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TR2/3/3A Bonnet Prop Rod Rattle

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PatGalvin

PatGalvin

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Well, my bonnet prop rod is like a physics experiment to prove out resonance. At certain RPM (especially around idle, unfortunately), that thing just starts to vibrate and rattle like nobody's business. It's such a long thin rod - no surprise it finds it's frequency and starts to vibrate like crazy. Has anyone come up with any way to stabilize this long rod and reduce the vibration/rattles? I know this has been discussed, my search for answers didn't result in much.

Thanks. Gonna drive this weekend. Should be nearly 80 degrees by Sunday.

pat
 
I am still struggling with that rattle as well. I have seen others tie a rag around the prop rod about in the middle, and it stops the rattle, but doesn't look stock. In mine, the rivet at the hood latch end is starting to become loose from excessive rattling. Since it only rattled at idle, and also dependent on exact idle speed, I tried to ignore it for a long time. My choke cable is adjusted so I can slightly pull on it to increase the idle, giving the idea that I have some control of the rattle. Eventually that bugged me too. Right now, I have an unused front end rubber bushing slid around the prop rod all the way up and wedged onto the hood latch end. It stops the rattle, but every time I lower the hood, the wedge slides back down and I have to reposition. It is a frustrating and loud rattle, so I'm eager to hear a better solution -I have been listening to that rattle on and off for 14,000 miles now.
 
Pat,

Is it the prop rod or the rear of the bonnet where that structural piece runs across? I know there was a piece of material under there in all of mine that I removed when painting and have yet to put back in....

Cheers
Tush
 
I have the same problem and have not found a solution. I have a temporary solution that works for a day or two, put a drop of oil on anything that moves around the latch and it quiets it down for a short while.
 
Hey David.

At the rear of the bonnet, where the inside bracing exists, I removed all the horsehair and sealed it with seam sealer. I don't think I've got anything moving around within the "well" created by the bracing. I'm pretty sure the rod is vibrating.
My bonnet catch hook also rattles, but I've put some electrical tape on that to quiet the rattle. That works and looks like $%##.

I wonder if I could glue a piece of rubber or foam to the bonnet internal surface that would bear on the prop rod, when it is parked in its position under the bonnet? Or, slide or wrap something on the rod to dampen the vibrations?

I don't think the spring that pushes up the bonnet is rattling. I initially suspected that, but the vibration isn't a high frequency buzz (like you'd hear in the spring). It sounds like a long steel rod vibrating.

Jer- 14,000 miles? That's spectacular. You built a fine car, you did!

Pat
 
Hey Pat. Could you try and wrap a small piece of fuel line around the Rod as a buffer where it is hitting?

Cheers
Tush
 
PatGalvin said:
It sounds like a long steel rod vibrating. You built a fine car, you did!Pat
Click to expand...

Thanks, it was a fine car before I rebuilt it, but now the rod rattles.

The way I determined it was the rod, and not the latch or spring, was to (almost) close the hood, but not latch it. It can rest on a small board or something, and you can see the rod reach a resonant frequency and vibrate like crazy -and hear the rattle. The rattle can be stopped by then reaching in there with a tooth-pick like stick and stop it. It doesn't take much to stop it, but a rag tends to slide up or down, and allow it to keep rattling. I tried replacing the rubber at the far end. It seems to me the rod needs another rubber in the middle to keep it resting against the hood. Each time I hear it, I can't tell if it's the framework of the hood, the rod or something else, but it sure seems like the rod to me. Maybe my approach is wrong and I should try to get it to idle smoother... I don't think that is going to happen.
 
I took about a foot long piece of very thin soft rubber drain hose maybe 3/8 id off a Saab and slip it over the end and worked it down the rod to about the middle. The hose is black and so is the rod, so I really do not see it. The noise is way better. I guess it acts like a vibration dampener.
 
