Adding height adjustment wheels to the bridge of the double bass or similar instrument makes it easy to keep the action at an optimum height. The aluminum wheels are readily available from violin parts suppliers. Here is one way to install them and a little bit of probing into the question of whether it is a good idea to install then at all.
Last updated: September 11, 2018
The short answer is that they might, but a better question may be to ask if adding the adjusters is detrimental to the tone of the instrument. Andrew Brown has published a paper entitled “An Acoustical Study of Double Bass Bridge Height Adjusters” which appears to indicate that adding adjusters does audibly change the tone of the instrument. It is the only work I can find that attempts to shed some light on this question. The study recorded and analyzed samples taken from a bass onto which was fitted a number of different styles of adjusters. Unfortunately the paper does not specify how or if a number of independent variables were controlled in the study, and lacking that it is difficult to conclude from the paper alone the extent to which the tone of the instrument was modified by the addition of the adjusters, since any of these other things could easily affect the tone. No indication is presented on how distance from the instrument to the microphone, bow speed and pressure, point along the length of the string bowed, etc. were controlled. Nor does the paper indicate whether the structure of the bridge itself was controlled for (i.e., if the same bridge was used for all of the adjusters tested) or how the fitting of the bridge(s) was controlled for - simply removing and replacing the same bridge could result in tonal changes. So it is not at all clear to me that one could conclude from this study that one could expect a change in tone with the addition of adjusters. But again, assuming you can expect a change, the real question is whether or not that change would be detrimental to the tone of the instrument. Data from Brown's study on the prevalence of bridge adjustment wheels on basses in North America (60 – 80%) may well indicate that no seriously detrimental change in tone is evident. Or, it may indicate that the convenience of having adjuster wheels outweighs any perceived change in tone.
I always fit the bridge to the instrument first, if it has not already been fitted. There are three potential starting places for this operation – where the bridge already fitted to the bass will be modified; where a new bridge will have adjuster wheels added to it for a bass that is currently fitted with a bridge; and where the instrument has never had a bridge on it before. It the latter case I recommend temporarily fitting the instrument with a spare adjustable bridge, just to get an idea of how tall the new bridge will have to be. Use this information when you scribe the feet when fitting the bridge to the bass. In the case where the bass already has a bridge and it is going to get a new bridge with adjusters, you can usually align the old bridge and the new one and then trace the arch of the undersides of the feet onto the new bridge. I use some wedges of wood between the bridges to line up the bottoms of the feet:
And then mark the arches of the bottoms on the new bridge using a pencil. The short bridge pictured is for one of my Canotto Savart style double basses. Note that, unlike typical bass bridges, this one is tapered equally on both broad faces.
Once the feet are marked you can rough them out using the bandsaw or a sanding drum. Final fitting is for the Canotto bridge is done by taping a piece of sandpaper onto the top of the instrument and pushing the bridge back and forth to sand the bottoms of the feet so they mate with the top. I like to keep the exposed part of the sandpaper just a little wider than the span of the feet. This provides a “track” which keeps me from moving the bridge side to side too much.
When the sanding dust trails are each as wide as the bridge feet you can check the bottoms of the feet to see if they are uniformly touched by the sandpaper. If not, more sanding is required.
I can get away with this technique because the Canotto doesn't have a sound post. If you are fitting a bridge to an instrument that has one, you'll have to use the more traditional chalk and scrape method. After the feet are fitted then you can trim the top of the bridge to the proper height.
Once the bridge is fitted to the bass the adjusters can be installed. They are generally installed in the middle of the leg part of the bridge. For short bridges like the one I'm working with here, they have to be installed near the bottom of the legs, high enough up so that the unthreaded part of the adjuster can fit into the foot. I like to leave about 1/4" of extra clearance between the bottom of the unthreaded post and the bottom of the foot. Although for standard height bridges the adjusters usually go in the middle of the legs, I like to install them lower in those bridges as well. To my ears, this makes adjuster wheel pickups like the Fishman Perfect Circle sound better.
