Frame Preparation

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Basic Frame Preparation

Whether you're starting out with a bare frame that you've just created, an old used frame or a raw frame from any one of several manufacturers there are some basic things that need to be done in order to get a good foundation for your project.

Alignment

The first thing we should do is to find out if the frame in question is even worth bothering with. If it’s an old used frame or even a new frame from a mass production shop, we need to know if it’s square and true. You’d be amazed at how many aren’t. In just the last few weeks I’ve read well over a half-dozen threads on the boards about misaligned frames or hardtail kits with problems. These problems usually don’t show up until the unfortunate owner is well into the project and by then it’s too late.

I usually start by checking the overall alignment fore and aft. Is the projected steering neck center point aligned with the rear axle centerline? This is the single most critical aspect of the entire build and failing to check that it’s correct is a mistake made by many builders, both newbies and seasoned builders alike who choose to trust that the frame they bought is straight.

There are several ways to do this. The first thing is to find a nice flat spot in the garage or shop and snap a chalk-line on the floor. If you look closely at the snapshot, you can just barely make out the line which is extremely faint in this photo.

Place a square between the floor and center of axle at the rear aligned with the chalk-line. Make sure the square intersects the axle exactly at the midpoint between the axle plates.

This is an old picture taken when we used a 4130-gauge rod that had been cut to a point on one end in the lathe. It’s held by ‘centering’ cones in the neck (available from automotive tool supply stores). Alternatively, you can just use a plumb bob with the string run up through the neck. It’s not quite as accurate but works in a pinch.

The hopeful results will be that both centers, front and rear hit the chalk-line perfectly. If they don’t then the frame isn’t worth using for a good project unless you want to spend the money to have it professionally aligned.

How good is good? When it comes to alignment you should expect perfection, but it’s seldom found unless you build the frame yourself. On factory Harley frames an eighth to five-sixteenths off is not at all uncommon. Some well used factory frames can be away from the line by a half-inch and people still use them and say they handle okay, but I wonder what they’re comparing their experiences to. Have they ever ridden a properly aligned bike before? It’s anybody’s guess. The factory considers .109” off center as the maximum acceptable discrepancy but they seldom meet their own criteria.

The next check that’s easy to do is to see if the transmission mounts are true and for this all you need is a tranny plate that you know for sure is ‘flat’ and has the correct bolt holes.

Before you bolt the plate in place use your fingers to see if the plate ‘wobbles’ on the mounting points. This one did, right at the spot where my fingers are so we know it’ll need shimmed, but the bolt holes lined up perfectly.

You can check the motor mounts in a similar fashion, but you’ll need a motor plate and if you have one you’ve probably also got the matching transmission plate. The parts shown below were provided by Bitter End Old School Choppers.

In this snapshot they’re just sitting there and not bolted in since I couldn’t find the bolts or the rear spacer on short order, but you get the idea. The plates will tell you immediately if the motor and transmission mounts are aligned and if the mounting holes are correctly positioned. We use these in our welding jig and at $220 they are a true bargain for the aggravation they alleviate.

The next thing I usually check is the alignment of the rear axle plates. This can be done just using a tape measure and a framing square.

First thing is to check the axle height on each side of the frame by measuring up from the shop floor to the bottom of the axle slot on each side.

This is simple, and in our case here both slots are exactly the same height. Next comes checking whether or not the axle plates are plumb and square to the frame.

Both sides are plumb for their full length. Finding warped or twisted axle plates is common but, in most cases, this can be fixed with some heat and bending if absolutely necessary. Square is checked with a long straightedge laid over the axle and then measured on each side relative to the frame centerline using a plumb bob from the backbone tube near the steering neck.

Another critical thing to check is whether or not you have enough clearance for the rear wheel/tire, brake rotor and sprocket with the chain (or belt) installed to fit between the rear axle plates and more importantly between the rear frame rails. You’d be amazed at how many don’t. I can’t understand why something so fundamental in frame design is often completely ignored by both frame builders and people who manufacturer weld-on hardtail kits.

For original rigid frames the distance between the rear axle plates should be between 8.75 and 8.84”. The distance measured between the insides of the frame rails should be 8.0”. from the rear transmission mount the rails ‘flare’ out slightly to an inside distance of 8.14-inches’ There is a half-inch dogleg in the left lower rail for chain clearance making an 8.64” space in this area. Keep in mind that these old bikes ran narrow wheels and used narrow drum brakes.

For modern mass-produced rigid chopper frames the distance between the inside of the plates should be 9.25”. The distance inside the lower frame rails should be at least 8.875-inches preferably 9. There is a sometimes a dogleg in the left lower rail as seen in the picture below.

The plates are distorted by the camera lens.

For weld-on hardtails intended to convert shovel frames to rigids the inside measurement between the rails should be 7.875” minimum where the lower rails meet the old frame rails. From this point towards the rear there should be doglegs or ‘flares’ out to the sides to provide a minimum of 8.875” clear inside rails. Shovels used 1.125” tubing.

The older factory manuals describe some other methods for checking alignment but in my opinion, they are sorely lacking so I don’t recommend that you follow them.

If you’re dealing with an ‘unknown’ frame it’s also a good idea to make sure the motor you’re planning to use will actually fit. You can get the key motor measurements from most manufacturers’ web sites. The tight spot on most frames is the distance measured along the slant of the seat post from the rear motor mount plate to the underside of the backbone. Even if this distance ends up being okay watch out that the upper wishbone tubes don’t drop down and infringe on this space in the motor bay. On our Paughco frame we’ve got 15” along the seat post but the wishbones come down at a steep angle effectively blocking the uppermost inch so we’re working with only 14” of clear area for the motor which is tight.

This frame checked out better than most with the exception of the forward right tranny mounting bolt location so we’re going to go ahead and start getting it ready to work on.

Unfortunately, many people skip these fundamental steps and then wonder why the finished bike doesn’t ‘track’ very well or steers ‘funny’. It’s all in the details and the up-front work whether or not you end up with a good machine. You’re going to get back exactly what you put into it.

In the next section we’re going to concentrate on detailing the frame.