Schwinn cantilever frame geometry, discuss!

[scott1race]

Hello, and thanks for adding me to the group. I have been building and riding these Schwinn cantilever frames for about 20 years, and I am very happy with them. I am working on my next frame build, and am getting much deeper into fork and frame geometry than before. This is for a 1963 model. Can anyone help with a set of frame specs, drawings with all the dimensions and angles? All modern manufacturers do this, so I know that someone has figured this out already.
This is one of my more unusual creations, for attention:

 

[bloo]

I don't think it exists. I asked about fork rake long ago and didn't get much response. I got enough information to fix my fork, but not a lot about typical Schwinn ballooner or middleweight geometry. I also want to learn about this. I suspect there is a lot of variation over the years of production. I'll be watching the thread.

 

[scott1race]

Thanks. I have been reading extensively for some time now, there are good discussions among the Klunking off-road groups about it. The earliest pre-1940 frames were prized because they have a higher bottom bracket for better ground clearance (fewer pedal strikes) and a slacker head tube angle which helps high speed handling. The forks were different of course, but both had a similar axle to crown dimension. I have been using the later style frames which of course are much more available. I did not like the quicker steering that seemed to come with the typical straight tubed BMX style forks, and so have been trying out a wide range of other options. The stock fork rides the best, generally speaking. However, riding in sand or snow makes the bike completely unstable, so I started working on alternate plans. Here’s the bike with the stock fork. It’s a 1958 Corvette, made on the same date as me.

 

[bloo]

I was aware of the higher prewar bottom bracket that has been described in some forums, and I suspect it is true, because the bottom bracket in my 61 Speedster seems low. All those prewar frames were not the same though, and although the C model did have a slacker head tube compared to the other Schwinns of the late 30s, it seems to be the DX that was most prized for Klunking. Allegedly it had a higher bottom bracket than the straightbar, but what about the C? I don't know. Sometimes I wonder if they knew.

Moving the front axle back, all else being equal increses trail. The quicker steering with a straight fork has to be an illusion to some extent.

 

[bloo]

1961 Speedster 26 (middleweight), non-drilled fork:
70-1/2* seat tube 70-1/2* head tube.
This one shouldn't be taken for gospel. Fork has had work, maybe frame too.

For comparison:

1940-41 Schwinn straightbar 26 with springer:
69-1/2* seat tube 70-1/2* head tube
(not 100% sure of head tube measurement, but pretty close. Also this was with no weight on bike, so it will be less slack in use).

1953 Huffy 24:
69-1/2* seat tube 70-1/2* head tube.

Trail and fork rake would be interesting...

 

[GTs58]

It's very rare to see a 1958 Corvette with that rear fender bridge. Sharp ride though.
There was a change on the middleweight cantilever frame between 1958 and 59. I don't think any of the frame dimensions change though, just the cantilever/seat stay bars change.

 

[bloo]

The spot where the seatstays crossed the fender moved by about 3/4" didn't it? That should have been super obvious at the seat clamp. I'll bet they moved the bottom bracket down.

61 Speedster: 24cm from ground to bottom of BB shell. 26.8cm from ground to center of crank. Bike is on 2.0 (oversize) tires,

Subtracting 0.6cm to compensate for the tires: 23.4cm ground to bottom bracket or 26.2cm ground to center of crank.

 

[GTs58]

The spot where the seatstays crossed the fender moved by about 3/4" didn't it? That should have been super obvious at the seat clamp. I'll bet they moved the bottom bracket down.

61 Speedster: 24cm from ground to bottom of BB shell. 26.8cm from ground to center of crank. Bike is on 2.0 (oversize) tires,

Subtracting 0.6cm to compensate for the tires: 23.4cm ground to bottom bracket or 26.2cm ground to center of crank.

They basically raised the arc of the seat stay/cantilever tubing. Not sure what else was changed, I didn't check everything out at that time. The bends of the seat stays were also bent outward after the seat post on the 59's, the stays are just slightly angled out on the 58 and earlier frames.

Red frame 1961, green 1957.

 

[scott1race]

It's very rare to see a 1958 Corvette with that rear fender bridge. Sharp ride though.
There was a change on the middleweight cantilever frame between 1958 and 59. I don't think any of the frame dimensions change though, just the cantilever/seat stay bars change.

View attachment 1440649

Thanks, GT. The frame came to me with a color matching corvette chainguard and fork, and I have seen at least one catalog listing that made it seem like you could get a Corvette with a coaster brake. I am not an expert on these details, and restoration was never part of my plan. I did manage to ride this bike 1800 miles in 2021, and since then I have built another klunker/cruiser using a 2 speed kickback hub. I managed 850 miles on that newer one in 2022, but I do have a lot of other bikes I also ride, including the green one here.

 

[Schwinn Sales West]

I have not seen any information on frame geometry on the middleweight frames like was common on the highter end lightweight frames. It should not take much to answer your question. Today you have low cost digital angle finders. Heck even some smart phones have an angle finder app. Just level the bike on the ground with some thin spacers to adjust to zero. Then measure the head tube angle and the seat tube angle. Done.

Imo, it's more accurate to measure the tube length's "center to center" to be uniform and consistent in your measurements. Most bicycle companies use the C to C way to measure frames. Schwinn, was odd because they measured their frame sizes from the center of the crank to the top of the seat tube. All of the other tube lengths were measured C to C.

If your really into this, I would make up a gauge tool. Use a 5/16" drill rod available at any industrial hardware store. Then make two pointers if you have a lathe. Drill the pointers with a 5/16" hole for the drill rod, and drill and tap them for a set screw. I would make it four feet long. Just move the pointer until it matches your center to center distance and snug up the screws. It's really handy for measuring frame tube lengths, of chain stay lengths (axle center to crank spindle center).

Keep in mind that it does not matter how many bends or curves the frame tube has in it, it still comes down to the "center to center" length that matters. For example, on a girls frame the top frame bar angles down and is much longer than on a comparable wheel sized men's frame. But the only measurement that has any real meaning to the rider fit and ride quality is the "effective top tube distance" or the center of the seat tube near the seat post clamp to the center of the head tube just below the top head cup. That distance will determine if the rider is comfortable sitting on the seat and reaching the handlebars.

John