Lotus Elan

Yaw sensor, emergency fuel cut off

PostPost by: h20hamelan » Tue Mar 24, 2020 5:03 am

I was gonna use a Ford inertia switch, because I like Ford.
Porsche etc use a yaw sensor, which detects any of too much movement. Whereas Ford is only up and down.
I will have the low oil pressure sensor trip the relay, to power the pump.
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PostPost by: john.p.clegg » Tue Mar 24, 2020 6:03 am

Will that help if you crash and the engine is still running?

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PostPost by: JonB » Tue Mar 24, 2020 6:47 am

The Ford inertia switch (which I use on mine) works on impact forces. It might be triggered by up/down movement as well, but when I test it I tap it on the side with a screwdriver (handle end).

From https://en.wikipedia.org/wiki/Inertial_switch :
"A small loose weight (called a proof mass) is trapped within a spring-loaded cage. A shock in any direction will cause movement of the mass relative to the cage. If sufficiently shocked, the cage will spring open which actuates an associated switch."

I think the main point for us is that it needs to be mounted it rigidly so that shock waves passing through the body in a collision may be efficiently transferred to it. It is also cheap and bulletproof. I think I'd rather have one of these instead of a complicated yaw sensor.
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PostPost by: HCA » Tue Mar 24, 2020 8:30 am

I would advise not to try and ape anything Porsche onto your Elan - unless you are a real self-harmer :)

The Porsche yaw sensor is linked into so many other sensors that form part of the PSM - Porsche Stability Management and is so intricate it claims to know you will have a smash before you do!

I agree with Jon, the simple Ford inertia switch fitted to the Focus et al is as good as it gets. And if you have EFI especially, use flexible teflon fuel hose without joins in conjunction with the switch and you are pretty well bomb proof.

My only concern, and I asked Jon's thoughts on this a while back, is if the elasticity of a GRP body might not transmit a knock to the switch as it should.

Instead of having a low pressure oil switch off the pump which is finite, how about the switch wired into a small piezo sounder that at least gives you a chance to look at the gauge in case of malfunction, allowing you to make the decision to switch off? Just a thought...
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PostPost by: JonB » Tue Mar 24, 2020 9:49 am

HCA wrote:My only concern, and I asked Jon's thoughts on this a while back, is if the elasticity of a GRP body might not transmit a knock to the switch as it should.


Indeed. But then, a modern Ford has crumple zones to absorb the shock waves. This will reduce the severity of the forces that the inertia switch experiences. The Ka switch is mounted inside the passenger compartment, too (that is, better protected). I recall that, in the Sierra Sapphire, (a saloon version of the Sierra) it was in the boot. So I would ask the question, is there an optimal place to put it on an Elan?

On my Plus 2 it is rigidly mounted on the bulkhead near the pedal box. I wondered about mounting it on the chassis but wouldn't this mean the body collapsing on impact would still soften the forces reaching it in a crash? Thus I chose to make it accessible.

As you might surmise, there are no perfect solutions to this problem, but it is much better to have a fuel pump cutout solution than not.

Regarding oil pressure cutoff switches, they might work (except there is a chance the engine will carry on running, and thus the switch won't cut out after a crash, as Cleggie says) but doesn't fitting one negate the point of fitting a pump (to a carburettor equipped car like mine)? It's there so that I don't need to turn the engine over excessively to prime the carbs when cold starting.
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PostPost by: nmauduit » Tue Mar 24, 2020 2:00 pm

JonB wrote:Indeed. But then, a modern Ford has crumple zones to absorb the shock waves. This will reduce the severity of the forces that the inertia switch experiences. The Ka switch is mounted inside the passenger compartment, too (that is, better protected). I recall that, in the Sierra Sapphire, (a saloon version of the Sierra) it was in the boot. So I would ask the question, is there an optimal place to put it on an Elan?

On my Plus 2 it is rigidly mounted on the bulkhead near the pedal box. I wondered about mounting it on the chassis but wouldn't this mean the body collapsing on impact would still soften the forces reaching it in a crash? Thus I chose to make it accessible.

As you might surmise, there are no perfect solutions to this problem, but it is much better to have a fuel pump cutout solution than not.

Regarding oil pressure cutoff switches, they might work (except there is a chance the engine will carry on running, and thus the switch won't cut out after a crash, as Cleggie says) but doesn't fitting one negate the point of fitting a pump (to a carburettor equipped car like mine)? It's there so that I don't need to turn the engine over excessively to prime the carbs when cold starting.


