Brakes forward and reverse

[Richard Lorenz]

Does Model A braking have the same efficiency going forwards and backwards? One of our members who has to back down a steep driveway thinks that braking is poorer when backing up. From looking at the design of the braking system, I don't think that there should be a difference. Comments anyone?

[ericr]

I'm just wondering...is the transmission gearing ratio different when backing up?

[Napa Skip]

The transmission gearing is different in reverse than in any of the three forward speeds.

Final drive gear ratios for transmissions such as are found in the Model A, are calculated as the ratio of the input gear (diameter or number of teeth) to the output gear, or where there are multiple gear engagements (such as in first, second and third gears) the product of such ratios and are expressed as the ratio of 1 revolution of the output shaft to the required number of input shaft revolutions.

Thus, for first gear, where the 16-tooth input shaft engages with the 31-tooth gear on the cluster gear, and where the 18-tooth cluster gear then engages with the 29-tooth low/reverse sliding gear, the number of input shaft revolutions to give 1 output shaft revolution is given by 31/16 x 29/18 = 3.12 (rounded down). Consequently, first gear is said to be 3.12:1 (that is, one rotation of the transmission output shaft for each 3.12 rotations of the engine crankshaft/transmission input shaft).

Similarly, second gear, where the 16-tooth input shaft (always) engages with the 31-tooth gear on the cluster gear, and where the 24-tooth cluster gear then engages with the 23-tooth second/high sliding gear, the ratio is given by 31/16 x 23/24 = 1.86 (rounded up), giving the 1.86:1 (sometimes seen as 1.85:1) second gear ratio.

For third gear, the forward portion of the second/high sliding gear internal teeth engage with the after portion of the input shaft external teeth and neither the low/reverse nor the second/high sliding gears otherwise engage with the cluster gear (which keeps spinning due to its permanent engagement with the input shaft). The after portion of the second/high sliding gear remains splined to the output shaft and consequently the output shaft is driven with no gear reduction and the final 1:1 “ratio” is achieved.

For reverse, the introduction of an idler gear is necessary to reverse the direction of rotation of the output shaft. Again, the 16-tooth input shaft engages with the 31-tooth gear on the cluster gear; the 15-tooth cluster gear engages with the 18-tooth reverse idler gear, and the 18-tooth reverse idler gear engages with the 29-tooth low/reverse sliding gear. Consequently, reverse gear is said to be 3.75:1 (i.e., the product of 31/16 x 18/15 x 29/18 = 3.75, rounded up) such that 3.75 revolutions of the input shaft gives one revolution of the output shaft.

Probably more than is needed for this discussion…

My knowledge of brake design is rudimentary so more knowledgeable individuals will correct anything that is stated incorrectly, but as to the efficiency of Model A brakes in reverse vs. forward, my understanding - given that both shoes pivot from one location (compared to more advanced drum-type braking systems where the shoes pivot from opposite ends) is that the braking efficiency would be the same regardless of the direction of travel of the car. Put another way, when the brake shoes contact the rotating drum on a standard Model A brake system, one shoe is "pulled" into the drum by virtue of where the shoe is pivoted and the direction of the drum rotation, while the other shoe - even though being "pushed" into the drum - is pulled away from the drum by the rotation of the drum.

Reversing direction of travel of the vehicle just reverses which shoe is the primary and which becomes the secondary.

More advanced drum braking systems, with independently pivoted shoes, are designed such that - for forward motion of the vehicle - each shoe is pulled into the drum by drum rotation, whereas when backing up and presumably going at a slower rate of speed with less braking demand (assuming you are not some Hollywood stunt driver, in which case you're probably driving a vehicle with disc brakes anyway) both shoes are pulled away from the drum, are less efficient, but need only provide a fraction of the forward-direction braking requirement.

At least that's what I recall from high school, some 54+ years ago.

I will await corrections to all this...

[Tom Wesenberg]

I agree. It looks to me also that the braking is the same either direction.

[Joe K]

Agreed not "self energizing" which would tend to make one direction superior to the other.

That said, any differences in equalizing between front and rear shoe (and the two do "self equalize as the wedge/cam move between front and back rollers) will be amplified as the brakes are adjusted. If one of these (either wedge or cam) get hung up, the owner may attempt to "adjust" to restore braking - and this will bring one shoe to wear more than the other.

Even this is not a deal killer. Still equal pressure between the shoes.

But, when backing, the entire assembly may move and upset the adjustment to the point of allowing only one shoe contact. (still binding on wedge or cam)

So in theory I agree. Practice may bring in the unexpected and his car may be subject.

I would check the equalization/wedge/cam areas.

Joe K

[Kurt in NJ]

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Stock brakes should work the same either direction, but if there have been 'modifications' anything can happen

[Richard Lorenz]

Thanks for the replies. It looks like there is agreement that Model A brakes in good original condition would be equally effective forwards and backwards.

A number of years ago I thought about building or using a commercial accelerometer to test for brake efficiency. I did not get much beyond the thinking stage, but with LapTop computers available now, it might be much easier to devise something to measure and record brake efficiency. Any ideas?

[SeaSlugs]

Gonna throw my input into this:

Accelerometers on the cheap: if you know someone with an iPhone there is an app that allows you to use it as an accelerometer in the X Y and Z directions. So you, your son/daughter/ or grandkids may have an iPhone you could borrow for a bit or have them ride along. Theres no wires or anything to run just secure the phone (tall cupholder) and run the application drive and brake. it puts all the data from all 3 directions in a graph with numbers and you can save it and run the test over and over. then you can compare the data and see what you get.

From my little knowlege of the brake system some of your guyses comments got me thinking. Napa Skip's idea of the brake shoes getting pushed/pulled into the drum depending on the direction since 90% of your braking is most likely forward and higher speeds than backing up braking that one shoe would get broken in differently than the other. So when one would back up and needs to stop mildly hard (steep driveway) its energizing that other shoe that isnt worn as much or worn in backwards from the drum rotation. (kinda like the top fin on a bluegill - go head to tail and you hardly notice it, go tail to head its a while nother story...)

And yes forward braking generates alot more heat/wear on the braking parts stopping from 40mph than it does stopping from 5mph backwards.

Also dont forget if your brakes are truley adjusted correctly the front wheels should lock up before the rears since the front of the car does 70% of the stopping power, when backing up your rear brakes are trying to do the same amount of work as the fronts but need more foot pressure to do so so it may feel to the driver their not working correctly.

I really dont have any hard evidence of any of what i said (besides the phone thing) but its just some ideas i had while reading this post.