Drivers behaviour on roads

Timing belt

https://www.boards.ie/discussion/comment/121120721#Comment_121120721

You can do that with any car once you have correct cable and software but need to be careful that you know old settings or you will f it up for good. The only real time you would need to do it is if you put adjusted wheel arches and put in bigger tyres.

GSBellew

https://www.boards.ie/discussion/comment/121120739#Comment_121120739

On the Mk4 Golf (PG34) platform some cars can have 18" wheels which are a fair bit larger overall than the 15 16 or 17 so they definitely need a separate setting, the others all have their own settings too even though the difference is much smaller.

liamog

https://www.boards.ie/discussion/comment/121120648#Comment_121120648

This has been weighing on me over the weekend :).

I think in the example you've provided the stationary car has become an immovable object, therefore the kinetic energy loss is as you described. In a real world collision between a 120km/h car and a stationary one the 2nd car will be accelerated in the direction of travel of the first car. We see this in incidents like the pile up on the m50 last week. In that case I think the overall loss of energy due to the impact will be very similar but we'll see the final resting place of the crashed vehicles in a different place.

Probably a good question for a physics forum

magicbastarder

yes, there's a further complication in that there's conservation of energy (which is squared in relation to speed) but also conservation of momentum (which is linear)

in terms of how much energy the collision 'dumps' into the cars (i.e. does not remain as kinetic energy) i assume that'd depend on how fast the two cars were travelling in the immediate instant after the collision, and i don't know what that would be. neither would come to a complete halt as they might do -as you mention - in a proper 'equal but opposite' head on collision.

of course, for the occupants, AFAIK it's mainly the peak decleration you would be concerned about.

liamog

Assuming no deformation, no friction, and equal mass (totally not real world). The combined cars would experience an acceleration of 60km/h. The moving car would decelerate 60km/h and the stationary would accelerate 60km/h, leading to the impact being the same as single car hitting a stationary immovable wall at 60km/h.

josip

However most walls aren't immovable and the real world reality of a 60kph head-on, is that the deceleration experienced will be equivalent to a much higher speed single-vehicle impact in most scenarios.

alan partridge aha

C'mon lads can we get back to ICE driver bashing.

liamog

The wall is a bit of a red herring, the comparison was questioning the effect of a dual car collision between one traveling at 120km/h and the second at 0km/h versus a head on collision between two vehicles traveling at 60km/h.

This started from a claim that a person traveling at 80km/h on a motorway was dangerous. It's my belief that the important factor is the delta between the speeds of the two car's. Therefore a car traveling at 120km/h colliding with a car traveling at 80km/h has the same impact force as two car's traveling at 20km/h colliding head on.

The kinetic energy explanation for why the 120km/h collision was worse only made sense if the 2nd car was an immovable object (i.e. fixed wall). In practice a real collision is going to be much harder to model and is never going to be perfectly aligned.

https://www.omnicalculator.com/physics/conservation-of-momentum

If we punch numbers into this calculator we see that a 1500kg car traveling at 120km/h hitting a stationary 1500kg car, the initial kinetic energy is 833kj after the collision it is now 416kj, this energy is absorbed by the heat and deformation of the cars with the net result the combined cars are now moving at 60km/h. If we set the same numbers for two cars at 60km/h, the cars have an initial kinetic energy of 416kj but due to the balance of forces the cars instantly stop losing the same energy as the initial higher speed crash.

Now it's fair to say the the 120km/h crash is likely more dangerous because we now need to slow the combined 3000kg cars that are traveling at 60km/h but the initial impact force that must be absorbed by the crash protection of the cars is effectively the same.

So back to the original conjecture, is a 120km/h car hitting an 80km/h the same impact force as head on collision between two cars traveling at 20km/h, the answer is yes. But after the collision their is still a lot more energy that needs to be discarded before the two cars come to a rest.

magicbastarder

i've sometimes wondered, in that hypothetical scenario where you're stopped in your car and see a car approaching from behind at speed, and you want to anticipate it hitting you - do you bury your foot on the brake, or take it off? i.e. will the brakes being engaged make any significant difference to the shunt you get (and would you also want to take into account possibility of injury to your leg, if it's pressed hard against the pedal?)

liamog

In terms of personal injury I think you need to reduce the acceleration on your body. In that case it makes sense to apply the brakes as much as possible.

magicbastarder

https://www.boards.ie/discussion/comment/121125507#Comment_121125507

yes, this is possibly an option, but let's presume the collision is inevitable.

i just wonder if the effect on the impact will be enough to justify bracing your foot against a pedal, possibly causing greater strain on your leg?

morritty

Hey OP.

By any chance does it happen to be a Ford Focus that tailgates and flashed you?

Also, did you previously drive a Lexus in a past life? Specifically with a Mark Levinson sound system?

If so? That explains it all