What's new
What's new

Deckel (FP4NC) Making Repro Brake drums

AlfaGTA

Diamond
Joined
Dec 13, 2002
Location
Benicia California USA
Been a bit slow here in the past several days and so thought some might enjoy seeing parts of a little job i have been running on my 3150 RPM FP4NC.
Had an OSCA MT4 in the shop and part of the needed work was replacement of the brake drums...

The OSCA uses an aluminum housing on the drums, finned and fitted with an iron friction liner. Did a bit of looking and nothing use able was available that fit the requirements.
So i set out to reproduce the originals....Measurements taken and sketches made of the relevant parts of the drums.

Good news here is that all 4 drums are the same, same offset , diameter and width...so only one design needed....

Outer body of the drum was made form aluminum. Originals were cast with the iron liner cast in place....Not an option here.

I choose to make the iron liners be shrink fit into the aluminum outer, and to be sure things stay put i joined the liner to the drum using a "spline" to assure a positive "anti-turn" result.....

Liners were blanked out from a Ford Taurus brake drum form the local Napa auto parts house.....

Once blanked out (boring lathe work) the liners were setup on the Deckel and held via the same chuck that i used tor the turning work.
Here the "Pie" jaws support the part (thin) without changing its shape.....

attachment.php


One of the nice things about having "D" style chucks is that by simply removing the cam lock pins (takes about a minute) you can easily set the chuck on the true face of the chuck adapter...
nice for mill work .
Here i use a simple through bolt to the table "T"... Tighten the chuck on the part. lightly , check to see if the part is flat, then secure the chuck grip, finally tighten the center bolt....

Program for the spline was worked out in SurfCam...this is a smooth curve spline...internal and external curves all the same....no sharp edges.

Cut begins. Overall height of the spline will be at .2" taken in two passes for full depth.

attachment.php


attachment.php


Final on the spline.....
attachment.php


Close view of finished spline...
attachment.php



More to follow
Cheers Ross
 
Before setting up and running the spline , the OD below the spline was finish turned to give a good shrink fit to the drum (+ .030" )

To see how all this went together...here is the brake drum body having its spline act . Geometry is the same as the liner...same numbers

attachment.php

The outside detail (fins etc) can be seen here.....
Finished ID on the drum and finished spline.....
attachment.php


Here is a detail shot of the installed liner in the drum outer....Of note is the drilled and tapped hole in the drum spline face....Purpose here is to fit two alignment pins (at 180*)
in the tapped holes. The pins are long enough to engage the spline of the liner and thereby timing the splines as you drop the liner into the hot drum during the shrink fit.....

attachment.php


For overall view...drum setup on rapid index fixture . Drilling in drain holes at the intersection of the liner and drum....Liner is visible

attachment.php


Front view of almost completed drum assembly....

attachment.php


Cheers Ross
 
Very nice! Did you try to go for interference on both the splines and cylindrical surfaces? It's nice working shrink fits on large diameters, because you can have a lot bigger margins and don't need to work so fast.
 
The good news here is that we pretty much decided that 30+ years back that we were not cut out to be "Concourse" show builders...Those guys are not what we are about here....
The show guys make everything "perfect" of so they think...giving way to crap like not making anything tight for fear of making a body line not perfect ..they don't lube anything...grease attracts dirt....
Makes the cars into real hangar queens..
Here we build and repair "drivers" Our largest niche is "Vintage racing cars"...Got to be faithful to the period and materials and design...no rocket ships here. But we don't over restore anything, and everything here
is used as a car...tours races the like.
Cheers Ross
 
Rich:
No shrink on the spline...pretty much both cut to the same program....Thought it might be pushing my luck to try for shrink on both plain diameter and spline.
Cheers Ross
 
Having seen a pair of mid 1930s vintage straight 8 Alfa's that Ross worked on go much faster than me on Canada's #1 Hwy in the mountains (and I was speeding), I can attest for the fact he works on "runners" :-)

And they sounded so nice!

