Old School Chopper Frame Fabrication
The easiest frame to build is a traditional old-school style chopper since there aren’t any complicated bends or compound miters to cut so we decided to show the chassis fabrication process from A to Z as we do it. The process described here may not be the best way to build a frame and it certainly isn’t the only way. Others probably have different techniques. This system has worked for us over the past thirty years and guarantees that a first-time builder can create a good straight frame using a minimum of special tools or really fancy jigs. About half of the frame can be constructed without a traditional building jig being used so this is a good project to start with if you want to build a frame jig but you don’t have a frame to use as a mock-up. As work progresses on the chassis you can fabricate a jig one piece at a time when it’s needed.
The frame for this particular project is derived from the stock Harley Davidson 1948 Factory Rigid design that has formed the basis of every chopper ever made. We stretched the down tubes about four inches over stock, the backbone two inches and raised the rear axle plates one inch and shifted them rearward 2” to lower the entire frame one inch and create a straight line between the steering neck and rear axle shaft when the bike is viewed from the side giving it a classic wedge-like chopper profile without an extremely high neck.
These instructions are actually part of the material that used to accompany our large-scale building plans and this is the first time we have published them separately. We’ve tried to make the small diagrams and photographs readable within the limitations imposed by the letter size page format but I am sure that some parts of the fabrication process will remain unclear without the prints. If you reach a stumbling block on your project larger format images of some assemblies, like mount dimensions for instance, are available in the appendix or on the website.
The frame we’re going to be building here will accept Panhead, Shovelhead and Evo motors and all transmission types from early four-speeds to modern six-speeds. It is designed to use a 140 tire with belt or a 150 with chain and the budget is set not to exceed $1500 up to the point of having a complete rolling chassis.
This figure is about $6000 less than many people spend on the typical low-end mass-produced rolling chassis kits available today that are offered in the mail-order catalogs
The final objective of this particular project is to have a complete running custom-built big-twin chopper for under $8500
Part 1
Depending upon where you live the materials cost for the project including tubing, a manufactured steering neck with cups and bearings, machined axle plates and miscellaneous plate stock will run between two and three hundred dollars or about half of what you’d expect to pay for a bargain basement pre-built frame.
To start with you need thirty feet of 1.25”x.120” ERW tubing. We’d suggest that you buy two full twenty-foot lengths since this will give you some extra material in case you make a few miscalculated cuts or bends. You’ll also need two feet of 1x2x.120 rectangular steel tube for cross members. To start the project you won’t need a steering neck or axle plates until further down the line so you can get this frame started with very little up-front cash outlay, in the range of 150 dollars.
After you’ve secured the tubing you can clean it and cut it into working lengths using an abrasive cutoff saw if you have one otherwise use a saws-all or even a small tubing cutter. As a last resort a regular old hacksaw will get the job done but will eat up a good chunk of time and give you some nasty blisters.
Figure 6.1
Figure 6.1 illustrates the primary chassis components of a typical V-Twin rigid frame that consists of the following primary elements:
1. Steering head. Also called the stem head, steering neck, headstock or neck-piece.
2. Frame Backbone. Sometimes called the Top-tube.
3. Seat post.
4. Backbone or Top Tube brace.
5. Wishbones. One left and one right. Also called the upper rear wishbones.
6. Wishbone cross member. Also called the upper fender mount.
7. Side tubes. Sometimes called the side rails, bottom rails or lower tubes
8. Seat post cross member.
9. Rear transmission mount/cross member.
10. Axle plates or side plates.
11. Front tubes or Down tubes which extend into the bottom rails
12. Front transmission mount.
13. Rear motor mount.
14. Front motor mount
15. Motor top mount.
The following table lists the pieces to be cut, the length and the quantity. Allowance has already been factored in to account for the bend lengths and to permit some room for possible cutting mistakes.
