Thread: Effect of electric brakes on the wrong side

Originally Posted by beefy

Looking at a photo of my electric brake backing plate, I seen a little white sticker with "LEFT SIDE" printed on it This brake is on the right side of my trailer. The curved solenoid arm is "pointing" backwards, but should be pointing forwards, which it would be if it was on the correct side.

Onto Google / Youtube I went to do a little research about the effect of the backing plate being on the wrong side. The general response was braking strength will be reduced but no one actually said why in any detail. Quite a few said that when the brake is one the correct side the leading (forward) shoe is pushed into the drum and the rotation of the drum helps pull it in. The thing is the trailing (rear) brake shoe is trying to be pulled away from the drum by the direction of rotation. When the brake is on the wrong side the situation seems identical. The leading shoe is still being pulled into the drum by the direction of rotation while the trailing shoe has the opposite effect applied.

Attached to the curved solenoid arm is a square cam. This pushes the brake shoes apart whether the curved arm is pushed forward or backward. At this point I cannot see mechanically how there will be less force pushing the brake shoes apart (i.e. onto the drum) whether the cam is rotated one way or the other. The cam is still turned, pushing the shoes apart.

The manufacturers obviously have a reason why they have a left side and a right side design. I'm just trying to understand what it is, because looking at the mechanics of the system, I can't see a reason YET why there will be reduced braking when the brake is on the wrong side. Old wives tales are rampant, as are explanations repeated by those that don't really understand them. One guy even said with such authority the brake shoes would be pulled away from the drum when the backing plates are on the wrong side. Does anyone fully understand the mechanics of electric brakes and can explain why you'd lose braking effect if they are on the wrong side.

I even contacted an electric brake manufacturer and the guy I spoke to clearly had no clue. Hoping some of you practical guys here might know.

Cheers,

Keith.

Keith,

You certainly would loose braking effect if the brake assemblies are on the "wrong side". The reasoning is down to the geometry of the brake shoes relative to the rotating brake drum.
ElectricBrakeBackingPlate.jpg
The picture at left shows a typical single actuator drum brake. On this type of drum brake (doesn't matter if it is hydraulic or electric), the actuator exerts force directly on one brake shoe only. In this case it exerts force directly on the brake shoe on the upper left side of the photograph. The upper left brake shoe, after hitting the brake drum, rubs on the second brake shoe on the lower right side of the photo to indirectly actuate the second brake shoe. The brake shoe that is directly actuated is called the leading shoe, whilst the indirectly actuated brake shoe is called the trailing shoe.

If you look at the photo at left, and imagine the brake drum and wheel rotating clockwise, you'll see that when the actuator pushes out on the leading shoe, and the shoe contacts the brake drum, the rotation force on the brake shoe caused by friction with the brake drum will assist the actuator in applying force to the brake shoe. The down side of this effect is that the brake can grab when applied, causing wheel lock-up. To counteract this in the days when cars had 4 wheel drum brakes, some mechanics used to bevel the leading edge of the brake shoe to minimise brake grabbing.

If you imagine the brake drum and wheel rotating anti-clockwise, the effect is opposite. The rotation force force on the brake shoe caused by friction with the brake drum will work in the opposite direction to the force being exerted by the brake actuator. The result is reduced braking effort, but little or no brake grabbing.

I have seen some instances where engineers have intentionally placed the drum brakes on the "wrong side". A typical example is on light aircraft drum brakes, where if the brakes grab when applied, it can flip a tailwheel aircraft on it's nose - so the engineers often install the drum brakes on the wrong side. The brakes are less effective than than they could be, but the brakes don't grab (which is far safer) and the brakes are still effective enough to do the job.

I may be wrong, but whoever built (or modified) your trailer, may have installed the brakes on the wrong side intentionally to solve a brake lock-up problem. If the brakes were found to be too powerful for the trailer, then putting the drum brake backing assemblies on the wrong side would have minimised the brake grabbing, and probably still left enough brake force to do the job.

So, that's my understanding. My advise ... if the brakes are working ok and are effective enough, don't worry about it. If the brakes are not effective enough, then check to make sure that the brakes are adjusted correctly and that shoes have plenty of meat on them, before doing any work to swap the brakes back to their "correct sides". If it's a commercially manufactured trailer, then the engineer who designed the trailer, may have inteneded the brakes to be installed that way.

Regards,

Roy

Roy, thanks a million, great info.

Haven't quite digested it all yet (wife is calling me away for lunch, must obey ) but after lunch I'm going to have a good study, and I'll probably pull the brake drum off and have a look there too.

Edit:
OK, had a chance to study what you've told me. Also checked out Wikipedia and the explanation was similar. I'm afraid I still cannot see a reason for reduced braking effort when the wheel is rotated backwards (same as the brake being on the wrong side when going forwards).

