The Physics of towed vehicles (1 Viewer)

[olley]

Hi Geo thanks for doing that, have to admit they work better than I thought they would. :Blush:

Olley

 

[scotjimland]

Thanks for taking the time to do the tests Geo,
that is reassuring, much better than I had previously thought..

 

[Braunston]

Thanks Geo

That is really helpful as I like a lot of people I'm sure had doubts if they would work at all, and to be honest i thought the cable idea on a braked "A" frame was little more than a visual feel good factor, if that makes sense. as from your tests it would seem that if adjusted correctly the over run device linked through a cable could well work or certainly provide some level of braking.

Thanks again

Braunston​

Not exactly laboratory conditions, but..
over all braking % figures without servo were very slightly less than with the servo, it is my considered opinion that in an emergency panic situation the

un​

servo,d brakes would stop the vehicle,in the event of a servo failure, even front wheel lock out was obtained without to much pressure being applied, pressure

reqd​

was approx 2 to 3 times greater and IMHO easily obtainable by the brake overrun system
vehicles with rear disc's as opposed to drums did not perform quite so well on the

un​

servo'd​

rears and were borderline but passable
Geo
Paul, if the Yanks use Recovery Dolly's it must be

ok​

:thumb:

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[Geo]

I will be the first to admit I was suprised at the results too, after years of towing in broken down cars on a rope, the brakes certainly didnt feel like they worked too well but you cant argue with test weights and rollers, when you think about it there has to be a back up of some sort, if all servos rely on is a rubber pipe not splitting
Geo

 

[Road Runner]

I have a design fault on my Chris Cox frame IMHO.

The brake cable is the furthest bit out from the front of the car so if someone bumper to bumper parks to you the cable can get damaged.

This can cause the brake to bind only slightly but there and I have been through a set of brake pads this way and melted a set of wheel trims.

It's only when driving the car i noticed a slight drag and a foot under the brakes releases it but next time you brake and it binds again, when towing blissfully unaware of it.

This is a real bad design as far as I am concerned.

Overall a good bit of kit but this is a weak spot in my book.

 

[pappajohn]

same on the Caratow frame john.

without major surgery you cant do nowt about it.:Sad:

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[PeteH]

"overun" Brakes

Hi

Interesting thread, Informative and useful from a point of view of amunition to confront the "A" frame sceptics, so, many thanks to Geo. :thumb: I still think the "elfs" and the "euro-crats" will get them banned though one day soon. `twill be a sad day. :Sad: Well constructed dolly`s are probably the best solution as the Yanks have proven. (and they can be reversed, not the case with "A" frames!!). BUT they have been proved to be illegal in the U.K. EXCEPT for GENUINE recovery work. (your "toad" does NOT constitute a broken down vehicle, even if you remove the "distributer cap"!!) More`s the pity!!. :Angry:

My Towtal rig could suffer the same cable damage problem as RR pointed out, but checking the cable before hitching up is part of the "pre-flight" checks. and a periodic check of the brake system can be achieved on hard level ground by taking off the car handbrake and pushing against the hitch head, if it doesent move more than a few inches then you have brakes and when you pull forward the car should move. It is also advised in the towtal literature NOT to lubricate the cable, as "road muck" sticks to the exposed bit and works it`s way down the cable causing frictional "jambing". Mind, we all make mistakes, I left the (car) handbrake on and "cooked" the rear shoes!!!. :Blush: Cost a new set of shoes in France oh:

pete

 

[Tony Lee]

Back to the original article that talks about the towed vehicle imposing nearly three times its weight on the towing vehicle when under brakes - I can't see how this could be unless the towing vehicle is braking at nearly 3g and the towed vehicle had no brakes. Even 1g would require perfect friction between the tyres and the road so 3g ...??

Any comments?

 

[Geo]

Im not into maths but I belive the same formula is used to explain why an unbelted rear seat passenger can crush the driver in an impact, I realise that in an impact the car stops almost dead but the maths are the same ie
x weight
traveling at Z speed
=3 times its mass (weight)

I dont think we need G force included to realise, that if you stood/jumped in front of a 1/2 ton car doing 1 mile an hour maybe you could stop it.
try it at 70 mph and see what happensoh:
I think thats where the extra weight comes in
Geo

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[Tony Lee]

That scenario is a little different because you have something flying freely through the air that then hits something that has been stopped (by being subjected to very high g forces itself).

In the case of two vehicles hooked solidly together, they both must decelerate at the same rate and the maximum deceleration is then something less than 1g (unless the tyres have long spikes on them of course). If the vehicle was being towed by a long rope and the tow vehicle jammed on the brakes, then the towed vehicle would slam into the almost stopped tow vehicle and give it a huge push forward (similar to an unrestrained passenger smashing into the driver - but again, that is not what the article was talking about.

On another point that he raised. If the total weight of tow plus towed vehicle is less than what the tow vehicle would normally weigh, then on the face of it there is no real reason for needing brakes on the towed vehicle from a statutory minimum braking efficiency point of view. The real reason is to prevent the load imposed by the towed vehicle from messing up the weight distribution (and hence total braking efficiency of the tow vehicle) AND more importantly, to prevent jack-knifing - which of course dramatically stuffs up braking efficiency and directional control.

I'm not suggesting that more brakes is not better than fewer, but I'll admit to forgetting far too much of my Physics and Engineering basics, so are there any more comments about the accuracy of the original article. Perhaps there is some dynamic effect I'm overlooking.

s=ut+½at², v²=u²+2as, Momentum=mv, E=½mv² etc etc all still work the same as they did 50 years ago I think.

 

[pappajohn]

In the case of two vehicles hooked solidly together, .

thats the point tony......they arent linked solidly (unless its unbraked)

the hitch has a sliding head albeit only a few inches of movement.

if they are both travelling at 50kph and the tow car stops the towed (toad) car will still be doing 50kph until the hitch has used up all the free travel in the sliding hitch head then the impact on the tow car will be greater than the mass of the toad which in my book is g force.

just the same as your body hitting the seatbelt in a frontal accident.
push yourself into the seatbelt...it dont hurt.
now try hitting a solid object at 50kph and see if the seatbelt hurts.

 

[coin99]

u could try the link below
Link Removed

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[Castaway]

thats the point tony......they arent linked solidly (unless its unbraked)

the hitch has a sliding head albeit only a few inches of movement.

if they are both travelling at 50kph and the tow car stops the towed (toad) car will still be doing 50kph until the hitch has used up all the free travel in the sliding hitch head then the impact on the tow car will be greater than the mass of the toad which in my book is g force.

just the same as your body hitting the seatbelt in a frontal accident.
push yourself into the seatbelt...it dont hurt.
now try hitting a solid object at 50kph and see if the seatbelt hurts.

Then there is the question of the difference between a (perfectly legal) 750KG unbraked trailer versus a 1000KG (for example) braked trailer. Which is going to be better to have on the hitch in the event of an emergency stop? Even at 50% braking efficiency?
I know which one I'd rather have!