Locking wheels question #2

[BluSTi]

Do we need to lock up fresh tires, or can we use older ones? I despise the idea of flatspotting a brand spanking new set of rubbers.

[PatClarke]

I agree.

The requirement to lock all four wheels to show the brakes work effectively is now obsolete.

With virtually everyone at the event having an accelerometer (in their cellphone) it should be easy to determine a car can stop in a straight line at more than one G without flat-spotting tyres!

It would also remove the need to supply 4 observers during brake testing.

Pat

[Charles Kaneb]

As of Lincoln 2012, you can do the brake test on any legal set of tires - dry if it's dry, wet if wet. We used a set that were right at the end of their life. One of the observers looked away at the critical moment and we had to retest. After the second one they were completely shot to sh...aving cream.

1g on dead-cold tires in the dry might be too strict, especially if you run a hard-compound tire. I think a .8g stop in a straight line is enough; that's as good as a passenger car.

[BluSTi]

Thanks you for the replies folks. I expect we'll be doing the same.

[jlangholzj]

Originally posted by PatClarke:
I agree.

The requirement to lock all four wheels to show the brakes work effectively is now obsolete.

With virtually everyone at the event having an accelerometer (in their cellphone) it should be easy to determine a car can stop in a straight line at more than one G without flat-spotting tyres!

It would also remove the need to supply 4 observers during brake testing.

Pat

Pat could you see any change in this for competition?

My thoughts:

Locking the brakes only really proves that the system can overcome the dynamic friction of the tire...which depends on the surface, tire compound, pressure, etc. An old hard tire on 40psi will lock a LOT easier than say a 25B with 10-12 psi. HOWEVER....without supplying any data would you agree that a softer tire (say even a LC0) with a reasonable pressure would give a larger magnitude of deceleration...correct?

So what are we really achieving with brake lock test? Whether or not we can overcome a crappy tire's friction or provide our car with proper deceleration....

[PatClarke]

I guess unless someone submits it as a possible design change for consideration, there will be no change.

But a well reasoned and presented submission to the Rules Committee might bear results (hint, hint) ;-)

Pat

[Charles Kaneb]

Using a 1-g toppling block on the floor of the car has the exact advantages and disadvantages as using an accelerometer for the test - the more the car pitches the less accurate the reading will be either way.

Use a pair of timing lights right before the brake test zone and a cone a known distance in front of the timing beam instead. Come through the traps too slow? A little red light comes on. Not have adequate brakes this time? You'll hit the cone!

If it's obvious that 1g deceleration will be unavailable to a car (awful tires, pouring rain, stupidly cold conditions) then you can just drive a road car through the traps at the test speed, stop, and place the cone at the nose of the car. If your brakes aren't as good as those on a twenty-year-old Miata you don't pass!

[jlangholzj]

Originally posted by Charles Kaneb:
Using a 1-g toppling block on the floor of the car has the exact advantages and disadvantages as using an accelerometer for the test - the more the car pitches the less accurate the reading will be either way.

Use a pair of timing lights right before the brake test zone and a cone a known distance in front of the timing beam instead. Come through the traps too slow? A little red light comes on. Not have adequate brakes this time? You'll hit the cone!

If it's obvious that 1g deceleration will be unavailable to a car (awful tires, pouring rain, stupidly cold conditions) then you can just drive a road car through the traps at the test speed, stop, and place the cone at the nose of the car. If your brakes aren't as good as those on a twenty-year-old Miata you don't pass!

coupling an accelerometer with a gyro should yield enough data to show that you're decelerating greater than 1 g. Of course you'd also need to know the CG of the car to properly determine if its hitting the 1G mark...OR a standardized location on the car would yield comparable results.

I've spent my lunch break today figuring some of this out http://fsae.com/groupee_common/emoticons/icon_smile.gif this is one of those tech things that really grinds my gears (as well as a few other things). I find it very disturbing that for an engineering competition there is so much subjective content in tech. Inspectors who can't make a decision on their own, certain templates being "smaller" than others!!! We were told this year that the templates were within "manufacturing tolerance" when there was a visible ~2-3mm difference between the two!!!

as pat stated, there's not going to be any changes until someone presents them.

