Safe Speed Forums

You keep a car THAT long. My God!


Intercoolers only work when the vehicle is moving! (Ramair)

I've known manuals that have dumped gearboxes at 70k with abuse so 100K's not bad at all.

I'm still not convinsed.

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Science won over religion when they started installing lightning rods on churches.

Unless I know that I am going to be stopped for a long time, I nearly always leave the transmission in D at traffic lights.

However, just out of sheer habit really (as I'm more used to driving HGV's these days), I normally slow to a crawl, about a truck length before the point I need to stop. So very often, the car never comes to a complete halt, before the traffic is moving again.

Not sure if the above is responding to my post but, if so, that's not what I said. I said I'd select P or N if stoppedfor over ~30 seconds AND there are several vehicles behind (i.e. no or virtually no shunting risk). On reflection, I think I'd rarely select P because have to go through N and R to get there. I mentioned it as a possibility to counter the objection to moving through R and flashing reversing lights.

It seems to me, if there is risk of being shunted, it's better to be in gear and ready to go for an escape if needed. In an auto that means being in D and holding on the footbrake. When that risk has passed, because of stationary traffic behind, there seems to be little difference between holding in D and applying parking brake and selecting N, apart from the trade-off between gearbox wear and fuel economy.

I wasn't aware thare was an argument against using P without the parking brake. I have to admit I sometimes don't apply the parking brake if I'm parked on level ground.

Picking up on an earlier point, if I am stationary in a queue of traffic and shunted from behind, with other vehicles in front, in which of the following scenarios will there be less risk of injury (NOT material damage):

(i) my vehicle is in neutral with hand/parking brake applied; or

(ii) my vehicle is in neutral without hand/parking brake applied?

Scenario (i) ensures that more of the impact is absorbed by the vehicles, such that a smaller acceleration will be caused and therefore less risk of injury. Furthermore, as your own vehicle accelerates less there is a reduced risk and/or severity of further secondary collision with any vehicles in front.

In fact, to take it to it's logical conclusion, if you become aware that a collision is inevitable with a vehicle approaching rapidly from behind, the right thing to do from the point of view of your own survival is to press the foot brake down hard to further increase this effect.

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

Hmm. Suppose the road ahead was empty, which would be better then?

It stays the same - at least from your own point of view. Imagine an extreme situation where your car was (say) bolted to the floor, such that the car hitting you from behind didn't move it an inch, but instead expended all the energy in compressing the crumple zones of the cars. You wouldn't feel a thing.

In the opposite extreme, where both cars don't crumple at all, but the front one is free to roll, then the collision will cause your car to accelerate violently to approximately half the speed of the other vehicle. You'd certainly feel that!

Of course there are a few other factors:

1. Your car will get damaged more if you resist the impact using your brakes (the energy has to go somewhere). In an extreme case this could cause deformation of the "safety cell" you are sat in, which might be worse than enduring the acceleration of a "free rolling" impact. That said, in a collision of that sort of severity the brakes wouldn't resist the impact anyway.

2. It ain't quite so good for the guy behind. If you resist accelerating then he has to decelerate more. In other words if it's less violent for you it is more violent for him, as you are effectively making your car into a harder object to hit.

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

Which IMO is best practise ANYWAY. You are already moving and have room in front if a rear end shunt is imminent, you're not applying the footbrake so there are no lights ti blind following traffic, you can move off faster when the lights DO change and it uses less fuel.

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Science won over religion when they started installing lightning rods on churches.

Aha - so it's not so simple then. If the energy to be dissipated is 'shared' between the [four] crumple zones of three vehicles (the one which shunts me, mine, and the one ahead of me), which will be the case if acceleration is not constrained by hand/parking brake, [instead of two crumple zones (front of of the car that hits me and my rear one)], then there is less risk of injury to me (and the person behind). Not so happy for the occupants of the car ahead of me, of course, who may have been unaffected if my brakes had been applied.

