Springer Measurement

Originally published in the March 2015 issue of Cycle Source Magazine

So you want to ditch the stock hydraulic front end for a custom springer. The first thing you need to know is how to take measurements for your new, built-to-order piece of hardware. This is one of the most important pieces of the puzzle to get right for many reasons. You want to ensure that the front end is the correct length so your ride is nice and level. It’s also important for the front end to work with the front wheel, tire, and frame. Taking accurate measurements not only ensures that the front end will be the correct length but also that the frame geometry is correct. Frame geometry is a critical factor when changing front ends. Simply having a front end that is the correct length to provide a level stance is not good enough. Rake and trail must be taken into consideration to ensure that your bike performs as well as it looks. For this reason, Twisted Choppers requires you, our customer, to provide specific measurements when ordering your custom springer.

As shown above, there are six key dimensions that need to be measured. All measurements should be taken with the motorcycle standing upright with the front wheel straight or with the frame supported at the correct ride height. It is also necessary to ensure that the measurements are taken on a consistently flat surface. A square or plumb bob should be used when taking all of these measurements to ensure accuracy; if a measurement is taken out of square, it will be longer as a result and will affect the overall calculation. The first measurement is the radius of the front wheel and tire. The distance from the ground straight up to the center of the axle is the (A) distance. The best way to measure this is to ensure the tire is properly inflated and the wheel is completely vertical when viewed from the front or back. Set one leg of the square on the ground and align the second leg with the center of the axle. Measure straight up along the square from the ground to the axle center, and you have the wheel radius (A).

The next two dimensions are the upper neck height (B) and lower neck height (C). Again, these measurements should be taken using the same technique as the first. It is important to note that if your frame requires neck cups for the bearings, these must be installed prior to measuring. When looking at the frame from a side profile, measure from the ground to the center of the neck cups (top for B and bottom for C). The most critical of these two dimensions is the lower neck height (C), as the bottom triple tree of the springer is fixed and cannot be moved.

The fourth dimension is the neck rake angle (D). In the US, this measurement is the included angle between the frame neck and plumb (true vertical). It is important to physically measure rake, even if you “know” what your frame or bike’s rake is (or is supposed to be). Wheel size, tire manufacturers, tire wear, and the way you have your bike mocked up can all affect the actual measured rake of a motorcycle. There are a few ways to take this measurement; the cheapest and easiest way is to buy an $8 angle finder from a hardware store and set it on the front of the frame neck. Note that this measurement will typically be between 28° and 40°. If your measurement falls outside this range, you either have a super raked-out chopper frame, or you are reading the wrong angle (meaning you measured from the horizontal instead of the vertical).

The fifth dimension is the frame ride height (E). This measurement is most easily explained as the distance from the ground to the bottom of the lower frame rails on a rigid frame. If your bike has rear suspension, ride height can be measured either with or without a rider. This measurement is not particularly important for determining the correct springer configuration but serves as a useful check dimension if something doesn’t seem correct in the others.

The sixth dimension is the length of the neck (F). It is important to note that if your frame requires neck cups for the bearings, these must be installed prior to measuring. This measurement is not particularly important for determining the correct springer configuration but serves as a useful check dimension if something doesn’t seem correct in the others.

Using the six measurements described above, the team at Twisted Choppers calculates the correct springer front end length for your application and verifies the trail calculation to ensure a properly handling motorcycle. Twisted Choppers offers different rocker arm configurations to allow the custom springer to be assembled with the correct geometry for your application. As always, we are here to help and happy to answer any questions you may have.