body structure or moving where there are only smooth roads.

 

[PeterBL]

My BN1 has recently received a full restoration (re-manufacture, a lot new bits plus welds). I haven't had a lot of opportunities to experience scuttle shake yet and I don't remember any when I drove it in the early seventies. New wheels (48 spoke) and tyres were part of the restoration. I also purchased some ceramic beads for balancing each wheel (and other parts), this way I would be able to rotate wheels if required. I haven't installed the beads yet. What I need is advice on whether to use the balancing beads. Is there any experience with this sort of wheel balancing for Healeys. If my schoolboy physics is any guide the drums, tailshaft and anything else rotating would balance.

 

[RAC68]

Hi Peter,

I have heard of the balancing beads but have not tried them. Do you have any experience with them? If so, would they work in the inner tube of a tire? Also, since many of the rear drums of our Healey’s are quite large and out of balance, would they work when quite an amount of counter weight would be required? Last, after stopping from 30 or 40 MPH in traffic, how quickly does rebalancing take place (I appreciate that there is no definitive answer)?

In the late ‘60s fabricating a tube around the inner edge of the rim and filling it with led bird shot to do this but couldn’t make it work.

Thanks.
Ray (64BJ8P1)

 

[Frameman]

There have been several items in this thread that I would like to address. Road and Track magazine, which test drove new Healeys, wrote about the scuttle shake.
Welding angle iron in the transmission tunnel area as suggested by Geoff Healey because of a lack of welds by Jensen. I wonder why if Healey felt there was a problem with welds done by Jensen , he didn't ask Jensen to fix the welds or find somebody who could do the welds better. Perhaps he didn't because he knew and Jensen knew it was not going to address the issue of scuttle shake.
I agree with Ray, you will never resolve this issue if you have the original chassis in your Healey. We solved the problem 30 years ago when we developed a replacement chassis. None of the cars with a Jule frame has scuttle shake. I am not trying to use this forum to sell my product but when you say there is no solution I have to let you know there is one. You may not want to choose it but it does exist.
I have driven many Healeys with original chassis where the owner has attempted to solve the scuttle shake . I do not agree and still feel scuttle shake and when the owners ride in a car with a Jule they also realize they have not resolved the issue. The scuttle shake is caused by the lack of rigidity throughout the structure and the incorrect engineering of the original chassis.
Martin

 

[andrea]

the lack of rigidity throughout the structure and the incorrect engineering of the original chassis.
I agree with Martin, add welding angle to the chassis is a loss of time and money, but the attemption to the complete welding points around the chassis, and balancing of drums and wheels can only help
to reduce this issues, the critical point of this chassis is the forward part, where the two main rail are very long and without any transversal reinforcement
the only attempt to add a reinforcement can be the reinforcement of the two platforms engine mounting, involving the engine block in the structure
NOTE the reinforcement must be accurately done- due the inertia of the engine block weight
this expedient was used with success on some motorcycle frame

 

[bob hughes]

I am currently about to venture down this path, scuttle shake is strong at 65mph, my engine has been balanced, with a new harmonic damper, so that should be clear.
The next in line will be the rear axle - new tyres, drums balanced and then the prop shaft, We will see where we go from there.

Pity I did not reinforce the bulkhead when I had the engine and gear box out, it would have been so easy. Never mind another day when I get the engine and gear box out to spring clean it



Bob

 

[Keith_M]

I am currently about to venture down this path, scuttle shake is strong at 65mph, my engine has been balanced, with a new harmonic damper, so that should be clear.
The next in line will be the rear axle - new tyres, drums balanced and then the prop shaft, We will see where we go from there.

Pity I did not reinforce the bulkhead when I had the engine and gear box out, it would have been so easy. Never mind another day when I get the engine and gear box out to spring clean it



Bob

Bob,
I am in almost the exact same situation. Engine was balanced when rebuilt, but I didn't know about the strengthening thing when I was doing the bodywork. As soon as the temps get warm enough for me to heat my garage up to a reasonable temp, I'll be pulling my rear drums and the driveshaft for balancing. My scuttle shake is not all that bad, but I want it to go away completely.

It will be interesting to compare results!

 

[Lotuswins]

Hi all,

I have been watching this thread with interest since I recently finished my BJ8 resto, which included frame repairs and sill replacement. I presently don't have the scuttle shake (knock on dash wood). The way I see it, from and engineering perspective (not an expert, just an ME who is interested) is:

Resonance: the frame apparently, as others have deduced, has a resonance at around 50-65 mph (we can calc the frequency if you wish). To determine the resonance of a structure we would do a bump test in the field and measure the 'response' with probes to determine its natural frequency. Some things that can affect the natural frequency response of a structure include mass, friction, boundry issues, configuration, etc. The suggested fix around the inside of the tunnel opening will help the issue by raising the resonant frequency slightly, and also limiting the 'whip' of the scuttle in the resonance range. Anyways, any stiffening of the structure will raise the natural frequency response of the structure. I was apparently successful by replacing the sills and outriggers with Kilmartin pieces (thicker material) along with full perimeter welding of the floor panels and around the scuttle 'hoop'.
One area that hasn't been mentioned would be examining and addressing any loose fasteners in the structure. Checking and tightening all fender/frame/dash/bracket bolts and rivets should help stiffen up the structure. Checking for metal fretting and weld failure/fatigueing also at that time and repairing those should help. This should help with minimizing the response to the forcing function.

