23 replies to this topic

[murpia]

Hi,

Does anyone know of a suitable cut-off switch that can be used to detect an impact and cut off a hydraulic system in a race car?

The ideal switch would be normally closed and latching open in the event of an impact. It would power a normally-open solenoid valve connected to the hydraulic accumulator. In the event of an impact the switch would latch open, de-powering the valve and releasing the hydraulic accumulator pressure. The valve requires 1.5A.

The 'g' range of the road car fuel-pump cut-off parts is a bit too low for comfort, we would like to use something around the 10->15g range to avoid any reliability issues.

Many thanks, Ian

[gruntguru]

What about collision sensors used for triggering airbags?

[cheapracer]

Many motorbikes have had mercury (or not actual mercury probably) switches for years (may even be legislated in some countries) so if the bike falls over the fuel pump is killed, maybe you could rig one up to suit?

[Tony Matthews]

The 'g' range of the road car fuel-pump cut-off parts is a bit too low for comfort, we would like to use something around the 10->15g range to avoid any reliability issues.

I was going to say the same, until I re-read your post. Standard ignition kill-switches are probably not what you want in that case. Sierra Cosworths have one mounted behind the front bumper, and scallywags persued by Cossie-equiped cops would stop and reverse into the police car, the fairly mild impact sufficient to cut the power. They would then drive on to continue their life of crime. The one you want sounds a bit drastic - is the idea that it only works after you've been through the Armco, and 3 metres into an earth bank? Or should I call you Officer?

[CSquared]

I was going to say the same, until I re-read your post. Standard ignition kill-switches are probably not what you want in that case. Sierra Cosworths have one mounted behind the front bumper, and scallywags persued by Cossie-equiped cops would stop and reverse into the police car, the fairly mild impact sufficient to cut the power. They would then drive on to continue their life of crime.

That is too funny.

[cheapracer]

. Sierra Cosworths have one mounted behind the front bumper, and scallywags persued by Cossie-equiped cops would stop and reverse into the police car,

The Crims in various countries have cottoned on to air bags now, sudden stop, cop car bangs into the back and 'bang' go the air bags - end of pursuit.

[gruntguru]

The one you want sounds a bit drastic - is the idea that it only works after you've been through the Armco, and 3 metres into an earth bank? Or should I call you Officer?

10-15g is actually not that drastic. A decel that spikes to 15G in the chassis might still only apply a few G to the driver when he hits the seatbelt or airbag.

EDIT. Excerpt from Formula SAE rulebook.

The team must submit test data to show that their Impact Attenuator, when mounted on the front of a vehicle with a total mass of 300 kgs (661 lbs) and run into a solid, non-yielding impact barrier with a velocity of impact of 7.0 metres/second (23.0 ft/sec), would give an average deceleration of the vehicle not to exceed 20 g's, with a peak deceleration less than or equal to 40 g's.

Total energy absorbed must meet or exceed 7350 Joules. Note: These are the attenuator functional requirements not test requirements. Quasi-static testing is allowed.

When using acceleration data, the average deceleration must be calculated based on the raw data. The peak deceleration can be assessed based on the raw data, and if peaks above the 40g limit are apparent in the data, it can then be filtered with a Channel Filter Class (CFC) 60 (100 Hz) filter per SAE Recommended Practice J211 "Instrumentation for Impact Test", or a 100 Hz, 3rd order, lowpass Butterworth (-3dB at 100 Hz) filter.

Edited by gruntguru, 06 January 2011 - 03:15.

[murpia]

Some more info:

The intent of the switch is to eliminate any danger from stored energy (in the hydraulic accumulator), should an impact be severe enough to risk damaging the hydraulic system. So the 'through the Armco into the bank' case is not far from the truth.

A bit of online research has suggested to me that road car fuel pump cut-off switches are rated around 10->12g. That may be good enough, we might have to test some. Any suggestions on suppliers? If you use a search engine the results are swamped by a thousand forum queries asking 'where's the inertia switch in my XXXX model car'...

Regards, Ian

[Tony Matthews]

10-15g is actually not that drastic. A decel that spikes to 15G in the chassis might still only apply a few G to the driver when he hits the seatbelt or airbag.

So the 'through the Armco into the bank' case is not far from the truth.

See - I was right!

[John Brundage]

An open palm hit to a rear quarter panel of a Ford (1980s) was enough to trip an inertia switch and disable the fuel pump. Most people didn't know why the car was a crank-no start, where the switch was, and that there was a button on top for resetting the switch.