A bottom cut line is drawn at this point across both legs of the bridge. Carry the line around to the other side of the bridge as well. The wheel portion of the adjusters I am using here are 3/16" thick, but I am going to mark the line for the top cut 1/16" higher than that. What this means is that, when the adjusters are installed and screwed all the way down, the bridge will be 1/16" shorter than it used to be. This is done so that the action can be lowered as well as raised. You may want to allow even more slack here than that (especially if it is winter time and the bass will live in a place that is hot and humid in the summer), but it is not a good idea to get too carried away. The top cut line is drawn parallel to the bottom cut line, on both sides of the bridge. I've marked the waste area here, too.
Next, the horizontal center of each of the waste areas is marked and then extended down to the bottoms of the feet. This is crossed with a line at the center of the thickness of the feet, to indicate the two points where the holes for the adjusters will be drilled. The holes will be drilled at the center of the legs, not at the center of the feet, which may not be the same. I usually don't actually draw the lines out on the bridge but here they are drawn for clarity.
The next thing that needs to be marked is the depth of the holes. Placing one of the adjuster wheels in position over the bridge, you can place a mark on the bridge at the point where the threaded part of the adjuster ends. Put another mark approximately 1/4" from that one, to leave room in the hole for the end of the tap which will be used to thread the hole.
The deeper mark indicates the depth that the hole will be drilled. If the bridge you are working with is shorter than a normal double bass bridge, it is possible that this would be too deep a hole. If that is the case some of the threaded portion of the adjuster will have to be cut off and the depth of the hole adjusted accordingly.
The holes will be drilled using a drill press. The bridge must be clamped upside down for drilling. I use a right angle plate, which makes it easier to be sure the bottom of the bridge is perpendicular to the drill bit. The bottom surfaces of the feet are aligned with the top surface of the right angle plate, and a small wedge of wood is placed between the plate and the bridge to keep things straight when viewed from the side, if this is necessary. You can see the wedge in this picture:
Again the bridge I'm working on here is tapered on both broad faces so the wedge is necessary to keep the bottoms of the feet perpendicular to the drilled holes. We need to drill two holes on the same axis in each foot. The first hole goes to the depth previously marked. This 3/16" hole will be tapped for the threaded portion of the adjuster in a later step. But while everything is lined up to drill that hole, we also want to drill a shallower 1/4" hole just through the foot on the same axis. So, when setting up to drill, be sure that you have enough room to chuck and un-chuck both the 3/16" bit and the 1/4" bit without disturbing the clamped bridge.
Chuck the 3/16" bit, align it with the center point on one of the feet bottoms, and drill to the depth marked on the face of the bridge. Then, chuck the 1/4" bit and drill to the depth of the waste portion that will eventually be sawed out from the leg. In the following picture I'm about to drill the 1/4" hole, and the bit is marked for depth with a piece of masking tape.
Repeat the drilling process for the other foot.
The next step is sawing the waste portion out of each leg. Because we're sawing across the end grain and I don't want to blow out the grain while sawing or flattening the cut surfaces I generally just saw the feet off through the waste section on the bandsaw. Since the bridge is wedge shaped, be careful that the bottom of the blade will remain within the waste area.
The new tops of the feet and the new bottoms of the legs can be straightened up any way you like. I use the belt sander to make quick work of it and avoid blowing out the grain.
Once everything is straightened the holes in the legs can be tapped. Use a 1/4” - 20 tap in a tap handle to tap these holes.
Make sure to clean out these holes after they are tapped. Before assembling the bridge the entries of all holes should be chamfered a bit. Then all sharp edges should be chamfered too, using either a scraper or fine sandpaper. Any pencil marks remaining on the bridge can be sanded or scraped away, too.
The bridge is ready for assembly. Thread the adjuster wheels into the legs using a little beeswax on the threads, and push the non-threaded ends into the holes in the feet. Be sure to get the feet on the right legs!