The forcing function, in this case the imbalance of the rotating parts or irregularities in the road surfaces, are linear in nature, i.e. they increase in force with increase in frequency (rpm/mph). The fact that the shake occurs between 50 and 65 mph, shows to me that it is a resonance issue with the structure, however, removing the exciting forces should be a priority. My BJ8 rear drums were significantly out of balance and the machinist who balanced them commented that he was barely able to balance one. Apparently not only was the casting out of round, but the material had inconsitant density. The rotors were fairly close, due mostly I believe to the way they are manufactured compared to the drums. Other issues like wheel imbalance, out of round or wobble need to be addressed. Note also that bubble balancing is generally only good to 55 mph or so. Having the wheel/tire balanced dynamically either on the vehicle or on a machine along with truing them up and insureing the tires don't have a flat spot or are out of round should have a significant effect, again, as others have mentioned.

Unfortunately we are fairly restricted on what can be done short of frame replacement due to the fact that the car is a ladder framed convertable with fairly poor torsional rigidity. I was amazed how much the frame flexed when jacking the car up on one corner near the tire. No wonder the aluminum shrouds fatigue and crack on these cars.

 

[AJAX50]

I'm not a sound or structure expert but I would have thought the vibration frequency was a result of harmonics so a number of factors will change the speed at which it occurs and severity of the shake.

 

[Keoke]

No NO 1! AjaX50--However here is a situation that will change the speed at which it occurs and severity of the shake>

On My Miss Blu the so called scuttle shake came at different speeds. I had checked everthing as defined in this post. In desperation I put the car up on jack stands and began checking the drive train again. When I got to the rear universal it checked OK but the drive shaft moved up and down at the pinion. This had never been seen before during similar checks. Now that ain't right so I disconnected the shaft from the pinion.The pinion continued to move up and down so I removed the pinon nut and a broken Split washer reveaed itself. Replaced the washer retorked the nut to 150 lb ft no more scuttle shake

.

 

[Frameman]

Scuttle shake is when the you feel the structure under neath you twisting which causes the car to shake when going over pot holes or rail road tracks. This is what Road and Track were talking about. Scuttle shake can lead to front shrouds being torn out of the front chassis rails, cracking in the front heal lamp areas of the front shroud, on 100-4 cracking on the upper front shroud hood area, front screws in the hood area being torn out of the aluminum shroud, cracked wind shields, convertible tops becoming loose at the hold clamps windshield pillars, doors flying open, broken welds at the rear inner super structure, main chassis cracking at weld beads, broken engine mounts, damage to outer body panels and the commonly felt vibration at 55-65 mph.
A solid structure is required for a smooth ride.
Martin

 

[Keoke]

Scuttle shake is when the you feel the structure under neath you twisting which causes the car to shake when going over pot holes or rail road tracks.

Well we experience it on the smoothest of freeways. And there is too much evidence that causes other than "Pot Holes Or Rail Road Tracks " produce the vibration. Jerry Rude has alluded to these additional factors in his excellent dissertation on Strength of Materials 101:friendly_wink:

 

[bob hughes]

Sounds as if I load the car up with a ton of weight to act like a damper things will smooth out, acceleration will be rubbish though. :excitement:



Bob

 

[AJAX50]

No NO 1! AjaX50--However here is a situation that will change the speed at which it occurs and severity of the shake>

On My Miss Blu the so called scuttle shake came at different speeds. I had checked everthing as defined in this post. In desperation I put the car up on jack stands and began checking the drive train again. When I got to the rear universal it checked OK but the drive shaft moved up and down at the pinion. This had never been seen before during similar checks. Now that ain't right so I disconnected the shaft from the pinion.The pinion continued to move up and down so I removed the pinon nut and a broken Split washer reveaed itself. Replaced the washer retorked the nut to 150 lb ft no more scuttle shake

.

I may be wrong but if, say, a rear brake drum was out of balance and the speed of rotation matched a harmonic of a resonant frequency of the body shell that would create a stronger vibration at that speed.

 

[Keoke]

I may be wrong but if, say, a rear brake drum was out of balance and the speed of rotation matched a harmonic of a resonant frequency of the body shell that would create a stronger vibration at that speed.

Well Yeah Unless it was 180 degrees out of phase
then it would cancel.

 

[AJAX50]

I may be wrong but if, say, a rear brake drum was out of balance and the speed of rotation matched a harmonic of a resonant frequency of the body shell that would create a stronger vibration at that speed.

Well Yeah Unless it was 180 degrees out of phase
then it would cancel.

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.