'27 JD Cut Down Project - SWAN

[Steve Swan]

Did You ever check out that 29 Partial JD lead Steve?

oh... sheeit. i have not.... and i accidentally deleted the pm you sent me. i just tried sending you a pm but your box is full...

[JoJo357]

Definitely worthy of seeing! Frankenhoser must have been back i am guessing in the days when we were poorer boys.

You hit the nail on the head Steve. Sell one, buy another days. I cleaned out my mailbox--fire away.

*M.A.D.*

[Steve Swan]

You hit the nail on the head Steve. Sell one, buy another days. I cleaned out my mailbox--fire away.

*M.A.D.*

pm sent....

[JoJo357]

Hi all,
Found this cool website when Cotten and i were reminiscing about those 'BoozeFighters'. Has an article 1/3 way down on Dee Cameron, under Table of Contents. It's worth spending the time looking over this site!
p1000244.jpg
p1000250.jpg

https://occhiolungo.wordpress.com/

*M.A.D.*

[aumick10]

JD Cut down disc Brake

Steve,
When trying to engineer a new system, there is a lot of issues that have to be taken into account. I will try to identify them, but will probably miss something. I have done this modification on other bikes and hope this post will help you not make the same mistakes I did.

Wheel hub.
I think the Honda hub you are looking at is not a good choice. For starters, it is set up for a 21" rim, with the spoke angles set for that. It is also set up for straight spokes on the disc side, and the under/over bent spokes on the other side. If you don't want to run a 21" rim, you will have to make 3 different types of spoke. I have done this and I can tell you I find it a real pain.
This hub is also for a 223 lb bike.

Disc diameter.
A large dia disc on a small dia wheel will have more stopping power than a small dia disc on a large dia wheel.
For the bike weight and the type of riding you are likely to be doing, This is my opinion only, but I think a disc diameter in the range of 8" would be suitable for this service. Unless you are riding two up at 100 mph. Then all bets are off.
It is easy to overpower the tire which only has a small contact patch on the road at any time by putting too much brake on the bike. Not a good thing.
If the disc dia becomes to large in dia, it is common for the caliper to interfere with the spokes.

Caliper.
There are many different types of calipers available as OEM and aftermarket. I think you should look at a caliper with a single active piston on each side. Two pistons moving toward each other by pressure. This arrangement allows the caliper to be solid mounted to the mount. With active pistons on one side only, the whole caliper must move to bring the pistons together. This is a much more difficult mount to make.
You may need to go to a small 4 active piston caliper to be able to match the area to the master cylinder as discussed below.

Caliper anchor.
As other posts have mentioned, the JD brake anchors have seen to be problematic in certain circumstances. I personally think the tubes are strong enough to take the increased forces with a disc brake. I have no personal experience with the JD forks, so you may want to be guided by someone who has.
I have a rigid Harley with a OEM springer that the factory reintroduced in the 70"s I think. The brake anchor on these springers were a stud drill through the back tubes and welded on each side. I have had no problems with this system. Colony do a similiar replacement stud for the rear brake. Pt no 9208-4
The closer you can get the anchor to the centerline of the leg, the less twisting force will be transmitted into the leg.

The caliper anchor will need to be able to rotate at the axle end, otherwise you will lock up the suspension.
Diagram attached.JD Disc barke GA-Model.jpg
In an ideal world, the brake anchor would be the same length, and parallel to the fork rocker. This is probably not possible. The anchor center to center will be longer than the fork rocker center to center. I would at least make it parallel to the locker.


Master cylinder.
The piston dia is a very important dimension. It must be matched up to the master cylinder on the handlebars to work correctly. To small a master cylinder, it won't move enough fluid to work. To large a master cylinder, the brake won't work until you squeeze the hell out of it and it will then probably lock up.
Brake feel is a very individual thing, but the brake people have come up with ratios that give different feel. They recomend a ratio of in the 20-25 range for good feel and operation. This is the cross sectional area of all active pistons divided by the cross sectional area of the master cylinder.

I am going to go metric here for a sec so bare with me.
A caliper with two active 28mm dia pistons has a total cross sectional area of 1231.51 square mm.
A master cylinder with a 12mm dia piston has a total cross sectional are of 113.1 square mm.
Dividing these together gives a ratio of 10.89.
Once you get below to 15-20 ratio range, you start getting a "wooden" feel to the brakes.
Also, the larger piston area, the larger the clamping force.

Something else to consider also is that a lot of master cylinders are for 3/4" bars, they won't fit the 1" ? JD bars.

Last, but not least is the aesthetics.
My personal choice would to not get to "space ship" with the parts.
See the attachment for the style of wheel i would use.(CB550 Honda) This is not too far from the original round hub with two spoke flanges on the ends design.
CB500 wheel.jpg
Some of the above are specifications you will need to follow to have a good system that is comfortable to ride.

This is your build so the actual selection of parts will be your choice.

Excuse the length of the post.

Cheers,
Mick.

[Steve Swan]

Thanks, Mick, for taking the time to write your very thoughtful post. Clearly there are a number of considerations to properly fit a front disc brake to this old fork setup. i will take into consideration what you've written and give this some careful thought. i shall figure this out!

i intend to run 21 inch WM3 (2.15) rims front and rear with Kenda K657 Challenger 90/100-21" tires; the overall diameter of the mounted K657 is 28.1 inches so this is 1/2 inch larger diameter than the 3.85x20 later tires and the same diameter as 28x3 earlier tires. The 21 inch rims will help me achieve the skinnier appearance of the the earlier twins. The width of these Kendas is 3-3/4 inches.

