Safe Speed Forums

Rather than further clutter the 'imminent accident' thread, thought I'd start another.

Did a car skid pan course (@Thruxton) many years ago. Great fun, but interestingly: taught to declutch during e-stops.

Obviously, this is contra to accepted 'DSA Test' standard.

In 1992-4 I took a lot of bike training with the MSF, a US organisation, and was taught to declutch during e-stops.

Obv . . . ditto.

Then I was pointed at this:
http://www.fmq.qc.ca/pdf/amorce-freinage_eng.pdf

Which clearly found a shorter stopping distance could be obtained by de-clutching.

Anyone know of an equivalent car research?


PS - Thruxton instructor suggested that one of the great benefits was that if the wheels locked you weren't stopping the engine, and bump-starting it again when releasing the brakes. Similarly, it made cadence braking a lot easier too!

PPS - MSF said that a rear wheel in straight line e-stops should not deliberately be locked, but if it did it should be left locked, as releasing the locked wheel could lead to a high-side if the wheel was 'out of line' when it regained traction.


Any thoughts? I realise that this is mostly 'car' oriented here, but I'm pleased to see some knowledgeable bikers coming out of the woodwork!

There was a fascinating topic on Pistonheads the other week about whether to engine brake or not, which is fundamentally the same question I suppose, and it proved to be quite a divisive issue.

In general the debate centred around whether to engine brake when driving RWD cars, with one camp being of the opinion that doing so would lead ultimately to unexpected rear wheel lockups, clutch wear, destroyed gearboxes, famine, pestilence etc; and the other believing it was a valid technique.

I have to admit I've engine braked as long as I've been driving, and I've never had a problem with any of the above issues. But when I waded into the aforementioned thread in my usual inimitable "angels fear to tread" style I actually had to sit down and consider the physics of it all a bit more than I perhaps had. It's always been something I've done because it "felt right", but when I actually tried to justify why I stumbled upon a couple of interesting points:

The first is that car manufacturers tend to err on the side of caution with the front/rear brake bias on cars, by shoving the bias slightly further forwards than would theoretically be optimal. In the general case they want the front to lock before the back, as most drivers are less likely to exit stage left into the greenery when a front wheel locks up.

This being the case (and I think we can assume it is), then under most heavy braking conditions when the front wheels are at the point of locking then there will still be some degree of "unexploited" grip available at the rear. Controlled engine braking can therefore "top up" the rear braking effort to bring it closer to the actual limit of grip.

So intelligent use of engine braking can give the driver the ability to effectively make minor rearward adjustments to the brake bias.

The second point is that the flywheel effect of an engine means that with the throttle closed it will only slow down at a certain rate. Depending on the gearing and engine revs at the point braking is initiated, it may well be possible to brake as fast or faster than the engine would slow down of it's own accord, thus the engine wouldn't actually be braking at all.

When we combine this effect with the first one we can see that it is to a degree self regulating: when we brake lightly the engine braking effect is relatively large, which shifts the brake bias to the rear. Given that under light braking there is very little forward weight transfer, this is no bad thing.

If we then brake more heavily, the rate of decelleration of the drive train will get a lot closer to the rate of decelleration of the engine, so there will be a much diminished engine braking effect. This also fits well with the fact that forward weight transfer will be large under heavy braking, so therefore we want the brake bias to move forwards too. Most car's braking systems will do this anyway, but it is interesting that engine braking reinforces this effect.

But of course all of this is completely irrelevant to motorcycles, where the brake bias is under the complete control of the rider at all times.

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We did skid-pan training in tankers and were told to de-clutch. I think the reason applies equally to modern cars:- Engine stalls, no power steering

When i did skid pan training another reason given for de-clutching was to keep engine running so you had more chance of getting out of way of any vehicles that are heading towards you.

I believe in 'two foot panic' absolutely. There are some notes on:

http://www.safespeed.org.uk/braking.html

Horse, I spent quite a few days on the Thruxton Skid pan in the 1980s. Did you meet an instructor called Richard Marriot, with an excellent 'Chris Tarrent manner'? I wonder what happened to him? He might still be there for all I know...

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Don't actually remember his name - although he was entertaining.

During one part of the course we were doing cadence brake/swerve stuff, and he was trying to break the 'cone fixation' some trainees had. He yelled at the girl I was paired with "Drive towards ME!"

So she did.

And when he jumped out of the way she steered and went after him . . . Just missed his leg.

Several years later I was talking to one of our bike 'L' trainees, and mentioned the course, and a particularly terrified lady on it . . .

. . . "That's my wife." he said.

Interesting thread, and i must admit, i don't know the answer...

On Met Advanced courses we are still taught to clutch at the very end of braking, in all situations....

In regards to engine braking.... a story......When on the course we were driving 3 litre Omegas on the skid pan. On cadence braking, a cone was placed on the 'pan' and we were told to imagine it was an old lady.