So where's the cross-over point?

Text in square brackts added

How about if you're at the back of the queue, leave emergency room ahead of you. Once you're in the 'body' of the queue its not so important, although technically you should leave yourself enough room to get out if the vehicle in front breaks down.

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Science won over religion when they started installing lightning rods on churches.

I don't think there's any doubt about your first point. The issue I'm exploring with JT is the 'received wisdom' that one should apply the handbrake when stopped in a queue of stationary traffic.

'Received wisdom' that I agree with, if for no other reason than not blinding the driver behind with your brake lights. I don't feel too comfortable with the idea of having no brakes applied whilst stationary for the reasons JT's already discussed.

I see your point about where the shunts stops - I suppose it has to be a compromise between every driver's responsibility to avoid causing / permitting to be caused damage to another vehicle and preventing one's own injury. TO be quite honest, if by having brakes firmly applied causes more serious injury to the person behind who caused the shunt at the benifit of the party who was hit then tough!

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Science won over religion when they started installing lightning rods on churches.

In general I'd say there isn't one - at least not from the "selfish" perspective of the "shuntee".

Let's say the car behind is doing 20mph. It will hit you with a fair old bang, and with your brakes off I'd expect you'll get jolted forwards quite violently. With your brakes on this will clearly be diminished, more of the energy will be dissipated by deformation of the vehicles, and risk of a secondary collision with another vehicle in front will be reduced.

So what if the car behind were travelling a lot faster? Clearly there reaches a point where the crumple zones alone cannot dissipate all of the energy, and it will start to deform the passenger cell(s) too. But in this situation you are talking about a pretty huge amount of momentum, which will more than overcome the retardation of the brakes of the vehicle in any case. In short I wouldn't expect the brakes to turn a non-passenger-cell-deforming accident into a deforming one.

To look at it another way, we know that the limit of traction of your average car's tyres is somewhere in the region of 1g, therefore using the brakes has the capability of reducing the impact felt by about that amount. For example, if the car behind shunting you would have caused your vehicle to momentarily accelerate at 3g then clamping the brake pedal to the floor would reduce that to 2g, and so on.

As regards the vehicle in front, I don't think shunting forwards into them is ever a good thing. As an example, imagine a car behind hitting an equal mass stationary one at 40mph. With no brakes applied, and no dissipation of energy both vehicles would now be travelling at 20mph. If there were a further vehicle in front then the middle vehicle would now have a secondary 20mph impact with them, but still with the full initial momentum, as there would now be the mass of both moving vehicles involved. So in terms of acceleration, the middle car has now received an initial strike absorbing half the initial momentum, then a secondary one involving the full initial momentum. If we take the alternate case of keeping the middle vehicle stationary then we "just" have one single full momentum collision.

I know this makes an unrealistic assumption - ie that no energy is dissipated by the collision - but I think it illustrates the point, that the secondary impact would be a very severe one, and indeed would be taking place with a car that is [presumably] already damaged from the first one.

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

That surely can't be right (although I think you acknowledged that later?).

If there are only two vehicles (and assuming no risk of secondary collision) surely the outcome for both shunter and shuntee will be better if the shuntee's resistance to acceleration is less rather than more (i.e. no handbrake) because the shuntee represents a softer target.

Let's take a scenarion with thee vehicles - shunter, shuntee 1 and shuntee 2. Let's say shuntee 2 is in contact with an immovable object and shuntee 1 is x metres further back. Shunter hits rear of shuntee 1 with force y.

Now take two scenarios (let me see if I've got this right):

(i) shuntee 1 has handbrake applied and force of collision pushes shuntee 1 up to but just short of shuntee 2, so no secondary collision. The total energy is dissipated by deformation of shunter's front crumple zone, shuntee's rear crumple zone plus the acceleration of shuntee 1 towards shuntee 2.

(ii) shuntee 1 has no handbrake applied so force of collision pushes shuntee 1 into shuntee 2. The same total energy is dissipated by deformation of five instead of only two crumple zones plus the acceleration of shuntee 2 into shuntee 1.