[cheapracer]

An open palm hit to a rear quarter panel of a Ford (1980s) was enough to trip an inertia switch and disable the fuel pump. Most people didn't know why the car was a crank-no start, where the switch was, and that there was a button on top for resetting the switch.

Lucky it wasn't one of these, idiot parked in my car park and I open palmed the left rear quarter to trigger the alarm and the whole quarter caved in! Didn't even trigger the alarm either - exit stage left (and rapidly) ............ (geez Greg, how thin are you making these things!).

[cheapracer]

A bit of online research has suggested to me that road car fuel pump cut-off switches are rated around 10->12g. That may be good enough, we might have to test some. Any suggestions on suppliers?

Avis or Hertz rent a car?

[Lee Nicolle]

As far as I know we do not have fuel cutoff switches on Australian cars. I have never heard of them, have seen pleny of cars started and driven with the airbags gone of. Have seen plenty of cars with major damage where the airbag has not gone off too. And a late model car in a gentle chain colision with about 5 airbags gone off.
What I am saying is I dont think I need that sort of very 'iffy' technology on a race car where it turns itself off after riding a kerb, or hitting a big bump. And racecars [and winged ones in particular] ride that hard to make that possible. That sounds like a huge crash waiting to happen.

[Catalina Park]

As far as I know we do not have fuel cutoff switches on Australian cars. I have never heard of them, have seen pleny of cars started and driven with the airbags gone of. Have seen plenty of cars with major damage where the airbag has not gone off too. And a late model car in a gentle chain colision with about 5 airbags gone off.
What I am saying is I dont think I need that sort of very 'iffy' technology on a race car where it turns itself off after riding a kerb, or hitting a big bump. And racecars [and winged ones in particular] ride that hard to make that possible. That sounds like a huge crash waiting to happen.

Lots of cars fitted with them in Aus. The early injected Falcons used to turn off with the right pot hole.

[jimmy8v]

Some road derived caterham race cars have them, included in the rover loom. My experience of these is that they can be triggered by running kerbs and other impacts that are par for the course. If all road sensors are similarly sensitive I'd steer clear. In the past we've cable tied a pebble onto the rubber reset switch so it's permanently resetting and cannot trigger. You do not what false positives.

[Lee Nicolle]

A little off topic but I have had isolator switches turn them selves off with moderate G forces when going through the dips at Collingrove hillclimb. And they were brand name ones too.
I have heard of driver airbags tripping on modified [low stiff springs and shocks]road cars in similar circumstances.

[Catalina Park]

I just remembered the Ford Falcons that if you slapped the side of the car the right way it would unlock the car and turn the hazard lights on...

[OfficeLinebacker]

I had a '94 Ford Escort. One night I was driving my friend Forrest home who's a strapping, hyperactive lad. He was bouncing up and down in the back seat and he basically bounced hard enough to trip the fuel cutoff switch in the back of the Escort. I had it towed to the dealer (it was under warranty) and they charged me a bunch to diagnose it because as the owner I'm supposed to know about these things (I was in uni at the time)

[Canuck]

To could always piece together a fairly inexpensive circuit with an accelerometer from VTI, a PIC processor and a couple of transistors. In all fairness I'm not sure how much their 12g sensors are - I know the SCA1000 runs in the $40 range.

[gruntguru]

If you find a switch that does what you want but is too sensitive, you can always reduce the sensitivity by using a resilient mounting and/or fine-tune by adding mass.

[DaveW]

I rather think I would prefer to use (average) wheel speed with a selectable defeat mode, given the presumed application & the fact that it will take a finite time to depressurize an hydraulic system.

[gruntguru]

I rather think I would prefer to use (average) wheel speed with a selectable defeat mode, given the presumed application & the fact that it will take a finite time to depressurize an hydraulic system.

How would the system differentiate a 4 wheel brake lock?

[DaveW]

How would the system differentiate a 4 wheel brake lock?

I would be tempted to argue that is exactly the condition for releasing stored energy...(no steering & no engine). I think that making the operation robust would be no more difficult than the alternative - identifying reliably an acceleration "crash" signature from a signal buried in, perhaps, 20gn rms engine "noise".

[jatwarks]

Hi,

Does anyone know of a suitable cut-off switch that can be used to detect an impact and cut off a hydraulic system in a race car?

Didn't Tyrrell & Brabham, amongst others, try using inertia switches to trigger fire systems on impact in F1? I seem to remember them going off under hard braking!

About the same time JYS & Black Jack messed with fighter pilot helmets.