Saw a 70's VW Beetle in a car park yesterday.
Something about it made my blood run cold.
Can it really be legal in Britain in 2001 for a car to have drum brakes on the *front*?
|
|
why not, trucks have them.
|
|
|
Dave,
What's wrong with drum brakes? If the car has an MoT, they will have worked well enough at the time. My first car was a '62 Beetle and the brakes worked very well. I would rather step on the pedal of well-maintained drum brakes than poorly maintained discs. Surely all those HGVs and PSVs out there can't be wrong?
|
It seems I'm wrong about this big style!
I appologise to all for my ignorance!
|
|
A friend of mine bought an ex-german Post Office Beetle in Germany in the 60s and that had cable operated drum brakes, and a two piece front windscreen. The brakes had to be very carefully adjusted or one wheel braked when you moved the steering, change of cable alignment or something. Nightmare!
|
|
|
|
|
actually, Dave, it made me think about the introduction of disc brakes on cars and those warning stickers (orange and yellow from memory but maybe not) that were displayed on the rear of the cars. I'm not sure whether the authorities were expecting trouble with drum braked cars driving into disc braked cars or whether it was a clever piece of marketing by the manufacturers.
|
|
Dad's '67 Jag Mk2 had a tiny chrome triangle on the bumper mentioning discs. Couldn't be seen from more than a few feet away. I think the idea was to politely warn other drivers to give you some clearance because of their superior stopping power. If you could read the wording, you were already having the accident.
|
|
|
|
|
I had a 54 cable braked beetle in the 70s. You didn't notice the brakes until you used them, and then you didn't notice them either. However it was great seeing the MOT inspector trying to figure out why the front wheels were locked solid as he didn't realise that the handbrake used the same linkage as the footbrake and worked on all 4 wheels. Happy days!
|
|
|
The beetle has a significant weight bias to the rear, and is a lot lighter than modern cars - it probably doesnt need disc brakes.
The biggest drawback with drum brakes is cooling - if they overheat you get massive fade and usually hit the car in front. But this problem can still be solved with good drum brakes. My 1965 Peugeot 404 had finned aluminium drums on the front, with cast iron inserts for the friction area. They were superb!
|
|
|
|
Actually big shift to discs on trucks/trailers reason maintenance costs and you can have ABS.
|
Working at Tesco i see quite a few trucks, and have noticed that most have disc brakes on front, not such about rear. Deffinatly the Scanias, had a nosey tonday
First Mainline's new busses also have disc brakes on front, again cannot see rear brakes.
Can someone explain why they still use air brakes? Are they better? If so, why cant cars have them?
Kev
|
Kevin
The main reason is that most trucks have diesel engines. Because they are fuel governed, diesels have very little manifold vacuum, unlike a petrol engine, so you can't use a vacuum servo. I suspect trucks have gone to the air pressure route (rather than vacuum pump route) for a number of reasons.
You can achieve a higher pressure with a pump (a vacuum system can never exceed -14.7 lb/in2 even if it drew a perfect vacuum), so the brake servo equipment can be smaller, and for the same reason it's possible to have a substantial reserve of assistance using a smaller tank. This existence of pressure reserve is also 'checked' by using the air pressure to hold off the parking brake. This can be set to come on even when there is a substantial reserve of air pressure.
Diesel cars tend to have an engine driven vacuum pump rather than an air pump. This is undoubtedly for compatability with the rest of the car and the vacuum servo which is standard fitting. Going to an air pressure system on diesels would add extra expense to the conversion. OK, all cars could have an air pressure system, but why bother with the cost and noise of an air pump when the vacuum is there for the taking?
Regards
John
|
|
|
|
|
Kev - I'm no expert, but I don't think airbrakes would be any better for cars. They stop the wheels easier, but car tyres run out of grip too easily to take advantage of this.
|
|
Guy's right, there are lots of other reasons why air braking would be impractical on smaller vehicles. You need a hefty compressor, storage tanks, expensive valves to control the effort and a means of stopping the vehicle in the event of pressure loss. Very expensive! Some smaller lorries have a combined system where the brake pads and shoes are actuated by hydraulics but the effort applied is via air pressure. The best and worst of both worlds. Both systems require a lot of maintenance including regular checks for air leaks, extra filters and so on.
|
|
|
|
I should think it is something to do with the size of the servo needed to operate a vast fluid braking circuit on a tractor unit plus trailer, don't forget. It is not practical to have a system such as this running on fluid purely from foot pressure amplified through a servo.
With ref to the Beetle - how many beetles do you know with drum front brakes that can realistically exceed 70mph so I doubt brake fade is an issue!!! They also have 6V electrics so headlights are also a bit suspect. The 1303S was the first beetle to have discs but by then it looked horrendous compared to the early models.
|
|
|
|
All good comments above about why air brakes on commercials, also for articulated vehicles it would be an absolute pig to connect up hydraulic circuits without having to bleed methinks. Its not impossible but.......
|
|
|
|
also, am i right in thinking that lorry brakes are fail safe, ie, the air pressure is used to keep the brakes in the off position so that if the pressure failed the brakes would be applied and stop the vehicle?
|
Michael
Yes, the parking brake is held off by the air pressure - see my post above.
Regards
John
|
|
|
|