I suppose that in scenario (ii), more energy will be transferred into accelerating shuntee 1 because there is less resistance. However, the greater energy transferred is then dissipated by deformation of the crumple zones at the front of shuntee 1 and the front and rear of shuntee 2. Whereas, in scenario 1, less energy will be transferred to shuntee 1 but the structure of shuntee 1 has to absorb more because less is being dissipated in acceleration.

The 'best outcome' for shuntee 1 can't be the same for all values of x and y, surely, solely because of handbrake applied?

Mrs Beard's autobox (always left in D at lights etc) didn't need a rebuild until 190,000 miles...

what's then point in having an auto and shifting it between D N and P every time you meet a queue? Just put it in drive and use the brakes.

I used to drive one of 4 autos, Peterbilt 377's with Allison HD4560P transmissions powered by 12.7 litre 60 series Detroits at 430hp. Once stopped, no matter how hard you pressed the throttle they wouldn't move with the parking brake applied. We just pressed D and drove. Oh and Case Number 1 engine oil meets all the specs for Allison automatic transmissions, just so you know.

Actually, I'm not at all sure that the handbrake on most cars will make much difference AT ALL to the deceleration the "shuntee" experiences OR, the distance his car gets shunted. (assuming similar vehicles involved rather than a Micra running into the back of an artic!)

Here's why I'm thinking that...

Most cars have handbrakes on the back wheels. When you get hit up the backside, the back of the vehicle tends to lift. As it is, there is very little weight on the back wheels of most common European cars anyway AND this reduces to "something-less-than-that" on impact. So in the first case, the rear wheels can't manage anything LIKE 1g retardation for the whole car by themselves and what little they CAN manage is reduced at the moment of impact anyway. What I DO agree, however, is that after the impact, having the handbrake on will stop the shuntee rolling further still. In the classic "T-junction" shunt from behind, (unless it's barely a "parking knock") I think the car would easily be pushed far enough to hit traffic on the main road that he was waiting for whether the handbrake was on or off. If it was "on", though, he wouldn't hit the stuff on the other carriageway too!

My other thought is that on modern cars, the fact that one can floor the throttle and not have the car move might not be any reflection on the handbrake. There might be some tricky "throttle-by-wire" system that doesn't let the engine pull too hard against the handbrake. (carefully optimised to allow hill starts though).

Absolutely, the whole point of an automatic is not changing gear and the ability to 'creep' when in a very slow moving traffic queue.

My father used to have XJ6s when he was alive. Every time one went for an MoT test it failed on the handbrake; a) because it was a fairly useless design anyway and b) because he never used it. He would stop and put the gearbox into Park.

I have an auto, a BMW 7-series, plus two manuals, an historic rally Cooper 'S' (1964) and my endurance-rally Rover 214Si.
In my BMW I always left-foot brake, use 'P' when stopped at traffic lights, etc, and virtually never use the parking brake as it's a foot-operated one with a manual release. Incidentally, when I 'hurry' in the BMW, I use the Steptronic shift, braking with the left foot and using a bit of throttle to smooth the down-shift and then enter corners with just a bit of throttle on to balance the car nicely.
Well, it works for me!

Was looking at patdavies post - got this tip from a Vauxhall foreman ( in the days when garages had foremen, not senior fitters) and have applied it sucessfully to other cars, including an AUSTIN MAXI ( where a MOT tester could not believe the figures on a handbrake test because of tip 2).

Tip 1 -- replace the operating arm ( arm attached to the cable , which operates the shoes ---over the year the slot where the arm enters the shoe wears and shoe and arm need to meet at 90 degrees.
tip 2 - failing new arms - build up wear with spot welds - in most cases this will last at least a year. My Maxi almost shot off the test ramp -

Of course replacing the arm is a better bet , the cost little and improve handbrake efficiency fantastically.