Overtaking with speed

[vonhosen]

I was approaching a much slower moving car with a clear road ahead and used my existing speed to complete the overtake safely (I believe). I didn't do the classic 'overtaking triangle' in as much as I didn't match the speed, take up a followng position, move offside without closing and then accelerate when clear.

Firstly is there a name for overtaking using your existing closing speed?

Secondly did I do right? If so, what are the particular things to note when doing this?

IIRC, R/c shows two different 'types' of overtake:
One where you follow, move out, GO!
The other where you continue on at your speed, move out earlier as you're closing (with time to abort if nec.), and pass.

Name for it? Dunno

Things to note? As mentioned elsewhere here, be aware that the driver may be unaware of you, especially if your closing speed is relatively high.

I call them "Classic" & "Momentum"

[jont]

Things to note? As mentioned elsewhere here, be aware that the driver may be unaware of you, especially if your closing speed is relatively high.

And the bike/caterham that's catching you up at even greater speed...

[jont]

Incidentally there's been more flooding today in Tenbury Wells, which is the start of 11 miles of the most demanding road I know. Hope it's clear for 28th.

So do I! it looks rather soggy at the moment:
http://news.bbc.co.uk/1/hi/in_pictures/6903501.stm

[7db]

I call them "Classic" & "Momentum"

Someone called one of the "triangulate" recently. It had me reaching for my compass and chart pencil. I can see that "triangle" makes sense.

In an argument about time exposed to danger, I ran the maths on PH some time ago about the time of commitment for the various types of overtake to show that with a momentum overtake you are committed much much earlier and for longer than you think (ie the period of danger starts a long way back from the target and before you have moved offside). The point being that you are committed as soon as you can't stop behind him if the target locks up.

If anyone's interested, I'll dig it out.

[James]

It is true 7db... and I suppose if for any reason you did have to abort (easily can happen with that amount of tim) it's all going to look a little messy too

[nuster100]

If here is a high closing speed, take a long straight for example, I would imagine you would move to the offside much sooner so that you can see the area infront of the target car.

If its not on, simpy drift up behind him.

Jay

[TripleS]

A likely story. Mr Plod probably did well to get past.

Yes OK, I've got my trousers seat padding ordered ready for 28 July.

Best wishes all,
Dave.

As it happens, I was driving my wife's Micra 1.0 so he had no problem, (anyway, do you think I wasn't using my mirrors?). I'm thinking of downsizing & wanted to see how I'd get on with a Micra or similar; the answer being, not very well, but 23 MPG is getting expensive; I may have to economise elsewhere,( e.g. food). I promise not to make you drive with me on 28th. Incidentally there's been more flooding today in Tenbury Wells, which is the start of 11 miles of the most demanding road I know. Hope it's clear for 28th.

Cheers, Chris.

Hiya Chris,

I'm sorry to hear about these downsizing type thoughts arising, but I think my inclination might be to do a little less motoring, but not reduce the quality of it - if you see what I mean.

Perhaps what you need is a nice carefully used 406, and it just so happens...... Old CUBby yielded 980 miles on the last tankful, which amounts to an average of 58.2 mpg over that distance. I might be hopeless in some ways, but I ain't a bad economy driver - and that's the one claim I will make.

Anyhow I'lll be happy to drive with you on 28th if you like, whatever the car, and I just hope the weather down there will not cause problems from a driving viewpoint, and more importantly, not be creating flooding horrors for people.

Incidentally, I think I'm scheduled to do a long route with Tony (Mr Toad) if that's still on, but we''ll see.

Best wishes all,
Dave.

[TripleS]

Things to note? As mentioned elsewhere here, be aware that the driver may be unaware of you, especially if your closing speed is relatively high.

And the bike/caterham that's catching you up at even greater speed...

I expect an MR2 Turbo could present a surprise or two, too.

Best wishes all,
Dave.

[Horse]

In an argument about time exposed to danger, I ran the maths on PH some time ago about the time of commitment for the various types of overtake to show that with a momentum overtake you are committed much much earlier and for longer than you think (ie the period of danger starts a long way back from the target and before you have moved offside). The point being that you are committed as soon as you can't stop behind him if the target locks up.

If anyone's interested, I'll dig it out.

Please!

Although there is a counter-argument that being offside sooner you might well see the danger earlier . . .

But as I said:
The other where you continue on at your speed, move out earlier as you're closing (with time to abort if nec.),

[TripleS]

If here is a high closing speed, take a long straight for example, I would imagine you would move to the offside much sooner so that you can see the area infront of the target car.

If its not on, simpy drift up behind him.

Jay

Yes, it needs using with care. The thing to particularly avoid is rushing up towards the tail of the target vehicle, with a large speed advantage and limited view, on the assumption that the overtake will be confirmed as being 'on' - and then finding it isn't.

