Last Poster #6

[Buka]

Because it's Saturday.

WEB EXTRA: T-rex Race

 

[Gerry Seymour]

Meh. Sometimes they just don't make sense.
My the wheels/tires on my Vette are staggered. The rears are larger in both diameter and width. You'd have to carry TWO spares, for a car in which you'd be hard pressed to find room for one.

True enough. If there were room for one, I'd assume a front tire would be better on the back than the other way around. Be fun watching you try to steer with that one monster on front, though.

 

[Gerry Seymour]

Because it's Saturday.

WEB EXTRA: T-rex Race

I loved the name of the winner.

 

[pdg]

And my thought is that if ABS activates when there's no skid, well, what ABS does is release the brakes to end the skid.

Close, but no cigar

ABS doesn't release the brakes - at least not in the same sense as taking your foot off the pedal... It will never leave you with no breaks, unless it's broke.

It pulses the line pressure at a fairly high frequency, allowing the wheel that is skidding to rotate again, catch up and contribute to slowing you down more effectively.

That's why, when it activates, you feel your foot vibrating (pressure pulse propagation).

It knows (should know) when to do this by monitoring the rotation of each wheel (just fronts in some systems) using a set of sensors and comparing. Better systems can restrict the pulse activation to a single wheel too.

Think about the different number of revolutions the wheels complete when you're on full lock turning (the different circles the wheel trajectories describe) - the system has to ignore that much difference at least or it'd activate every time you park. Some more modern/complex systems have steering wheel position and speed sensors to adjust sensitivity for this.

This difference can be 25% or more, so to mess with the system your toy spare tyre would need to be less than 3/4 the diameter of your standards.

Exactly the same sensor and monitoring system is used for traction control - to apply the brakes in conjunction with cutting the throttle by some degree to reduce/eliminate wheelspin.


So, if the brakes just didn't brake, something is broken - not just a bit confused.



(Just a little caveat - I don't actually know if US ABS systems are built following a federal ruling that NO brakes are a less litigious solution than better brakes - somewhat like that time a certain US car manufacturer decided it was cheaper to settle the lawsuits caused by deaths rather than issue a recall and fix the problem...)

 

[pdg]

Meh. Sometimes they just don't make sense.
My the wheels/tires on my Vette are staggered. The rears are larger in both diameter and width. You'd have to carry TWO spares, for a car in which you'd be hard pressed to find room for one.

That car's not really designed as a shopping cart/commuter though

 

[pdg]

Informative. All the more reason to discontinue their use.

A little more support to my opinion...

An average space saver tyre here is something like a 125/85r16, rated at 99M.

This means it's manufacturer rated to 81mph, at 775kg. (The full size normal tyres on my car are 91V rated, so 149mph at 615kg.)

There's no reason at all that the actual tyre itself can't cope with running at the legal speed limit continuously until it's bald...

 

[Dirty Dog]

True enough. If there were room for one, I'd assume a front tire would be better on the back than the other way around. Be fun watching you try to steer with that one monster on front, though.

Not really. The difference in size left to right would screw up the differential.

That car's not really designed as a shopping cart/commuter though

Sure they are. I use mine for a 120 mile daily commute.
Yeah, there are Corvette owners who act like they melt in the rain. But they really don't.

 

[pdg]

Not really. The difference in size left to right would screw up the differential.

Limited slip I assume?

That'd get stroppy with a size mismatch...

Sure they are. I use mine for a 120 mile daily commute.
Yeah, there are Corvette owners who act like they melt in the rain. But they really do

I didn't mean that so much as general practicality.

I used to use my 1972 land rover as my daily driver by working around it's shortcomings in that application - it was doable but not ideal.

If I could afford to run a vette (fuel prices here are the killer) it'd be the same thing.

 

[pdg]

If I could afford to run a vette

I'll expand on this

Let's say I drive like a pensioner and get 15 mpg and base it on DD's 120 miles per day...

It'd cost me about £250 per week to fuel it, for a 5 day week with no other use. That's 300 USD at the current exchange.

 

[Dirty Dog]

Limited slip I assume?

That'd get stroppy with a size mismatch...

Be pretty tough to race it with an open diff...

I didn't mean that so much as general practicality.

I used to use my 1972 land rover as my daily driver by working around it's shortcomings in that application - it was doable but not ideal.

If I could afford to run a vette (fuel prices here are the killer) it'd be the same thing.

It's got 2-seats (which is all we need for the commute), enough room under the hatch for Sue and I to put suitcases, and gets as much as 32 MPG if I stick to the posted speed limits. Seems pretty practical...

