Editor's Note:Here's another tech taken from a chapter of a WolfGang Publication book–How To Build A Cheap Chopper. This covers frame basics, but there's more in the book. Check it out.–Bandit
Frank Pedersen from Motorcycle Works in Olathe, Kansas (just south of Kansas City if you’re wondering) likes to compare the motorcycle frame with the foundation of a house. “If you are building a house and the foundation is crooked you are fighting it all the way through. But if the foundation is good then the whole job goes well.”
So take Frank’s advice and buy a good one. By good we mean a frame that’s straight, with all the brackets welded on in the right locations. A frame that won’t require engine leveling shims or enlarged holes to correctly locate the major components. No less an authority than Donnie Smith commented that, “People try to save money when they buy the frame, but it always ends up costing them more cash and time in the long run.”
The best judge of good and bad frames are the guys and gals who use them day after day, folks who build motorcycles on a regular basis. If the frames aren’t straight, or if the drive-train doesn’t line up easily there’s trouble ahead. Guys who build bikes everyday work with reliable components and know how to check a frame quick.
Building a quality frame requires a stout jig to hold all the components in the correct relationship to each other – on every frame, every time. Because most frames are constructed from round tubing, each junction requires a mitered cut. Seldom do the two pieces of tubing come together at exactly 90 degrees, which means the beveled joint is a complex shape.
Some companies use a fixture and a Bridgeport mill to precisely cut these mitered joints, at least the major joints where the top tube and down tubes meet the neck. There are even “miter boxes” designed to cut concave miters of various sizes and angles to afford tight, strong connections for even stronger welds. Fancy laser cutters are also available for fabrication shops that can justify the expense. Commonly though the cuts are done with hand grinders, which is fine if the fabricator is careful about the fit between the two pieces of tubing.
The long-winded explanation is necessary because of the fact that heat moves metal. And loose joints require more filler material which requires more heat which means more movement of the metal during the welding. All of which comes back to the fact that some frames are better than others, it’s up to you to find a good one.
How To Build A Cheap Chopper
By Timothy Remus
Custom bikes, choppers and bobbers in particular, don’t have to cost thirty thousand dollars. S chopper or bobber built at home can be had for less than five thousand dollars. The key is the use of a donor bike for most of the components. How to Build A Cheap Chopper documents the construction of five inexpensive choppers and bobbers with complete start-to-finish photo sequences presented in full color.
Least expensive are the metric bikes – like the Honda and Kawasaki projects documented here. Next up, price wise, are bikes built using a Buell or Sportster as the donor. Check the sequence from Redneck Engineering or Motorcycle Works to see how quickly a Buell can be converted into a simple hardtail.
Most expensive, but still dammed cheap by comparison with most of today's bikes, is the big twin chopper built in the shop of Dave Perewitz from carefully chosen aftermarket parts.
If you're long on desire but short on cash, this is the book to buy. The first think you need to assemble that new chopper or bobber is this book – How To build A Cheap Chopper. Cheap Chopper uses 144 pages and over 400 color images to completely explain each assembly. Available for $24.95 + $5.50 S&H from Wolfgang Publications
217 Second Street N.
Stillwater, MN 55082.
Order from the web at www.wolfpub.com or by phone at 651.275.9411.
(See Frank Pedersen’s interview in Chapter Seven for more on frame manufacture.)
Definitions
Before delving further into a discussion of frames it might be helpful to define some terms. Rake is the angle of the fork assembly as compared to vertical. With choppers the rake at the neck may be different than the rake of the fork, this discrepancy is done in order to achieve the best trail figure.
Trail is the distance between the front tire’s contact patch and the point where the centerline of the bike’s steering axis meets the ground (see the illustrations).
Motorcycles have positive trail. Much like the caster angle of an automobile, positive trail provides the straight line stability that allows us to take our hands off the bars while rolling down the road. The more trail the more straight-line stability.