Quantity
Item
Material
Cut Length
2
Down-tube/lower Rail
1.25”x.120 ERW
84”
1
Backbone
1.25”x.120 ERW
35”
2
Wishbones
1.25”x.120 ERW
38” each
1
Wishbone Cross-member
1.25”x.120 ERW
12”
1
Seat Post
1.25”x.120 ERW
23”
1
Seat Post Cross-member
1”x2”x.120 CREW
12”
2
Spacers
1.25”x.120 ERW
12”
1
Transmission Cross-member
1”x2”x.120 CREW
12”
Table 6.1
To make this an easy project and to minimize the need for a full blown frame jig until later in the process we’re going to basically build what can best be described as the upper half of the frame with the seat post and seat post cross member as one unit and later we’ll add the down tubes and bottom rails as a secondary assembly.
When the first set of tubes have been rough cut and the wishbones have been bent we’ll have a group of parts that should look very much like those pictured on my driveway in Figure 6.2 below.
Figure 6.2
This assembly actually represents about 70% of the overall chassis tubing and while it may not look like much in this condition it represents a tremendous amount of work and very few ‘do it yourself’ builders get even this far on their first project.
The primary sticking point is always the bends. If you can’t afford a bender what can you do?
Getting off track for a minute, if you don’t have a tubing bender you can probably find a chassis or welding fabrication shop somewhere near your locality to bend the wishbones. Try posting a message on one or more of the chopper forums and you might find a fellow builder near you who’d be willing to help you out with the bending. I do not recommend that you attempt to make these bends with a hydraulic pipe or conduit bender and if you try to heat the pipe and bend it by hand the radius will be to small and the tube will probably flatten anyway.
If all else fails and you can’t possibly find anybody to make these bends you may be able to successfully accomplish the task by using a 1” thick-wall (rigid) conduit bender with a long extension handle as the outside diameter of one-inch rigid electrical conduit (EMT) is very nearly the same as the O.D. of 1.25” ERW tubing. The results of resorting to this stopgap measure are usually superior to resorting to the cheap bottle-jack pipe benders but again they’re just not really very good.
To be completely honest and straightforward however I have to say that if you can’t manage to get the bends done in a real tubing bender it is better not to start the project to begin with. Trying to take the cheap way out on making tube bends usually ends up with a piece of crap frame that you’ll never be happy with so why do it to begin with.
If you can live with the reality of that notification you’re ready to get started on an adventure that will very probably change your life and this is no hype. Chopper frame building is addictive and most people can’t stop with just one. Several visitors to our site who originally just wanted to build a single frame for them selves now own and operate successful chopper shops. Believe me when I tell you that this stuff just gets into your blood once you’ve successfully built that very first frame.
To get back on track the first thing we have to do is to decide what style of connection we want to make between the backbone or top tube and the seat post. There are three joints that are commonly used. The first, seen in Figure 6.3 is probably the most common and the one I personally prefer as it allows easy access for running wiring inside the tube. It seals off the seat post to water intrusion yet the open end of the top tube is naturally sloped to drain. This isn’t the easiest connection to make and for that reason it has fallen out of favor by shops doing high volume work but it is a strong connection and permits the builder to easily change the slope angle of the backbone by simply deepening the seat post notch on the back side of the tube. This is the connection method most typically used by custom builders.
Figure 6.3
The second method is almost as popular but a much older style of connection going all the way back to the days when the seat post really served a purpose. The problem with this style is that water can get down inside the seat post tube and even if you provide a drain hole it will eventually get clogged up with rusty gunk.
Figure 6.4
The third connection style as shown in Figure 6.5 is becoming more and more popular because it is easier and quicker to make than either of the two methods show above that both require a coped or fish-mouthed joint in one of the two tubes.
Figure 6.5
I have mixed feelings about this last connection method, which is correctly called a fully mitered joint. Even though it can be made quickly with a chop saw to my eye it just doesn’t look right but it definitely has the advantage of sealing off both the top tube and the seat post. This is a common connection in high volume mass production shops.
For a first-time builder this is probably the method I would select but on this particular build we’re going to use the first style where the top tube extends over the seat post.