Anyway, i come steaming up the ramp and onto the pan. Cadence braking, steering like a man possessed, past the cone, all pleased with myself on having avoided the cone. Instructor says, "open your door and look back at the cone". I open the door.......no cone to be seen.......it was stuck solid under the rear wheels......doh!!!

One advantage of not putting the clutch in is the fact that the engine can stop the wheels from locking up. I have found this to be the case where you are braking on snow and ice. You do need to know when to put the clutch in though to stop stalling the engine when you eventualy stop.

Apart from that my instict is to put the clutch in in an emergency stop.

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Speed camera policy Kills

hi, newish here, just meandering round the forums...

couple of points of interest for you:
- manufacturers abs tests are often carried out with the vehicle in neutral for repeatability (no engine braking, no clutch drag)
- some ABS/ESP systems aim to prevent wheel lock caused by engine breaking (on ice) by intervening on your behalf with the throttle (how kind).

for my rospa stuff we kept the clutch in as long as possible, only dropping it to avoid stalling the engine. even emergency braking toward the nice lady who turned across a red light in the wet the other day my right foot stood on the brake and lef abs pick up the peices, my left foot was still on autopilot ready to catch the engine.

in this country at least having engine braking lock the wheels on ice, or spin the wheels up on snow is a fairly academic discussion... aquaplaning from engine braking seems a more likely possibility!

Great to see intelligent comment from another new user!



I'm a big fan of choosing to hit surface water on a trailing throttle - A little forwards weight transfer sure seems to help to prevent aquaplaning and the rear wheels simply run in the dry tracks left by the front.

My only real aquaplaning experience was while overtaking a coach on the M4. I've given the company Cavalier (FWD) a fair squirt to get past the coach and the spray. The spray has prevented me from observing the surface water. For about 2 seconds (maybe less) I was a passenger with the front wheel(s?) spinning and the revs climbing. When I shut the throttle it soon came back to me. I theorise that the power tended to drag a film of water under the front wheels and the forwards weight transfer of shutting the throttle helped dig through it. With FWD the engine braking effect is tending to help push the water forwards and away rather than dragging the water film under the wheels. (Ever tried to lift the front of a RWD car onto ramps?)

Anyway, less speed makes aquaplaning less likely and a couple of mph may make the difference. I always lift of before I hit known surface water and so far it's worked every single time.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

Many years ago I had a job as a delivery driver. One of our vehicles was a new (at the time) Mitsubishi L300 diesel, RWD with a 2.5L engine.
Dropping to 2nd gear at over 40mph could lock the rear wheels in the dry....

....bad Homer....bad bad Homer....

Not sure if today's turbo diesels have quite that level of engine braking and considering the mainshaft in the van's gearbox sheared at less than 18 months old I'm not inclined to try.

I've been working my way to the end of this thread, wondering when somebody would mention diesels!
My last four vehicles have been diesel, each one different in several respects.
In the context under discussion, the Maestro 2 litre was impressive - it was basically an Austin Princess petrol engine, converted by Perkins, and used in Sherpa vans. It was very weighty, and all the weight over the front wheels made for excellent traction under all conditions. The braking effect with the diesel meant simply changing down and lifting off the throttle took lots of speed off quickly, so much so, that it was wise to use the brakes, simply to bring on the lights to alert the vehicle behind.
The present mount, a 2.0 Peugeot 406 has ABS, with a horrible habit of appearing when it's not welcome - like going down a hill with weight on board, and loads more grip left in the tyres.
However, Pauls trailing throttle works well in this car, and standing water [a common occurance in Cumbria] has never been a problem, and as yet, I have yet to have it aquaplane. Understeer is a frequent difficulty when you first drive it, but practice soon enables one to guage it and control the power to correct it.

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Time to take responsibility for our actions.. and don't be afraid of speaking out!

just out of interest... how so ?

I presume he means that the ABS is intrusively taking the brakes off when his experience tells him that the tyres still have plenty of grip left.

A lot of people think that all ABS systems are the same, but they absolutely aren't. Particularly the early systems were quite crude, not independently monitoring each wheel (eg if one wheel on an axle locks then both wheels have their braking reduced) and intruding at very low slip values.

The classic situation is braking to a halt at a junction on a "tar and chippings" country lane, where there is often an accumulation of loose chippings where everyone has turned. As you brake to a halt your inside wheel runs over the loose "marbles", which in a non-ABS car would cause no problem at all - the affected wheel would partially lock as it brushed the chippings out of the way down to the surface beneath.

But with a crude ABS system the effect can be quite dramatic. All hell brakes loose as it detects a wheel locking, and it unloads all the braking effort from both front wheels. The sensation is like a giant hand pushing the car on from behind, just at the point when you had more or less stopped!