I've added a Ford crash sensor when I installed an electric fuel pump in the trunk (plus braided lines to the carbs): it's in the trunk attached by screws into the fiberglass, upper rear left corner where an existing unused wire was conveniently lying (for aerial antenna probably). I played with it a bit before proceeding, it goes off relatively easily but should do fine as it since passed a track session without going off. My reasonning was that if the crash is low enough that it does not go off, then I should still be awake enough to turn off the main circuit if need be.

I'm planning to implement an oil cut-off circuit as well, but have not yet done it as not as high priority (it requires a by-pass switch too, without cluttering the dash yet rather accessible in case of emergency...). I see it as an extra safety mostly for the engine (not for start-up issues but for the risk of oil pressure failure while driving).
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PostPost by: john.p.clegg » Tue Mar 24, 2020 2:37 pm

" it goes off relatively easily but should do fine as it since passed a track session without going off. "

Then you need a driving course :lol: , mine went off during some " spirited " public road cornering ...

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PostPost by: prezoom » Tue Mar 24, 2020 3:14 pm

I have installed Ford's inertia switches on both the S2 and the Plus2. They are simply a "ball" switch, that when displaced will cut the current to an electric fuel pumps. In addition, I have installed a fuel cut off switch in the fuel line between the fuel tank and the fuel pump on the Plus2, because fuel is drawn through a port on the bottom of the tank. The solenoid is operated to allow fuel flow when the ignition switch is in the run position. Both the solenoid and the fuel pump are controlled through the inertia switch.
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PostPost by: nmauduit » Tue Mar 24, 2020 4:25 pm

john.p.clegg wrote:" it goes off relatively easily but should do fine as it since passed a track session without going off. "

Then you need a driving course


I sure do - but this is my street elan, I keep it within the track limits to avoid the vibrating kerbs as much as possible, as I suspect these are not the best for wheel bearings and steering play.

I've gone off the track a number of times as it was a wet track day, though not maximum speed (say less than 100km/h) into the grass and without hitting major cement banking - I think I should be fine and it may only go off on a sudden impulse (hitting something, falling off from or climbing onto something brusquely, sidewalk, large pothole...). There was a dry track part also for which basic telemetry indicated about 0.9G all around on Yokohama A048 without going off, and I don't anticipate pushing the car that much on opened roads anyway.
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PostPost by: h20hamelan » Tue Mar 24, 2020 5:10 pm

Dealing with accidents, the Fords dont normally trigger when I figure they should have. They normally only trigger from a very forceful (say 20’ drop off a cliff or something) Unfortunately this is the same case for air bags (in any vehicle), and more of the time, similarly they just dont work. The opposite is also true at times. I have seen or a fuel cut off/inertia and airbags deploy from hitting a curb.

I believe Bosch (Audi BMW Porsche etc) yaw sensor is more accurate, I might get around to cutting both open at some point as I dont have access to the schematics. I also dont understand if the Bosch yaw is as simple to make operate as the Ford.

I am off to my yard to collect a few more Ford inertias. I think there will be a magnet on top of a spring, which the ball is attracted to. If the ball gets dislodged, it makes contact with the metallic sides, or if the straight down force is enough. It triggers it through the spring. The other option might be the ball is the electrical path, and the metallic contact is through the spring, magnet, ball then contact patch above the ball.

Either way, vehicles that have gone off a 5-10’ cliff dont trigger the switch. Seems more force is required in a straight down accident. Whereas when a vehicle hits a curb. Maybe the front right, where the sensor is. Takes more of the force... all speculation
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PostPost by: pharriso » Tue Mar 24, 2020 6:57 pm

I don't know why you guys are talking about using a yaw sensor, Yaw is the angle between the centreline of the vehicle & direction of motion. Would you want to cut fuel if you were oversteering? (rear end out having fun...)

What is needed is an inertia switch which senses rapid deceleration.

As an aside my favorite quote is "“Speed has never killed anyone. Suddenly becoming stationary, that's what gets you.”
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PostPost by: JonB » Tue Mar 24, 2020 7:59 pm

The “yaw” sensor probably has a gyroscope / accelerometer in it like modern smartphones. If so it’s more than capable of detecting a collision. But it’s OTT for an Elan fuel pump cutoff... if you can get it working.

Re: quote: nice one, Phil! :lol:
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