Lucky7
 
Rich:
Thought it might be pushing my luck to try for shrink on both plain diameter and spline.

My thoughts exactly, but thought you might have a secret sauce!

By the way, have you ever run a circle/diamond/square test on your FP4NC to see if it makes good geometry? I have not on my FP2NC. Ignorance is bliss, and it still makes good parts.
 
Hi All,

Before making my comments about Ross`s posts, perhaps I should explain to the few that probably don`t know why the "Alfin" type brake drums were fitted given that the aluminium and cast iron components that make up these drums have dissimilar rates of expansion.
The normal way to manufacture these drums is to cast the aluminium around the cast iron liner.

These type of drums were developed during the 1920`s and 1930`s for two reasons, the first was to dissipate the heat developed by heavy braking in the cast iron drum lining via the aluminium fins on the outside diameter of the drum. Secondly, the weight saving over a using a solid cast iron drum could be considerable and this weight reduction would reduce the unsprung weight of the suspension to give a more comfortable ride and better road holding. These expensive type of drums were only normally fitted to racing or sports cars. not to price driven standard cars.

There has been a revival of this type of drum in recent years as people now have more income to lavish on their "dream cars". These recently manufactured drums have had their fair share of problems due to the manufacturers not fully understanding the problems that can occur. Typical faults are that the cast in linings do not have adequate adhesion between the liner and the aluminium due to the dissimilar rates of expansion between the two. Also, if the liners are not cast in centrally then the drums cannot be satisfactorily balanced! There have been reports of catastrophic failure with the drums exploding during extreme braking.

Ross, while I can get excited just by looking at the high standard of your workmanship and the excellent photos, (that unfortunately tend to disappear after a while), I hope that your shop has adequate insurance in case of damage/loss of life due to faulty components fitted during the restorations. Your task was a lot more complex than just making a spindle brake for your FP2.

I hope that you will agree with me that any modifications made to the braking system on a car should be engineered as to be 120% safe in operation, to be able to perform properly during the racing and mountain pass storming that the testosterone driven owners of these machines will do.

My personal opinion is that the drums that you have produced will not be suitable for this as you have used an untried design in their manufacture and risk an unpredictable performance.

You should have explained to your customer that the safest way to go forward would be to make the drums from cast iron or steel and shrink on the aluminium fins. This configuration would take the punishment and although not looking 100% kosher would not be seen as the side of the drum would be hidden by the wheel.

Alan
 
My personal opinion is that the drums that you have produced will not be suitable for this as you have used an untried design in their manufacture and risk an unpredictable performance.
Because Ross is too modest to toot his own horn, lemme just say that this particular design has worked well at high speeds on race tracks such as Sears Point and Laguna Seca for more than thirty years. Ross'es work is not untested. In fact, his company was instrumental in creating the historic racing niche.

You don't get to play with these if you don't know what you're doing ...

231650d1179507175-new-protoform-shadow-1-8th-body-uop-shadow-glen.jpg




But I did hear the proper way was to rough the drums in a K&T first :D
 
Bentley1930 How about you post more of your own content and less commenting on Ross' work. Ross has provided this site with a massive amount of quality content and knowledge, where as you seem to just come in here trolling him.
 
You should have explained to your customer that the safest way to go forward would be to make the drums from cast iron or steel and shrink on the aluminium fins.
Alan

Is that not exactly what Ross did? He said he had a 0.030" interference. The coefficient of thermal expansion of aluminum is about twice that of gray iron (not sure what iron the Taurus drums are made of), so there would need to be about a 200°C (360°F) temperature increase to kill that interference, and that's assuming the aluminum is as hot as the iron, which it will not be; the iron will be hotter, and therefore the aluminum will not expand as much as it would under a uniform temperature distribution. Even if the aluminum loses interference altogether, it is still captured between the wheel and hub. Something to check is whether the Taurus drum has enough meat to be strong enough all by itself, forgetting any help from the hoop strength of the aluminum.
 
