I have swept the internet, including Saab fora, to find out what scuttle shake is rather than what causes it and which vehicles exhibit it, to no avail. Given that having a roof would prevent the longitudinal shearing of the A pillars, this sketch is what I think it is:
Does this fit what you guys are experiencing? It would manifest as an oscillation of about 2 Hz after e.g. hitting a pothole with one front wheel.
As you can see below, the Saab actually has a scuttle at the base of the windscreen, whereas the MX-5 has some scuttle like structure linked to the engine bulkhead under a plethora of plastic panels.
Mazda’s technical review suggests that the brace is intended to resist the twisting of the chassis rail due to the offset force going into the suspension tower. This need would exists even with a roof, so the brace is possibly not intended for Scuttle Shake, but given the stiffening across the node of the mode shape, it would help.
How does the cross brace help?
It strengthens the the strut brace, other manufacturers do likewise
Ah, Blue, those are a different kettle of fish. The cross bracing and fillet plates stiffen the structure. A single strut stretching across the towers doesn’t seem to do anything to increase overall torsional stiffness nor local scuttle shake.
This is a good thread. The brace that blueAgave is showing is even better. I have looked at shuttle shake on convertibles & all answers come back that a cross strut, tower to tower is virtually u.s… A brace from towers to scuttle improves the shake, the fact that the doors on mx5 have bushes helps stiffen the body is good. At the moment I do experience slight judder so I will give a brace a go. The “ DaveFab “ product looks good.
Hi, Paul, is the slight judder actually all the plastic vibrating, or do you feel it in the steering? The stiffened brace looks good but it is so expensive.
Hi Steve, yeh I get a slight judder in the steering but only when moving off or flooring to get a quick response, blinking idiots don’t seem to see a small sports car. I had a lotus 7 kit car which never had that problem but there were no doors & the stainless steel chassis wrapped all around the cockpit. I did look at a lot of other things but the car only has 13000 miles. So I don’t mind spending a few bob as I love this car. I’m not generally a fast driver at 73 but have always loved sports cars.
Elsewhere I have seen many recommendations of this brace.
David
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The standard Mazda brace has single bolted joints where it interfaces with the scuttle. Single bolted joints will never EVER be as stiff as a welded one piece joint.
First _rider_off, totally agree with what you are saying but at some stage you need to bolt. Looking at the Dave fab is very close. Something I did neglect to mention was that one of the things I looked at was the gearbox, took car back to Mazda & was assured that the gearbox was not the problem. Checked a lot of things such as all the plastic stuff even steering column & rack. Changed gearbox & diff oils using specified oils, no difference so this is probably last throw of the dice. If this does not cure I will live with it, I’m not looking at this for no other reason than safety, at my age I’m quite happy with the performance. Just feel that manufacturers could do more with convertibles. I don’t think the RF would have this problem. Well it’s not a real problem but I personally would like to see if it is curable. I’ve also noted that there are a few threads that talk about gearbox problems & I was wondering if those problems are really scuttle shake. Just wondering. Anyways I’m going to look see & in a month or two I’ll let you know how I get on.
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“gearbox problems & I was wondering if those problems are really scuttle shake”
The gearbox issues were very real and absolutely NOT connected.
The problem with judder coming through the steering would in no way be down to skuttle shake which is caused by the floor flexing because a convertible car obviously doesn’t have a metal roof. I would suggest looking at tyres/tracking, steering department. Also do you have the same make/model of tyres all round?
In the sales literature, I see the mention of strength. I don’t understand why. The design of braces is a stiffness issue, not strength, which makes aluminium irrelevant in these tubular concepts. A 25mm 1mm thick steel pipe is the same weight and stiffness as a 25mm aluminium pipe, and a fraction of the cost.
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The Mazda brace is designed to brace between the tower and the point on the scuttle. The bolt is probably enough, as there are just 2 8mm studs at the tower end.
Blue, I don’t mean that the judder comes from the steering. I presume the point of worrying about scuttle shake is that the vehicle handling is affected by the flexure that you refer to.
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Malc & BlueAgave, sorry if I gave the wrong impression but I also said just wondering. Vibrations & causes are hard to diagnose. In my case it happens in cold & warmed up engine etc. Not sure where steering column passes through to steering rack but if the scuttle shudders albeit briefly it could be felt through the column. If you get heavy judder at say 30mph that fades out after hitting a few mph more it’s probably wheel balancing. Surely it’s a good thing to suggest looking at other would be faults, if nothing else but to eliminate. Anyways as I said I’ll come back & I will give an honest account of what happens. Some years back ( 20 ) I had a convertible Astra, really was a nice looking car anyway I had the same vibe, judder when top down but never did get to the bottom of it. So for me it’s worth a try.
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I would be interested to hear about the RF. As with the roadster, there is no rigidly connected roof. However, the loop over the B pillars might add some rigidity.
Strut brace versus chassis brace.
I had a strut brace on my Eunos but it was jittery on rougher surfaces. I fitted a Mr Fast bar underneath and it was much better. Going back to around 2010.
Got the impression it dialled out some oversteer.
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However, I love the design. This is what I have in mind to do for a DIY job.