I have a very interesting work project where we are designing a large diameter water wheel in the LA River. One of the exercises the structural engineers did was to analyze the vibrational frequencies for this large metal object, as it turns. I learned that objects have natural frequencies (resonant frequencies) and if those frequencies are induced, the vibration can be severe. For the wheel, it was a certain RPM that would create vibrations. So, the prop rod has a natural frequency - when your engine operates at that frequency, it induces a vibration. If the frequency is just right, it can cause it to vibrate fairly violently. A dramatic example of this is the bridge failure at Puget Sound Narrows. Check out this YouTube video, if you have interest and a few minutes to spare.

https://www.youtube.com/watch?v=j-zczJXSxnw

In any event, slipping a piece of tube over the prop rod will change the natural frequency of the rod. Maybe that's all she needs? I was thinking of gluing a piece of rubber or foam to the underside of the bonnet such that the prop rod would bear on the rubber/foam, stopping the vibration.

Good ideas. Interested to hear from others. Sounds like many of us have this issue. For me, it's bad. Sort of embarrassing, when at a stop light.

Pat
 
Our last TR3A rattled, but this one does not. I put some rubber pieces in the spring and or some other miracle happened. Let me look today at the TR3A and I will bring my ideas on the run tomorrow. But I think we are bringing the mini instead of the TR3A.
Jerry
 
Pat, when you come up with your solution, please let us know. I haven't got mine on the road yet but hope to in the next few months.

Cheers
Tush
 
There is supposed to be a wavy spring washer in the prop rod pivot. That plus the rubber clip should keep it from rattling. I don't recall offhand whether I replaced the rivet with a bolt & nut, or just crimped it tighter, but that was all it took to stop mine from rattling. An easier solution is to wrap some cotton string or thread into the joint to take up the gap.

A friend of mine came up with some interesting solutions for some of the other rattles. He installed two more rubber buffers, same as the ones used at the front but at the rear of the hood. Properly adjusted, they hold the hood up against the hinge pins, so it cannot rattle against the hinges. He also machined the lift rod bracket so he could add nylon sleeves (push-in bearings) for the lift rod to run in. Eg https://www.mcmaster.com/?m=true#snap-grommets/=wp1w8i (I think). I don't recall the size offhand, but notice that there are different sizes involved.

Probably varies from hood to hood (ISTR there were at least 4 different hood assemblies, maybe 5), but many of them originally had some sort of felt or horsehair between the rear brace and the sheet metal. You can replace the original with synthetic felt (from MMC if you can't find it at Home Depot any more) or silicone caulk seemed to work well for me. Any kind of elastomer should help dampen the resonance.
 
Mine rattled on my car also . It rattled at the forward pivot of the rod in the little metal loop welded to the bonnet . I fixed it by taking a piece of shrink tubing and shrinking it over the pivot to hold it and cushion it. I thought it was kind of a half baked kind of fix until I bought an untouched TR3 and saw the factory had wrapped it the same joint with electrical tape.
 
Sounds like a very common problem. Mine was no different. As long as you don't mind the looks I cured mine by taking a square piece of somewhat dense foam rubber and glued it to the underside of the bonnet. When the prop rod is put away it first rests against the foam placing tension on the rod then it snaps in the rubber piece. never heard a peep from it since. I'd take a pic of it but I have the car all apart again. I have been contemplating what I was going to do because I'm trying to make a # 2 condition car and I like Randalls fix. It's worth a try for me.
 
I'm feeling lucky now that mine does not rattle...

As for the engineer in me, if you have a resonance problem, you have to change the stiffness and/or mass of the part to change the resonant frequency...dampen the part....or, change the operating frequency to avoid the resonance. Some thoughts...

Heating and quenching the rod would stiffen it, therebye raising the frequency that resonance would occur.

Gob paint on the rod to dampen the resonance.

Lower or raise the idle speed to prevent the resonance from developing.


I am curious at what engine speed the resonance is happening. Mine idles at 600-700RPM. What speeds are those of you with resonance idling?
 