Most motorcycle designers and builders like trail figures in the 4 to 6 inch range. Most factory bikes fall into this same range. In fact, the chart for the current model line up from Milwaukee lists trail figures from 4.1 to 6.2 inches. In general, sport bikes run less trail which tends to make them quicker to change direction (some would call it twitchy), while cruisers and factory customs run more.
With most standard-issue front fork assemblies and standard triple trees an increase in rake will result in an increase in positive trail. With choppers this means a rake angle of 45 degrees can result in a trail dimension of 8 or more inches. Too much trail means great straight line stability, but a lot of what’s called “flop” at slow speeds, shaky off the line. The more straight line stability the less a bike wants to turn. At speeds the bike wants to stay straight and you’ll need a larger turning radius and more pull to handle corners. That’s why a balance is important unless you’re building a bike for Bonneville or the drags.
The trail dimension can be altered without altering the frame, with the use of “raked trees.” (Check the illustrations here for clarification.) In fact, some of the factory bikes from Milwaukee use raked trees to help them achieve what the engineers consider an ideal trail figure. Before bolting a set of raked trees on your bike remember that raked trees reduce trail. Exactly how much depends on the angle and height of the neck, the size of the front wheel and where the triple trees position the fork tubes relative to the neck.
Because raked trees reduce trail you can’t just bolt a set of seven-degree trees to a stock Sportster to give it that long raked look. Raked trees on a stock bike can easily move the machine into a negative trail situation. This means you’ve lost all straight-line stability and your bike will be prone to high-speed wobbles. We’re talking major instability here.
Experienced chopper builders might buy a frame with 40 degrees of rake and then use a set of “five-degree trees” (this is an example, don’t duplicate these numbers) for a total of 45 degrees of rake. By using the five-degree trees they reduce the trail to a more manageable level (approximately 4 inches) and get rid of that really heavy feeling, most noticeable on slow speed turns, that results from extreme trail dimensions.
What all this means is that you have to buy a rolling chassis kit from a quality manufacturer who has already figured out the trail. Or you must carefully determine the trail for yourself. There are some web sites that have trail calculators, see the nearby side-bar for more.
StretchThere are two types of “stretch,” out and up generally. Typical out stretch refers to material added to the top tube – sometimes called a stretched backbone. Downtube stretch refers to the material added to the downtubes to raise the neck. On the street people say a certain frame is “4 inches out and 3 inches up,” meaning the neck is positioned four inches ahead and three inches higher than a “stock” frame.
That’s no all, though. Some builders stretch the rear out to install the battery behind the transmission. There’s gooseneck stretch jobs, you name it.
The LookFrame dimensions are very important for a number of reasons. For one thing, the motorcycle frame needs to fit your frame. Big people are going to want more stretch and possibly a taller neck. Put a short person on a bike with a raised neck and they won’t be able to see past the front of the gas tank and the upper triple tree.
You need to do more than just reach the controls and put you feet flat on the ground at a stop. The bike should be comfortable enough to ride, if not all the way to Sturgis, then at least for most of the day. Some corrections can be made with different bars and risers, but the basic chassis size, seat and bar positions all need to be situated to match your inseam and riding style.
Frame dimensions are important for another important reason. The dimensions of the frame (and to a lesser degree, the shape of the tank) go a long way toward determining the “look” or silhouette of the motorcycle. While all the bikes we’re discussing are choppers, they don’t all have the same appearance. There’s a hell of a difference between a short stubby chopper with a dropped neck, and a long stretched chopper with raised neck. So figure out which are the choppers you like the most, and roughly what their dimensions are. Now look for a frame or rolling chassis kit with those same dimensions.
Brian Klock suggests you keep a photo file, or clip images from magazines and Bikernet.com. When the pile gets to a suitable size, sort out the bikes you really like the best. Now figure out what it is that those bikes have in common, and what their likely frame dimensions are. Also not the manufacturer’s name behind the frame.