Dear Ross,

Nice work as always!

Like Alan, I was also wondering about what would happen if the brake drums are heated by repeated or hard braking. Is there a danger that the aluminium would expand so much more than the steel or cast iron so that the liner comes free? So I did an estimate as follows:

Thermal expansion temperature coefficient for steel, about 1.2 x 10^-5 per degree Celsius
Thermal expansion temperature coefficient for aluminium, about 2.4 x 10^-5 per degree Celsius

Assuming your brake drum liners are about 10" in diameter, and using your value of 0.030" of interference in diameter, to make the linings come loose we need a temperature increase T such that

T x (2.4 - 1.2) x 10^-5 x 10" = 0.030"

This gives a temperature rise of T = 250 Celsius = 480 Farenheit. As a reference point, I think that's around the boiling point of DOT4 or DOT5 brake fluid. How does this compare with the typical maximum operating temperature of brake drums?
When I looked on the web I found this handy listing:

550°F - 650°F Brake resin odour is present
850°F Brakes begin to smoke
1,100°F Brake oxidation occurs at parts of the brake open to air rushing by
1,250°F Drums become cherry-red internally
above 1,250°F Danger of run-away due to excess drum expansion

So a crude estimate seems to show that this is a concern. Am I overlooking something?

Cheers,
Bruce
 
The iron lining is where the heat is generated and the aluminum outer shell is always going to be cooler that the cast iron. The air flowing over the fins as the air flows over it guarantee this.
As a side note I used to do rally racing in Datsun 240Z and it had stock from the factory aluminum/iron rear brake drums. (disk front)
 
What feature in the original cast design prevents the thermal expansion from becoming an issue? It would be interesting to know what the temperature differential between the inside of the liner and the cooling fins would be in use.

Brake fade with drum brakes at those temperatures has to be pretty severe and I would think the driver would have the sense to ease off.
 
Some thoughts
The aluminium gets cooled by the passing air more as the CI liner inside
The cast iron liner will expand because of the force of the brake pads
The hotter the CI liner gets the easier it expands through the force of the brake pads

Peter
 
Bentley 1930 -

I think your comments are more than a little alarmist. There will be a large temperature differential from the inner cast iron drum friction surface to the outer alloy part. The alloy outer should be cooled reasonably effectively by the air flow over the fins, so the temperature differential should be maintained at a very high level in normal racing conditions. The overall temperature rise for the composite drum that would be required under these conditions to allow the shrink fit to be lost would be very large indeed, and I would expect could only be induced with static heating of the drum assembly. The alloy section and drum is trapped in any case, and the splines add further security.

The Alfin process is a little more complex than meets they eye and I am not surprised that people who have recently tried to replicate it have had some problems. I think the original patents belonged to Fairchild Aviation, but there were several licensees. In UK, Wellworthy developed the process for motorcycle clinder barrels, but it proved to require really exacting metallurgical and mechanical design for success and the process was expensive. Norton and JAP cylinders used this process as (I think) did Velocette. It is unfair to imply by association that the problems small suppliers might have with replicating a complex process for cast in Alfin type liners would also apply to Ross's design. They are entirely different in concept.
 
Because Ross is too modest to toot his own horn, lemme just say that this particular design has worked well at high speeds on race tracks such as Sears Point and Laguna Seca for more than thirty years. Ross'es work is not untested. In fact, his company was instrumental in creating the historic racing niche.
But I did hear the proper way was to rough the drums in a K&T first :D

Hi All,

Seamoss, Can you supply us with the information that shows that Ross`s method of building those brake drums is used on racing cars successfully

All of us that drive fast vintage cars know that drum brakes cannot achieve the same performance as disc brakes! That`s the reason why disc brakes are used on all modern cars.

We are engineers and we ask questions about things that don`t appear to be right, this is what we do! All Ross has to say is that he copied a successful design and give details of this so that we can all be satisfied that things are alright and that would be the answer to the question.

Alan
 








 
Back
Top