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i think you'd struggle to find a new car (or even up to 5years old) that doesnt have 4-channel abs, although even then legislation prevents fully independant control of the 2 rear wheels (i think). so i'd argue most current systems are comparable, the manufacturers are after all in direct competition for business.

in most case modern abs will acheive >105% of the stopping distance of a trained driver holding the tyres on the limit (but then you've compromised your steering which abs is designed not to do).

my most regular 'false' intervention is braking up to a roundabout over a recessed personhole cover, although the only telltale of this is the pedal feedback.

my personal opinion is that the system is worth the occasional 'false' intervention since even i cant claim optimum braking on all surfaces especially when faced with an emergency manoeuvre. quite happy to plant my right foot and wait.

Spot on JT.
I hope Horse will back me on this, but riding a motorcycle where you have full control over braking effort, gives you a keen sense of when your braking effort is balanced, and when you need to ease off on front or back.
When you go downhill, too much effort at the front lifts the rear wheel - which you overcome by slacking off the front until both tyres are making FIRM contact with the road.

Similarly, in my car - a W reg 406 estate, when empty, going down a hill, the weight is transfered to the front more so than when on the level, but when carrying a load, and the rear remains firmly planted, the ABS insists on chipping in when there is a lot more adhesion in the tyres than the manufacturers allow for.
Of course, they have to allow for drivers running with the wrong pressure, or worn tyres, but my argument is, there should be a switch for competant drivers!
I believe some new cars have tyre wear sensors which presumably adjust some of the settings.

My Renault Espace, and L reg had no such vices, but of course was set up differently because it was a people carrier - not as long as the 406 estate, (yet loads more room!) and front wheels nearer the corners of the vehicle.

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Time to take responsibility for our actions.. and don't be afraid of speaking out!

again just to clarify.. what do you mean by chipping in?


the grip available at the tyre is proportional to tyres load which will vary with vehicle load, gradient, weight transfer due to braking, CofG height etc.
the peak of force vs slip changes in magnitude with tyre load, but the location (optimum slip) does not vary greatly.

abs monitors wheel speed and hence wheel slip and aims to control the slip within this optimum region.

hence in my understanding it is not possible for a hill, or different loading to adversely effect its operation.
for abs to intervene it will have seen a certain level of slip on one of the tyres, which suggests independant of loading (pressure, wear) that the tyres force is approaching its peak value.

Hmm. The wheel speed, below which the ABS starts is presumably programmed in.
If I had new high quality tyres, or cheap Korean worn tyres, the level of grip the tyres are capable of will vary. The point at which the ABS comes on does not.
The ABS stops me applying a level of braking capable of stopping my vehicle from the speed at which I am travelling.
This is more apparent downhill than on a level road.
On the hill on which I live, going uphill, it is sufficient to simply lift off the accelerator under anything other than emergency braking!
I assume the ABS is linked to wheel speed AND braking effort, otherwise it would come on just as the vehicle slowed to a halt from any brake/stop operation, and not just when the brakes are applied more firmly?

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Time to take responsibility for our actions.. and don't be afraid of speaking out!

ABS will inherently compensate for the quality of the tyres, and "braking effort" isn't really a direct issue.

The way it works is to monitor the speed of the wheels under braking. If any one wheel revolves more slowly than the others then it deduces that it is beginning to lock. Typically it will allow something like 15% slip, ie it begins to intervene when the locking wheel slows to 85% of the speed of the others. At this point it temporarily releases the brake on the affected wheel, allowing it to speed up slightly, before resetting. It can repeat this process several times a second, which is what leads to the "pulsing" feeling you get through the pedal, as the wheel actuator effectively pushes the hydraulic fluid back up the pipe against the pedal, to release the brake.

Thus it measures actual wheel rotation, so operates irrespective of tyre quality or level of braking effort - or indeed the incline of the road.

But this also illustrates how different systems can be of varying efficiency, in terms of how intrusive and/or effective they are. The allowable slip percentage may vary, typically a system on a sports can will allow a higher degree of slip before it intervenes - I was reading recently that the system fitted to the Lotus Elise intervened at such a late stage that some professional test drivers were unable to tell it was active until it was later proven to have intervened via a telemetry readout. The number of channels may also vary, along with the quality of the electonics, the reaction time of the system (which determines how many times it can pulse the brakes off in a second etc) and the degree to which the actuators actually release the brakes.

All these variables (and no doubt others) contribute to the overall quality and performance of the system, so it is wrong to assume that all cars fitted with ABS will all work to the same degree, in the same way it would be wrong to assume that the 2 litre engine in a cheap car will have the same power output and refinement as the 2 litre engine in an expensive high quality car.

_________________
CSCP Latin for beginners...
Ticketo ergo sum : I scam therefore I am!

Got ABS and EBD on my car (electronic brake force distribution).

When it comes to acronyms I'm your man.

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Speed camera policy Kills