 

[granfire]

I don't know. In a movie, I'm not sure I'd believe it. Some things that really happen are just too stupid for a movie.

I think Owen Wilson and Vince Vaughn have pulled stuff like that in movies though.
(yeah, slightly on the unbelievable side though)

 

[AngryHobbit]

@gpseymour discovered the new level of tired. When you see a piece of fruit on the counter and think, "What's wrong with this apple?" only to take a couple of sips of coffee and realize it's a tomato.

 

[Gerry Seymour]

Close, but no cigar

ABS doesn't release the brakes - at least not in the same sense as taking your foot off the pedal... It will never leave you with no breaks, unless it's broke.

It pulses the line pressure at a fairly high frequency, allowing the wheel that is skidding to rotate again, catch up and contribute to slowing you down more effectively.

That's why, when it activates, you feel your foot vibrating (pressure pulse propagation).

It knows (should know) when to do this by monitoring the rotation of each wheel (just fronts in some systems) using a set of sensors and comparing. Better systems can restrict the pulse activation to a single wheel too.

Think about the different number of revolutions the wheels complete when you're on full lock turning (the different circles the wheel trajectories describe) - the system has to ignore that much difference at least or it'd activate every time you park. Some more modern/complex systems have steering wheel position and speed sensors to adjust sensitivity for this.

This difference can be 25% or more, so to mess with the system your toy spare tyre would need to be less than 3/4 the diameter of your standards.

Exactly the same sensor and monitoring system is used for traction control - to apply the brakes in conjunction with cutting the throttle by some degree to reduce/eliminate wheelspin.


So, if the brakes just didn't brake, something is broken - not just a bit confused.

Thanks for the primer. I knew a tiny amount about ABS, but significantly less than that. From a fuller description the Hobbit gave me today, I think it was an actual fault in the brake system (ABS or some other component), perhaps (or perhaps not) exacerbated by the tiny spare. I'm going to measure it out of curiosity, but my quick estimate is that it is, in fact, less than 75% the diameter of the standard ti(y)re. It's startlingly small, to my eyes.

(Just a little caveat - I don't actually know if US ABS systems are built following a federal ruling that NO brakes are a less litigious solution than better brakes - somewhat like that time a certain US car manufacturer decided it was cheaper to settle the lawsuits caused by deaths rather than issue a recall and fix the problem...)

Are you implying that there might be things created in the US of A that are less than stellar because of foolishness? There's no foolishness in the US of A, mister!

 

[dvcochran]

The difference in the tire sizes is pretty dramatic. And my thought is that if ABS activates when there's no skid, well, what ABS does is release the brakes to end the skid. So, if it's entirely confused, it seems it could (not sure if it's possible in the programming, but seems possible) release the brakes at an inopportune moment just as you need to brake.

I really can't think of what else would have cause that momentary failure, when nothing shows up on a visual inspection and the brakes operate fine afterwards.

The best explanation I have ever heard of is that ABS operates when it senses a skid (ground speed & wheels turning slower than ground speed) and/or sudden and hard application of the brakes. I get the latter in my F-250 from a hard stop on pavement sometimes. Scares the crap out of me.

 

[Xue Sheng]

Why not?

Don't want to.

 

[Gerry Seymour]

Don't want to.

What's that got to do with anything?

 

[AngryHobbit]

Close, but no cigar

ABS doesn't release the brakes - at least not in the same sense as taking your foot off the pedal... It will never leave you with no breaks, unless it's broke.

It pulses the line pressure at a fairly high frequency, allowing the wheel that is skidding to rotate again, catch up and contribute to slowing you down more effectively.

That's why, when it activates, you feel your foot vibrating (pressure pulse propagation).

It knows (should know) when to do this by monitoring the rotation of each wheel (just fronts in some systems) using a set of sensors and comparing. Better systems can restrict the pulse activation to a single wheel too.

Think about the different number of revolutions the wheels complete when you're on full lock turning (the different circles the wheel trajectories describe) - the system has to ignore that much difference at least or it'd activate every time you park. Some more modern/complex systems have steering wheel position and speed sensors to adjust sensitivity for this.

This difference can be 25% or more, so to mess with the system your toy spare tyre would need to be less than 3/4 the diameter of your standards.

Exactly the same sensor and monitoring system is used for traction control - to apply the brakes in conjunction with cutting the throttle by some degree to reduce/eliminate wheelspin.


So, if the brakes just didn't brake, something is broken - not just a bit confused.