Get out as early as possible for the best possible view, and do watch that speed differential, is what I would emphasise.

Perhaps Von, or somebody, would care to give a more comprehensive guide to using this technique properly, as IMHO it can be very useful and should not be discounted completely.

Best wishes all,
Dave.

[JamesAllport]

Not a comprehensive guide, but some thoughts:

(1) The requirement to confirm that the overtake is "on" before commiting is the same whatever the method. You still need time on the offside before commiting to check for side entrances, road surface etc etc. So move out early if safe.

(2) Think about your speed differential as you pass. Yes, you want to maintain your flow - that's the point of a "rolling" overtake - but equally you don't want to "blast past" with a huge speed differential. Sometimes, conditions allowing, it's worth easing past with a gentle differential and then making progress away once you've passed.

(3) Think about your gear choice. Even if you don't slow, it's worth thinking about whether you need a lower, punchier gear, so that if you need to complete the overtake as quickly as possible (e.g. a bike doing 90+ appears around the corner in the distance, as happened to me last week) you have the ability to "squirt" on some power.

(4) The earlier you go offside, the more you need to be sure that the overtaken driver has seen you and understands that you are going to come through. If in doubt, don't do a rolling overtake. Hold, give a warning with horn or lights and wait for evidence of a reaction before committing.

James

[TripleS]

Not a comprehensive guide, but some thoughts:

(1) The requirement to confirm that the overtake is "on" before commiting is the same whatever the method. You still need time on the offside before commiting to check for side entrances, road surface etc etc. So move out early if safe.

(2) Think about your speed differential as you pass. Yes, you want to maintain your flow - that's the point of a "rolling" overtake - but equally you don't want to "blast past" with a huge speed differential. Sometimes, conditions allowing, it's worth easing past with a gentle differential and then making progress away once you've passed.

(3) Think about your gear choice. Even if you don't slow, it's worth thinking about whether you need a lower, punchier gear, so that if you need to complete the overtake as quickly as possible (e.g. a bike doing 90+ appears around the corner in the distance, as happened to me last week) you have the ability to "squirt" on some power.

(4) The earlier you go offside, the more you need to be sure that the overtaken driver has seen you and understands that you are going to come through. If in doubt, don't do a rolling overtake. Hold, give a warning with horn or lights and wait for evidence of a reaction before committing.

James

Thank you, James, there's one or two fair points there.

Point (2) reminds me of something that was said about the Jaguar E-Type when it first appeared in 1961:

'This is one of those rare cars where the prudent driver finds himself slowing down to overtake!' - or words to that effect.

That kind of driver doesn't deserve to have our NSL inflicted upon him.*

Best wishes all,
Dave.

* This is the opinion of the writer, and it may not be shared universally.

[7db]

Please!

I can't find it, but I can reproduce it.

* * * *

The example was an overtake on a car travelling at 50mph. Everyone has 1g braking and 1s reaction times. The overtaking car can accelerate at 0.2g (that's 0-60 in about 14s) at 50mph. Cars are 3m long.

The danger zone is defined so that a car is in the danger zone when it cannot stop behind the target car when it exerts maximum braking, until the time when it is clear of the nose of the target car.

In each example we calculate time in danger.

Momentum overtake
Approach at 70mph and clean pass.

i) How far behind does danger start?
50mph car stops in (v^2=2as)
70mph car stops in (vT + v^2/2a)
So danger starts 56m behind target.

Relative to first car, danger zone extends 62m and is travelled at 20mph so
TED = 6.9s

More generally - http://tinyurl.com/2q57wz


Triangle overtake
Safe offside following position is at 50mph thinking distance = 22.3m

So distance to clear is 28.3m, covered relative to target at a starting speed of 0 and with acceleration of 0.2g. (s=1/2 at^2)

TED = 5.4s

More generally - http://tinyurl.com/yulumz


Edited to correct braking value in Momentum example and html formatting.

[jont]

I can't find it, but I can reproduce it.

<maths>

That makes for very interesting reading, even with the large assumptions about acceleration/reaction times etc. Given that, is there an optimum speed differential to use for a momentum overtake, or is minimum TED always starting from 0 initial differential speeds? (sorry, I'm feeling lazy although I probably could do the maths myself). And how does that speed vary with different accelerations available? (for instance a quicker car with maybe 0-60 in around 7s).

[7db]

Just factorise the difference of the two energies in the banana example and you'll be able to extract and cancel a factor of speed differential throughout.

You'll have the answer to your question staring you in the face then and no need to figure out the partial differentials.

You might also spot the weakness in the underlying assumptions. You might not.