The fi
Old School Chopper Frame Fabrication
The easiest frame to build is a traditional old-school style chopper since there aren’t any complicated bends or compound miters to cut so we decided to show the chassis fabrication process from A to Z as we do it. The process described here may not be the best way to build a frame and it certainly isn’t the only way. Others probably have different techniques. This system has worked for us over the past thirty years and guarantees that a first-time builder can create a good straight frame using a minimum of special tools or really fancy jigs. About half of the frame can be constructed without a traditional building jig being used so this is a good project to start with if you want to build a frame jig but you don’t have a frame to use as a mock-up. As work progresses on the chassis you can fabricate a jig one piece at a time when it’s needed.
The frame for this particular project is derived from the stock Harley Davidson 1948 Factory Rigid design that has formed the basis of every chopper ever made. We stretched the down tubes about four inches over stock, the backbone two inches and raised the rear axle plates one inch and shifted them rearward 2” to lower the entire frame one inch and create a straight line between the steering neck and rear axle shaft when the bike is viewed from the side giving it a classic wedge-like chopper profile without an extremely high neck.
These instructions are actually part of the material that used to accompany our large-scale building plans and this is the first time we have published them separately. We’ve tried to make the small diagrams and photographs readable within the limitations imposed by the letter size page format but I am sure that some parts of the fabrication process will remain unclear without the prints. If you reach a stumbling block on your project larger format images of some assemblies, like mount dimensions for instance, are available in the appendix or on the website.
The frame we’re going to be building here will accept Panhead, Shovelhead and Evo motors and all transmission types from early four-speeds to modern six-speeds. It is designed to use a 140 tire with belt or a 150 with chain and the budget is set not to exceed $1500 up to the point of having a complete rolling chassis.
This figure is about $6000 less than many people spend on the typical low-end mass-produced rolling chassis kits available today that are offered in the mail-order catalogs
The final objective of this particular project is to have a complete running custom-built big-twin chopper for under $8500
Part 1
Depending upon where you live the materials cost for the project including tubing, a manufactured steering neck with cups and bearings, machined axle plates and miscellaneous plate stock will run between two and three hundred dollars or about half of what you’d expect to pay for a bargain basement pre-built frame.
To start with you need thirty feet of 1.25”x.120” ERW tubing. We’d suggest that you buy two full twenty-foot lengths since this will give you some extra material in case you make a few miscalculated cuts or bends. You’ll also need two feet of 1x2x.120 rectangular steel tube for cross members. To start the project you won’t need a steering neck or axle plates until further down the line so you can get this frame started with very little up-front cash outlay, in the range of 150 dollars.
After you’ve secured the tubing you can clean it and cut it into working lengths using an abrasive cutoff saw if you have one otherwise use a saws-all or even a small tubing cutter. As a last resort a regular old hacksaw will get the job done but will eat up a good chunk of time and give you some nasty blisters.
Figure 6.1
Figure 6.1 illustrates the primary chassis components of a typical V-Twin rigid frame that consists of the following primary elements:
1. Steering head. Also called the stem head, steering neck, headstock or neck-piece.
2. Frame Backbone. Sometimes called the Top-tube.
3. Seat post.
4. Backbone or Top Tube brace.
5. Wishbones. One left and one right. Also called the upper rear wishbones.
6. Wishbone cross member. Also called the upper fender mount.
7. Side tubes. Sometimes called the side rails, bottom rails or lower tubes
8. Seat post cross member.
9. Rear transmission mount/cross member.
10. Axle plates or side plates.
11. Front tubes or Down tubes which extend into the bottom rails
12. Front transmission mount.
13. Rear motor mount.
14. Front motor mount
15. Motor top mount.
The following table lists the pieces to be cut, the length and the quantity. Allowance has already been factored in to account for the bend lengths and to permit some room for possible cutting mistakes.
