Why do Ford still use Drum brakes?

[akirby]

What do commercial trucks use? Tractor trailers, I mean.

 

Drums I think. Really big f****ing drums.

[351cid]

 

Drums I think. Really big f****ing drums.

Yup.

 

There are a **few** going to disc set ups, but most trucks are drum with antilock. I've not pulled any trailers with disks so I can't comment on how they function. The few drivers I've talked with says that they take some getting used to. Pedal effort is a lot more than we're used to.

[silvrsvt]

The reason larger trucks use drum brakes is because they are air powered systems vs hydraulic systems. No air in the lines means the brakes are engaged, which is helpful with heavy loads.

[akirby]

That's just a failsafe system - if there is a leak the brakes lock up instead of failing. I don't see how it provides any other advantage in normal driving even with heavy loads.

[silvrsvt]

That's just a failsafe system - if there is a leak the brakes lock up instead of failing. I don't see how it provides any other advantage in normal driving even with heavy loads.

 

 

Here is a good link explaining the differences between the system, and why to use them. It looks like Hydraulic brakes are limited to GVWR of 33K pounds

 

http://www.worktruckonline.com/channel/maintenance/article/story/2012/03/air-brakes-or-hydraulic-brakes-that-is-the-question/page/1.aspx

[akirby]

I didn't take into account that air brakes can provide significantly greater pressure than hydraulics and are more applicable for heavier loads. And I misunderstood your other comment. We're in agreement.

[SoonerLS]

FWIW, I just took a quick look at the Ford Parts site, and my F150 does have those "supplemental drums," but the Focus (at least the '14 Focus) has no drums at all on the rear discs, just what appears to be the same spring assembly on the caliper as my LSes, SHOs, and T-Bird have/had. That would appear to blow that theory out of the water...

[YT90SC]

Sooner, That type of caliper (e-brake equipped) is very problematic. Drum in hat park brakes are more reliable and cheaper to service.

[SoonerLS]

Sooner, That type of caliper (e-brake equipped) is very problematic. Drum in hat park brakes are more reliable and cheaper to service.

 

I've had six Fords/Lincolns with discs and no supplemental drums, and no problems with the parking brakes (aside from the occasional erroneous "service e-brake" complaint in my LS). I realize that the plural of "anecdote" still isn't "data," but I'm not really concerned with which design Ford chooses to use.

 

At any rate, my point is that Ford is not using the "drum in hat" brakes on the Foci with rear discs, so the costs associated with that design can't be a consideration that would cause them to use drums on some trims and discs on others.

[xr7g428]

Sooner, the engineering requirement for the parking brake has a lot to do with weight and the type of transmissions available. The Escape, Explorer, and of course F150 and Expedition as well as Super Duty all use the drum in disc combination. The engineering requirement for rear brake capability is also weight based. If you have a light vehicle, with little towing capability, then probably a drum offers more than adequate capacity. This would be particularly true where you have a front wheel drive vehicle with a weight bias to the front of the vehicle. As the vehicle gets heavier, the weight is more evenly distributed, and towing capacity becomes more of a factor, disc brakes would offer more capacity. I suspect that the spring activated disc parking brake system is limited in holding power where towing ratings start to be a factor.

[SoonerLS]

Sooner, the engineering requirement for the parking brake has a lot to do with weight and the type of transmissions available.

Again, I'm not arguing any of that. All I'm saying is that Ford does not use the "drum in hat" or "supplemental drum" design on the rear discs of the Focus or Fiesta, therefore the costs associated with that design cannot be a consideration in why using drums would be cheaper for those cars (and, presumably, the EcoSport).

 

In theory, using drums could be simpler/cheaper than using discs with a "drum in hat" design, but Ford didn't use the "drum in hat" design on the relevant vehicles, rendering that argument irrelevant. That's all I'm saying.

[xr7g428]

Not arguing, but not communicating very well either... (on my part). I think this is a great discussion. From a marketing standpoint, 4 wheel discs sell better.