Mine has rattled for over 50 years at idle. Two years ago, I squeezed a piece of natural sponge between the rod and the bottom side of the hood and that stopped the rattle. The problem with that is :- 1. It's not original and 2, Every time I lifted the hood to prop it up, the sponge would fall onto the top of the engine. To me, the rattle is Ă’RIGINAL. But only at idle.
 
Thinking about this from a technical point of view. Does't it take TWO pieces of metal vibrating against each other to cause the noise we are getting? Just having a rod should not produce the rattle. So if the rod is vibrating it must be loose enough where it contacts another piece of metal to produce the rattle. The one end is clamped into the rubber, so unless the rod is rubbing somewhere on the hood, the pivot end would have to be the source. It seems very likely if there was wear in that pivot over time it would loosen enough to allow the rattle.
I don't have the solution, but shouldn't this be the location of the actual problem?
On the TR4 I was able to stop the rattle by using a couple of spring washers to better fix or clamp the pivot end of the rod. But the TR4 and TR3 are different.
Charley
 
I think John is spot-on with his analysis of the resonance (rattle) problem. My car idles closer to 1000 rpm. if I was at 600, the darn think would stall when the fan comes on. Fix one problem create another....

CJD said:
I'm feeling lucky now that mine does not rattle...

As for the engineer in me, if you have a resonance problem, you have to change the stiffness and/or mass of the part to change the resonant frequency...dampen the part....or, change the operating frequency to avoid the resonance. Some thoughts...

Heating and quenching the rod would stiffen it, therebye raising the frequency that resonance would occur.

Gob paint on the rod to dampen the resonance.

Lower or raise the idle speed to prevent the resonance from developing.


I am curious at what engine speed the resonance is happening. Mine idles at 600-700RPM. What speeds are those of you with resonance idling?
Click to expand...
 
I have thought about this rattle more than once, but this thread has me revisiting my efforts. For the majority of the 7 years of restoration, I never noticed the rattle because as Don stated, it happens "only at idle". When I first had the car running with the hood on, it had what I assumed to be an original rubber retainer for the stay rod. The rubber was old, covered with multiple coats of paint, brittle, very hard, and began to tear shortly after it was installed. I had other rubbers equally as original, but they either started to tear after installation, or were unable to hold the rod, so a new one was purchased. Also, the rivet and bracket at the other end were rusted and needed help to move before I painted. By the time they were assembled, the rivet was again tight because of the paint that managed to seep around the edges. I don't know when I first noticed the rattle, but the rattle was erratic, even after the idle was stable. With the ability to bump the idle a little with the choke pull, the rattle will still come and go at different idle speeds.

If I understand resonance correctly, it is rather finicky. Either end of the rod could effect it, as well as anything touching the center, as well as the stiffness of the rod and probably even the temperature of the rod. My idle can be between 800 to 1200 but I haven't found a sweet spot where the rattle will not (ever) happen. I think Johns 600rpm idle is amazing low and doubt mine can reliably idle that low. I have a TR4 cam in mine, but under 900rpm or so, it searches a little, almost lope-like, probably the result of the distributor springs over-reacting. -anyway, the rattle doesn't always happen when I stop a red light. It does seem to always happen when a pedestrian is showing interest in my TR3 while it is idling.

My rattle may have become more noticeable for several reasons. 1)it is capable of idling 2)the idle is relatively smooth 3)other functional issues are comparatively small 4)the paint has worn away from around the rivet on the front 5)the rubber stay rod holder is soft rubber. -Giving this much thought to a single rattle in a toy car over 50 years old, makes me realize just how lucky I am to have it.

I may add a rubber o-ring on a string instead of the "original" holder for the stay rod and hope that resolves the noise completely -or I might just try to remember how lucky I am to have it rattling.
 
I took the 3A on the club rally this weekend and found out it does rattle, but only at idle. I had put rubber washers on the top and bottom of the spring. This helps keep the spring rod tight, but I see the vibrations seems to be from the front edges of the bonnet. Up with the buffers!
Jerry

PS: We had to drive about 55 miles on the freeway and got lots of stares and thumbs up on the car. Nice to see people appreciate old cars.
 
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