Quantity
Item
Material
Cut Length
2
Down-tube/lower Rail
1.25”x.120 ERW
84”
1
Backbone
1.25”x.120 ERW
35”
2
Wishbones
1.25”x.120 ERW
38” each
1
Wishbone Cross-member
1.25”x.120 ERW
12”
1
Seat Post
1.25”x.120 ERW
23”
1
Seat Post Cross-member
1”x2”x.120 CREW
12”
2
Spacers
1.25”x.120 ERW
12”
1
Transmission Cross-member
1”x2”x.120 CREW
12”
Table 6.1
To make this an easy project and to minimize the need for a full blown frame jig until later in the process we’re going to basically build what can best be described as the upper half of the frame with the seat post and seat post cross member as one unit and later we’ll add the down tubes and bottom rails as a secondary assembly.
When the first set of tubes have been rough cut and the wishbones have been bent we’ll have a group of parts that should look very much like those pictured on my driveway in Figure 6.2 below.
Figure 6.2
This assembly actually represents about 70% of the overall chassis tubing and while it may not look like much in this condition it represents a tremendous amount of work and very few ‘do it yourself’ builders get even this far on their first project.
The primary sticking point is always the bends. If you can’t afford a bender what can you do?
Getting off track for a minute, if you don’t have a tubing bender you can probably find a chassis or welding fabrication shop somewhere near your locality to bend the wishbones. Try posting a message on one or more of the chopper forums and you might find a fellow builder near you who’d be willing to help you out with the bending. I do not recommend that you attempt to make these bends with a hydraulic pipe or conduit bender and if you try to heat the pipe and bend it by hand the radius will be to small and the tube will probably flatten anyway.
If all else fails and you can’t possibly find anybody to make these bends you may be able to successfully accomplish the task by using a 1” thick-wall (rigid) conduit bender with a long extension handle as the outside diameter of one-inch rigid electrical conduit (EMT) is very nearly the same as the O.D. of 1.25” ERW tubing. The results of resorting to this stopgap measure are usually superior to resorting to the cheap bottle-jack pipe benders but again they’re just not really very good.
To be completely honest and straightforward however I have to say that if you can’t manage to get the bends done in a real tubing bender it is better not to start the project to begin with. Trying to take the cheap way out on making tube bends usually ends up with a piece of crap frame that you’ll never be happy with so why do it to begin with.
If you can live with the reality of that notification you’re ready to get started on an adventure that will very probably change your life and this is no hype. Chopper frame building is addictive and most people can’t stop with just one. Several visitors to our site who originally just wanted to build a single frame for them selves now own and operate successful chopper shops. Believe me when I tell you that this stuff just gets into your blood once you’ve successfully built that very first frame.
To get back on track the first thing we have to do is to decide what style of connection we want to make between the backbone or top tube and the seat post. There are three joints that are commonly used. The first, seen in Figure 6.3 is probably the most common and the one I personally prefer as it allows easy access for running wiring inside the tube. It seals off the seat post to water intrusion yet the open end of the top tube is naturally sloped to drain. This isn’t the easiest connection to make and for that reason it has fallen out of favor by shops doing high volume work but it is a strong connection and permits the builder to easily change the slope angle of the backbone by simply deepening the seat post notch on the back side of the tube. This is the connection method most typically used by custom builders.
Figure 6.3
The second method is almost as popular but a much older style of connection going all the way back to the days when the seat post really served a purpose. The problem with this style is that water can get down inside the seat post tube and even if you provide a drain hole it will eventually get clogged up with rusty gunk.
Figure 6.4
The third connection style as shown in Figure 6.5 is becoming more and more popular because it is easier and quicker to make than either of the two methods show above that both require a coped or fish-mouthed joint in one of the two tubes.
Figure 6.5
I have mixed feelings about this last connection method, which is correctly called a fully mitered joint. Even though it can be made quickly with a chop saw to my eye it just doesn’t look right but it definitely has the advantage of sealing off both the top tube and the seat post. This is a common connection in high volume mass production shops.
For a first-time builder this is probably the method I would select but on this particular build we’re going to use the first style where the top tube extends over the seat post.
The fi
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