On the Trail to Stability
The best way to know for sure what the trail is on that new chassis you just purchased is to check it. Option number two (and a good aid to planning a chassis) is to use the very slick trail calculator at perseperformance.com. RB Racing also has a calculator at: www.rbracing-rsr.com/rakeandtrail.html. And if that’s not enough, there’s a rake/trail/tube length chart in the Drag Specialties catalog. Back to the photos. The shop images come from American Thunder in Prior Lake, Minnesota (a certified Wolfgang Publications affiliate shop). The trail-checking tool is a simple tool that Neal and crew assembled from aluminum stock – designed to screw into the threaded hole found in the bottom of most triple trees.
As you can see, there’s lot of difference between a bike with a 33-degree neck and another with a 40-degree neck. Vince from Redneck Engineering likes to see the trail between three and five inches, the Perse Website uses a figure of 3.5 to 5 inches. Some stock Harley-Davidsons come with 6 inches, so maybe that should be the upper limit.
The important thing is to be aware of the impact that trail has on the bike’s handling, and avoid building a bike that falls outside what most chassis experts and professionals consider to be the acceptable range.
A Conversation with Vince Doll of Redneck EngineeringWhat started as a very small motorcycle shop only a few years ago has evolved into a sprawling fabrication facility responsible for the manufacture of over 1000 frames per year, and an undisclosed number of chassis kits and turn key custom bikes. The creation of Vince Doll, Redneck is like a hungry growing animal, always looking for more space to house more welders, fabricators and frame jigs. Follow along as Vince tells us how he feels about rake, trail and raised necks.
Vince, how long have you been making motorcycle frames?
Three and a half years.
How does a guy without much background in Motorcycles have so much success in such a short period of time?
My whole goal was to build stuff anyone could afford. We got aligned with people who treated us right and could help us build a good product at a very reasonable price. Our stuff isn’t really cheap. Our fit and finish is very good. It takes no more time to do something right. For example, it actually takes less time to run the wires through the frame than to run them on the outside.
Tell us a little about how the Sportster/Buell frames came about?
For my first project I wanted to build a Sportster frame for the guy who was left out, a guy with an old Sportster in the garage. But we did a big twin frame first. And the first time we displayed that frame, at Biketoberfest in 2000, we sold 185,000 dollars worth. We were so busy after that we couldn’t back up and get to the Sporty chassis. So it wasn’t until Sturgis in 2003 that we debuted the first Sportster/Buell bike, the one that Mike Marquart built.
How is your frame different than some of the other Sportster frames on the market?
Most people start with a big twin frame and then put a sporty engine in it. We actually took tubing and built a frame around a Sportster engine. My initial idea was to give Sportster guys something to do with the bike they had in the garage, but when we built the first frame we had a Buell in the corner.
The factory doesn’t set the engine in exactly the center of the frame, so we put a Sportster motor on a piece of angle iron until we found the exact center of balance. Then we marked the cases so we could set the motor right in the middle of the frame. Our new frames are perfectly balanced and will still accept a 200 rear tire.
How hard is it to title a bike built from a Redneck frame and a donor Buell or Sportster?
We give them an MSO, and they also need the title to the bike the engine came out of. In most states it ends up being titled as a re-construction bike.
What about insurance, how much of a hassle is it?
It depends mostly on the individual and their relationship with their insurance company. Some guys do it for $400 per year and some are spending $2000 for the same coverage. Because these bikes are so inexpensive, you might want to just run liability coverage and leave it at that.
Frame dimensions, what’s safe and what’s not. Every-body wants these extreme choppers, when do they become unstable, What do you like to see for rake and trail?We won’t go over eight up on a chopper and seven out. We prefer to see the rake at 42 to 44 degrees. Then we work with the trees to get the correct trail. A good riding bike is six to eight inches up, Eight is cool for a big guy. I like to see the trail at 3 to 5 inches. I try to put them at 4 inches. On our springers we change the length of the rockers depending on the rake, that way we get the right amount of trail for any rake angle.