[aumick10]

Steve,
The 21" wheels will probably flex more than the original wheels. The spoke length doesn't help, and I have never seen a 21" wheel with heavy gauge spokes, although they may exist.
Did you get a PM from me?
Cheers.
Mick

[exeric]

Late H-D factory 21" wheels are laced with heavy gauge spokes.

[Steve Swan]

Steve,
The 21" wheels will probably flex more than the original wheels. The spoke length doesn't help, and I have never seen a 21" wheel with heavy gauge spokes, although they may exist.
Did you get a PM from me?
Cheers.
Mick

Mick, I didn't get your pm.

[aumick10]

Eric,
I stand corrected,
Cheers

[Tommo]

There's also un-sprung weight to be considered.
What effect does the increase in un-sprung weight have on the original design of a set of un-damped front forks.
I suspect Matt Olsen's mishap in a previous Cannonball might have something to do with the increase in un-sprung weight.

[Steve Swan]

There's also un-sprung weight to be considered.
What effect does the increase in un-sprung weight have on the original design of a set of un-damped front forks.
I suspect Matt Olsen's mishap in a previous Cannonball might have something to do with the increase in un-sprung weight.

Tommo, what was the nature of Matt Olsen's mishap?

[Tommo]

Sorry for the delay but I've been trying to find a picture of Matt Olsen's 1913 Sears he rode in the first Cannonball in 2010 and I can't find one.
It's reported that on a downhill section of road he hits a pothole and falls off. Broken bones are the reported injuries due to the bike going into a major wobble.
A picture of the bike will reveal a twin leading shoe Triumph conical hub front brake. A huge increase in un-sprung weight.
This is where I say I SUSPECT that the major increase in un-sprung weight caused a front suspension designed in the teens to lose control of the front wheel and in effect pogo Matt off the bike.
Weight and un-damped forks are a recipe for disaster, so when you start adding modern technology to old bikes all sorts of problems come into play.
I'm not saying don't do it but I am saying be aware of the problems and pitfalls you may be getting yourself into.
Harley had problems when they first put a front brake onto forks that were basically a 1908 design. Yes there were changes between 1908 and 1928 but what I'm trying to point out is that they had problems with that design back then and now we are going to ask a troubled design to accept an even stronger braking force. Hello!
The JD fork post elsewhere on this forum shows another one of the strengthening mods that Harley did to help solve the problem and when the VL came out they were not prepared to use a tried and true design so they used the I beam fork until they could come up with something better and that proved to be the fork that came out in 1936 on the knuckle.
Maybe I'm wrong with my take on this issue and I'd welcome anybody else's opinion on the subject. Feel free to criticise if you think I'm wrong.

[TechNoir]

I have been following this debate on the pro's and con's of adding a front brake to a 20's HD with interest.

It ocurs to me that george Brough "borrowed" Harley's fork design and modified it and used it on his bikes which also had better brakes then the Harley bikes. He called them "Castle Forks" Is there any merit of using Brough type forks?

John

[Steve Swan]

Thank you Tommo and thank you John for your thoughts.

Tommo, certainly adding unsprung weight to these old forks requires thoughtful consideration and i'll do my best to do so. that i why i am thinking the hydraulic disc parts off a modern dirt bike might be best, they are small and they are light weight. using the master cylinder, caliper and disc from the same model will be a "matched setup" so the braking power should be there. another option would a lightweight drum brake off a small displacement dirt bike such as Kevin runs on his #97 Cannonball bike on his Powerplus. i think the bottom line is at least three considerations; not too much brake and not enough as well as is the brake on this old design going to be fit and work properly and be safe. maybe i wold be better having no front brake and go with a non-factory brake on the rear. although the majority of stopping power is on the front brake, i am thinking in modern bike terms and i probably don't want to get stuck thinking having a front brake on my project is the only option for improving braking ability... what i do know is the JD accelerates just fine for modern traffic, certainly on back roads, but braking ability, albeit not quite adequate for modern traffic, is not something i want to have to be constantly thinking about when i am going down the road.

John, after the initial mention of Castle forks was made, i did a brief amount of internet "research" on the subject. pics i found don't show well the changes Brough made to the fork to (i assume) accommodate his front brake. i'm not sure where i could obtain a picture of Castle forks that would give me an idea what mods i might consider to a set of the 25-27 J-type forks.

reflecting on the past posts of this thread, especially considering the exploits of Mssr's Cameron and Tidwell, i think maybe we are in agreement running a front brake on J-type forks is doable but not without problems if not given the amount of consideration necessary for a front brake setup to be fit right, work well and above all be safe.

PS - i had heard from a local buddy about Matt Olsen's fall at an earlier Cannonball. Although i personally know nothing this particular fall or about the factors causing his fall, i had heard he had a rear hub failure subsequently locking up the rear wheel. this mishap may have been while riding a different bike at a different Cannonball. Assuming the twin-leading shoe front brake fitted to late 60's-early 70's era Triumph's and BSA's is what i am thinking of is what Matt had on his Sears, these units were were not only a large diameter, they were heavy; i am guessing something at least 10 pounds if not closer to 15. certainly more brake from a weight standpoint, (if not stopping ability and aesthetics) than i would be comfortable with, especially on forks that were at least 12 years older and lighter in design than 25-27 J-type forks.