I understand everything you have told me about how the brake operates, apart from why it's different when going backwards.

In the picture you supplied that is a left side brake (curved solenoid arm facing forward).

So wheels turning in the forward direction (anti-clockwise as we look at the image):
Going over what you explained, the electric solenoid is pushed to the right, but due to the see saw pivot point of the curved arm, the leading brake shoe (left one) is pushed to the left at the top. The anti-clockwise rotation of the drum is in the same direction, thereby assisting this push. The leading shoe grips the drum and wants to rotate with it. It presses on the left side of the adjuster mechanism at the bottom. The adjuster mechanism is actually free to slide so that it can transfer some "push" to the trailing shoe (right side) at the bottom. The trailing shoe is then also assisted by the anti-clockwise rotation of the drum. The top of the trailing shoe presses hard up against the stop on the backing plate, and unlike I previously thought, is not actually contacting the cam of the curved arm.

Wheels turning in reverse (or same as brake on wrong side when going forward):
OK this is where the operation seems identical to me, except everything is in the opposite direction. The drum is now turning clockwise. The solenoid will be pushed to the left and therefore the cam pushes on the top of the right side shoe (this is now the leading shoe seeing as we are going the other way). This right shoe is assisted by the clockwise direction of drum rotation. It grips the drum and also wants to rotate clockwise. The right shoe pushes on the sliding adjuster at the bottom and transfers some "push" to the left shoe at the bottom. The push on the left shoe is assisted by the clockwise direction of drum rotation. The top of the left shoe presses hard up against the backplate stop, above the cam.

Do you see where I'm coming from. I cannot see mechanically any reason why the braking power in the reverse direction will be any different. Forward or reverse the mechanics APPEAR identical.

Please don't get me wrong, I'm not saying they are the same, I simply cannot see why from a mechanical perspective. Have I missed something in your explanation.

Regarding the trailer manufacturer deliberately installing the brakes backwards, I hope they didn't. I've got a total weight of 2.3 tonnes so I need all the braking power I can get

Cheers,

Keith.

Have a look at the brake shoes, ie the brake material, you'll see one has a longer surface area which is why they must be installed the correct way as explained by AussieRoy.

You should have two identical steel bits but one will have less friction material which is what Roy is talking about, the shorter side grabs first, then the weight transfers to the longer side to get maximum braking effect

Thanks very much Acco.

Indeed I never picked up on that. At least now I can see one reason why things will work different in reverse. After reading Roys post I guess I was looking more at the metal of the shoes and they looked the same.

Cheers,

Keith.

At that weight am assuming that you have a tandem setup? Didnt notice it being mentioned on your writep. This is on the rear brake?

Asking cause it would probably effect the park brake (assuming one is connected) if it was on the park braked axle.

It is very common for electric brakes to be installed on the wong side.....if this is intentional the installer did not know what they where doing....mostly is is carelessness pure and simple.

Twin leading shoe designs are the norm for electrc brakes...they depend on the leading shoe design to provide a braking advantage because the magnets could not possibly produce sufficient force without.

people are often confused about the arrangements and claim that they are not indeed twin leading shoes....you need to look realy close at the arrangement....it is cleverer than it may first appear.....probably patented at one time.

back before power assisted brakes and indeed drum brakes it was usual for the front drum brakes to be twin leading shoe designs..for the same reason.

The basic princible is...due to the design of the geometry..when the shoes are pressed againt the drum, friction pushes the shoe arround and thus harder into the drum....this is moderated by the force applied by the magnet, slave cylinder or cable as the case may be.
When the preasure is released from the magnet or other actuator the drum pushes the shoe away.....properly designed and mainteined they do not grab in motion.

NOT all leading shoe designs provide this mechanical braking boost..it is a particular geometric arrangement.

If the system is poorly maintained or badly designed it may indeed grab...but that is not a good reason to reverse the brake drums.

It is normal for electric brakes to bind a little if realy pressed hard to a stop...some drvers use this to advantage......they press hard as they come to a stop..the brakes bind a little giving a bit of breaking delay when they take off.......if the brakes are realy tight it may take driving the vehicle forward to make then release....remember frward motion pushes the shoe away if there is no force from the actuator

trailing or one leading one trailing shoe designs will be designed differently...they will not be leading shoe designs reversed.

in extreeme cases a leading shoe design installed the wrong way arround will produce little or no braking.

It is well known that electric brakes work poorly at best in reverse.

reassemble your brakes and put them on the correct side

cheers

Thanks Jatt & Soundman,

yes it is a tandem setup.

I've always known how to adjust electric brakes properly or so I thought. Just read this thread:

https://www.woodworkforums.com/f184/electric-brakes-locking-114075

and Yonnee mentions about loosening the backplate bolts before using the brake shoe adjuster to push the shoes against the drum.

Just when you think there's nothing left to learn.......................

Keith.