 

This is getting lost in the discussion of why drums are cheaper than discs:

The point of all of this is that the rear brakes have a dual purpose that makes it incrementally more expensive to use discs than drums.

 

 

If you have a drum brake you don't have much added cost to produce a highly effective parking brake. (Basically one lever and a link between the shoes) If you have a rear disc you have to add a supplemental secondary parking brake mechanism. On a light weight vehicle with an automatic transmission with a minimal tow rating, you can get by with a secondary mechanism to acute the caliper, but it is far more complex and expensive than the lever and link in a drum brake. If you add a manual transmission, additional weight, or towing capacity you have to add a complete secondary rear brake to the mix to hit the target. It is sort of a spectrum of choices starting with a simple drum on one end and a combination disc drum on the other. I am just trying to explain from an engineering standpoint what the "go to" is for different applications.

 

For small cars, a rear drum brake is adequate to get the job done and less expensive when you factor in the parking brake regardless of how the parking brake is configured on a disc brake based system. Drums are not universally cheaper than discs. They are cheaper where they are small, adequate to the job and the cost of the parking brake is taken into consideration.

 

Given all of that, why hasn't someone designed a less expensive way to add a parking brake to rear discs? Maybe a pad that contacts the outside diameter of the disc? Drop a pawl into one of the vent slots and it won't go anywhere.

[RichardJensen]

Are the rear discs of compact cars vented?

[SoonerLS]

Are the rear discs of compact cars vented?

From the drawings on the Ford Parts site, it looks like the Fiesta's rear discs are not vented (the drawings of the front discs show vents), at least on the SE. One would think that the advent of differential braking for yaw control would increase the usage of vented rear discs, but I suppose it's probably still an unnecessary expense for (what pass for) lightweight, inexpensive cars these days.

Edited September 14, 2014 by SoonerLS

[SoonerLS]

Not arguing, but not communicating very well either... (on my part). I think this is a great discussion.

Oh, I agree--just because I'm not arguing the point, that doesn't mean that I don't think it's an interesting discussion.

[xr7g428]

Some what off topic, but in the ball park. The very early 428 Cobra Jet Mustangs and Cougars were so nose heavy they actually had to use narrower rear shoes to keep the rear end from locking up. The stopping distances for those cars is laughable today.

 

Good point Richard, I don't think that they are vented. Disc brake pads actually work better with some heat. From the design stand point, if a pawl type lock did make sense, you could easily add teeth to the outer edge.

[Biker16]

Again, I'm not arguing any of that. All I'm saying is that Ford does not use the "drum in hat" or "supplemental drum" design on the rear discs of the Focus or Fiesta, therefore the costs associated with that design cannot be a consideration in why using drums would be cheaper for those cars (and, presumably, the EcoSport).

 

In theory, using drums could be simpler/cheaper than using discs with a "drum in hat" design, but Ford didn't use the "drum in hat" design on the relevant vehicles, rendering that argument irrelevant. That's all I'm saying.

Drums are cheaper. Because the parts are cheaper to make.

 

Your "drum in hat" idea is missing the point. Look up the price of a slave cylinder on a disc vs a drum. Look at the cost of parts.

 

Disc brake caliper is 120 while drum slave cylinder is 20 on the Focus.

 

On the focus and fiesta the drum assembly holds the wheel bearings and an integral hub carrier while the discs require a separate hub carrier.

 

This design saves alot of money at the factory. And has been used on EU ford for the last 16 years.

 

Vs the traditional drum

[SoonerLS]

Your "drum in hat" idea is missing the point.

It's not my idea, and I'm not missing the point. All I've said about them (and I don't know how many times I need to repeat this) is that Ford doesn't use them on the Focus and Fiesta, so the costs of the design are not relevant to the discussion of why Ford is using drums on those cars.

 

That's all.

 

Period.

 

End of story.

 

-30-

Edited September 16, 2014 by SoonerLS