(Just a little caveat - I don't actually know if US ABS systems are built following a federal ruling that NO brakes are a less litigious solution than better brakes - somewhat like that time a certain US car manufacturer decided it was cheaper to settle the lawsuits caused by deaths rather than issue a recall and fix the problem...)

Ok, you sound like you really know what we are talking about. Here is what went on.

We have one of those super-steep mountain driveways you could launch space-shuttles off of. There is a bump in the driveway closer to the top - there to help divert some of the water when we have torrential downpours (every day in the summer). Normally, going down, I stay very gently on the brake because of the steep slope, but not jamming it. Just easing it down.

Here is what happened this time. The donut spare was rear passenger side. I backed out of my parking spot, like I normally do - no problem stopping there. I am still on the flat parking area at the top of the driveway. I turn her to point nose down the driveway. I let off the brakes slowly - she starts to roll before my foot is fully up. At that point I am already feeling something odd. She starts to roll and accelerate. I tap the brakes - nothing. I push all the way down (she had hit that little ridge and is now accelerating fast, is halfway down the driveway, and there is another car coming down the road) - nothing. Now, I know what the pedal feels like when it's a cut line, a leak, a bubble - it's not like that. The pedal feels COMPLETELY NORMAL. In my brain, I know the brake SHOULD be engaged, but it's not. Screw it, I have to handle this thing.

We are flying down the driveway and I am trying to figure out how to crank her to keep from hitting that other car and to keep from flying across the two lanes into the creek (there is not much of a shoulder there). I come barreling to the bottom of the driveway at Mach 2 and crank her as hard as I can to the right. And that's when I clip the mailbox and slice off my passenger side mirror. But she slows down - and I can finally spare a hand to grab the emergency brake (I couldn't before because, at that speed, going downhill, I had trouble controlling the car and needed both hands on the wheel to make that turn). I am not on top of the other car, I am not in the creek - big breaths. I let off the emergency brake very gently and let her roll. I tap the brake pedal and it works. I am out of English and Russian cusswords by that point, so I just put the blinkers on and limp to the mechanic at minimum speed.

 

[granfire]

 

[Gerry Seymour]

Ok, you sound like you really know what we are talking about. Here is what went on.

We have one of those super-steep mountain driveways you could launch space-shuttles off of. There is a bump in the driveway closer to the top - there to help divert some of the water when we have torrential downpours (every day in the summer). Normally, going down, I stay very gently on the brake because of the steep slope, but not jamming it. Just easing it down.

Here is what happened this time. The donut spare was rear passenger side. I backed out of my parking spot, like I normally do - no problem stopping there. I am still on the flat parking area at the top of the driveway. I turn her to point nose down the driveway. I let off the brakes slowly - she starts to roll before my foot is fully up. At that point I am already feeling something odd. She starts to roll and accelerate. I tap the brakes - nothing. I push all the way down (she had hit that little ridge and is now accelerating fast, is halfway down the driveway, and there is another car coming down the road) - nothing. Now, I know what the pedal feels like when it's a cut line, a leak, a bubble - it's not like that. The pedal feels COMPLETELY NORMAL. In my brain, I know the brake SHOULD be engaged, but it's not. Screw it, I have to handle this thing.

We are flying down the driveway and I am trying to figure out how to crank her to keep from hitting that other car and to keep from flying across the two lanes into the creek (there is not much of a shoulder there). I come barreling to the bottom of the driveway at Mach 2 and crank her as hard as I can to the right. And that's when I clip the mailbox and slice off my passenger side mirror. But she slows down - and I can finally spare a hand to grab the emergency brake (I couldn't before because, at that speed, going downhill, I had trouble controlling the car and needed both hands on the wheel to make that turn). I am not on top of the other car, I am not in the creek - big breaths. I let off the emergency brake very gently and let her roll. I tap the brake pedal and it works. I am out of English and Russian cusswords by that point, so I just put the blinkers on and limp to the mechanic at minimum speed.

For those trying to picture it, this is the view from the top of the driveway.

To get some scale, here are two points of reference:

1. That first railing you see on the left is about 8 feet off the ground and about 20 feet along the driveway from me. It is below my feet.
2. The house is 2 stories at the base, which is approximately halfway down the drive. At the top of the drive, the waves are at knee level.

In other words, “steep”. And that tiny strip of green on the other side of the road is all that separates the roadway from a creek about 3’ below.

 

[AngryHobbit]

Another pickling marathon today. On the menu:

- Dill cucumbers
- Dill yellow squash
- Zucchini relish
- Onion jam
- Pickled veggie mix

Onward and upward!