Frame Stretch

There are three areas in a frame that can be stretched or extended to change the profile of the chassis.

First the rear wishbones and lower side rails can be extended rearward, lengthening the wheelbase and allowing a lower seat height. Secondly the front down-tubes can be extended or stretch upward and lastly the backbone can be stretched forward.

The frame can be separately altered in any one of the individual areas or can be stretched by making a combination of extensions, which is the usual case.

When I first started building bikes the objective was to create a longer and lower chassis, one well suited to street drag racing so we'd stretch the rear out 2 aft and 1 up lowering the whole frame and dropping the seat height. To stretch the steering head we would extending the top tube and add a few more degrees to its slope angle creating almost a straight tubing line when viewed from the side from headstock to rear axle.

A few years later when long forks came into vogue we began extending the front down-tubes upward which resulted in the steering neck needing to be raked to keep the frames long and low.

By the mid sixties the classic chopper profile developed into the wedge shape that we all enjoy today.

When people now talk about stretch in the down-tubes or backbone the dimensions they refer to are all referenced back to the stock Harley hard tail frame introduced in 1936-37 which had a 25 backbone sloped at an 18 degree angle from the horizontal and 21 down-tubes sloped about 11 degrees off the vertical, 10 degrees, 41 minutes and 13 seconds to be exact.

It is sometimes assumed that you can take a stock frame and stretch it by just cutting the backbone and down-tubes and welding in some segments of new tubing but it's actually a little more complicated than that because moving the location of the steering head also changes the overall angles of the tubing runs.

The following diagrams illustrate the various methods of stretching frames but note that the sketches are drawn at an exaggerated scale for clarity and in reality the modifications result in less distortion in the frame tube runs as they intersect the steering head.

Figure 2.5

Figure 2.5 shows the effects of stretching a frame rearward three inches with the axle plates also moved upward by another inch, which drops the whole frame down for an overall lower profile.

Figure 2.6

In Figure 2.6 we've moved the steering head forward by four inches while maintaining a stock angle on the top tube. Note how the angle has changed on the down-tubes and the kink where the stock steering neck meets the down-tubes.

Figure 2.7

Figure 2.7 illustrates stretching the down-tubes four inches along the stock angle, which causes the top tube angle to change.

Figure 2.8

The picture in figure 2.8 shows a frame with stretch in both the top tube and down-tubes.

Figure 2.9

Figure 2.9 illustrates a frame with top tube, down-tubes and the rear wishbones all stretched and the angles to the steering neck connection aligned with the frame rails to create the classic chopper profile.

Remember that when chopping first started nobody had access to the modern angle finders and digital protractors that we enjoy today. The methods used had to be reasonably accurate, easy to perform using just a ruler and fast, A quick and dirty method to describe stretch was almost universally adopted by most chopper builders and today we have the commonly accepted 'up and out' terminology. Remember however that stretch always refers back to a stock Harley rigid frame.

The worse thing you can do when stretching a stock frame, which is where this all started, is to do in such a manner that you end up with an ugly and weak 'kink' in the down-tubes as they approach the steering neck a seen in figure 2.6 above. To eliminate this problem fabricators developed a couple of simple rules of thumb for calculating the amount of offset needed in the jig to produce the desired results. One rule says that for every 4 inches you stretch the down-tubes upward the steering neck will move 1 inch forward, this is the classic 4 up and 1 out stretch, (0.25:1 ratio). Other possible combinations using this method are 8 up and 2 out and 12 up and 3 out and so on. Jigs built to use this method have the headstock fixture sloped at an angle of 14 degrees from the vertical (14 degrees 2 minutes and 10 seconds). Another school of thought used jigs with the head-posts sloped at an 11-degree angle so you ended up with popular stretches like 5 up and 1 out and 10 up and 2 out.

Today most frame builders and large manufacturers have standardized their jigs to build frames with a fixed angle to the down-tubes that range anywhere from 10 to a much as 18 degrees, 15 degrees being very popular. When you order a frame today, unless it is completed custom fabricated you'll end up with a 'fixed' angle and stretch length on the down-tubes and a variable angle and stretch on the backbone.

None of these methods actually produce a frame that has mathematically perfect 'up and out' stretch figures. There are always fractions involved and everybody just rounds their published figures to the closest whole number like 4 up and 2 out when in reality it is more like 4 up and 1.75-inches out.

For frames that are completely custom built the stretch figures have no meaning whatsoever since you can change the angles of the down-tubes and backbone to suit any design that your heart desires and while the location of the steering neck itself could be referenced back to a stock frame doing so wouldn't have much practical meaning.

The point of the story is that you should never order any part for a bike based upon 'published' stretch measurements since they vary significantly from one builders methods to the next and from one type of frame to the next. Unfortunately many advertisers and marketers, and even some frame builders, 'exaggerate' their real stretch figures to make a frame seem more 'radical' than it really is. I've seen frames cataloged as being 12 up and 3 out when you could plainly see that they were much closer to being 8 up and 2 out. Use the numbers only as a reference or a rough guideline since that's what they really are to begin with.

Also please keep in mind that as a frame gets more radical with long lengths of tube runs for lots of stretch it also gets inherently weaker unless you increase either the diameter or wall thickness of the tube being used. A frame having an 8 up and 2 out stretch for example is significantly less rigid than a frame being 4 up and 1 out if the tube specifications on both frames are identical. A good example of this can be seen almost every day in the huge number of tank mounts that fail due to frame flexure in bikes with large amounts of stretch.

There are practical reasons beyond the structural reasons for not ordering a frame with large amounts of stretch. For starters there is a limit on up-stretch where it becomes a matter of safety when you can't see over the fuel tank to dodge potholes. Large amounts of stretch can also cause some very uncomfortable riding positions for many people and as the steering head gets higher and higher you have less leverage, physically, in your arms and shoulders to handle turning forces in the forks themselves. Large amounts of up-stretch also dictates longer forks and longer forks mean more weight, more flop and more steering effort. From my own personal experiences and the input from other chopper owners it appears as if 6-inches of down-tube stretch is about the maximum amount where good looks, good comfort and good handling start to diverge.

For people who on insist on having that ultimate 'wedge' or 'high-neck' look it is far more practical to have a frame with 6 up and 2 out that incorporates a dropped seat than a frame that is 8 up and 2 out with a standard seat position.

 

 

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