[Charles Kaneb]

J,

If you'd like another piece of equipment to be used for the brake test at competition, build an inexpensive prototype, test it with a couple of different vehicles, write up a quick report, send it in as a rules question, and provide an exact parts list. I'm not sure whether the accel/gyro device would allow quick testing and easy verification like a pair of timing lights and a cone, but it can't be too bad to build and improve, so do it! A 4.5 V battery, a pair of 2g accelerometers, a pair of low-pass filters, a summing-in-quadrature amplifer setup, and a "one-shot" comparator trigger to turn on the green LED if adequate deceleration is obtained should get you most of the way there.

As for templates, if you need them to be precise, make 'em and donate 'em in advance. If your mill's big enough to machine them, plywood and MDF should be able to hold tight tolerances.

[jlangholzj]

Originally posted by Charles Kaneb:
J,

If you'd like another piece of equipment to be used for the brake test at competition, build an inexpensive prototype, test it with a couple of different vehicles, write up a quick report, send it in as a rules question, and provide an exact parts list. I'm not sure whether the accel/gyro device would allow quick testing and easy verification like a pair of timing lights and a cone, but it can't be too bad to build and improve, so do it! A 4.5 V battery, a pair of 2g accelerometers, a pair of low-pass filters, a summing-in-quadrature amplifer setup, and a "one-shot" comparator trigger to turn on the green LED if adequate deceleration is obtained should get you most of the way there.

As for templates, if you need them to be precise, make 'em and donate 'em in advance. If your mill's big enough to machine them, plywood and MDF should be able to hold tight tolerances.

The templates we use personally we get laser cut from one of our sponsors, that includes cockpit and nose template, percy we still used plywood and angle iron! HAHA. I don't necessarily "need" them to be precise...its just a little disconcerting that an engineering competition, where most of the vehicles hold a pretty tight tolerance (maybe +/- 0.001" on critical parts), would have something that.....wonky

My initial idea was to make a box that could give you a yes/no on the decel rates, kind of like a go-no go datta ack box. After re-reading your post thought I've also thought of a similar solution. Pretty much any car with a known mass at a given initial and final velocity will have a known stopping distance. (essentially what you pointed out) I think though instead of a timing light, simply have a speed trap with a big green light that will light up when you hit the desired speed. For X weight you must stop within Y distance.

I've been playing around with all these thoughts over the last week or so. Hopefully have something put together between summer testing!

[mdavis]

Originally posted by jlangholzj:
coupling an accelerometer with a gyro should yield enough data to show that you're decelerating greater than 1 g. Of course you'd also need to know the CG of the car to properly determine if its hitting the 1G mark...OR a standardized location on the car would yield comparable results.

I've spent my lunch break today figuring some of this out http://fsae.com/groupee_common/emoticons/icon_smile.gif this is one of those tech things that really grinds my gears (as well as a few other things). I find it very disturbing that for an engineering competition there is so much subjective content in tech. Inspectors who can't make a decision on their own, certain templates being "smaller" than others!!! We were told this year that the templates were within "manufacturing tolerance" when there was a visible ~2-3mm difference between the two!!!

as pat stated, there's not going to be any changes until someone presents them.

Every car has to go through tilt before going through brake. Scales in the tilt table would give you enough information to get the CG height, plus it could take care of official weight at the same time. Killing 2 birds in 1 stop? I'm a fan.

As for the subjectivity of tech, I couldn't agree more. This year in Lincoln, we had a tech inspector that questioned the FIA legality of our seat belts (they are the Schroth Hybrid II belt that I saw a lot of teams using and the same ones we used at MIS and FN), the attachment of our impact attenuator (even after seeing the approved report), and the wall thickness of a non-required tube (even after seeing the SES). We got through tech in Michigan in ~45-50 minutes, Canada took ~30 minutes, and yet we were in tech in Lincoln for an hour and a half mainly because of this 1 tech inspector. We legitimately had a couple of things to fix (safety wire on bolts where you couldn't see the 2 threads sticking out of a locknut) that I'm surprised we didn't get caught on before, but the other stuff was just obnoxious. I think that had something to do with only 34 teams (or thereabouts) getting into tech on the first day at Lincoln.