The importance of a good motor mount

(May 31, 2015) I’ve been very limited in my riding since early May due to a bulging disk in the lumbar (lower) region of my spine. The problem started last summer when I rode 7,400 miles in 13 days, from Cary, North Carolina to Banff, British Columbia and back. However, I suspect it was more than merely the number of miles in such a short time that injured my spine.

Somewhere just short of the 100,000-mile mark, my original front motor mount failed (rubber pad disintegrated) and the upper mounting bolts fell out, or that order reversed. The resulting vibration to the frame likely exacerbated the stress to my spine. In August last year, Kevin at Jockeys Cycle replaced that original front motor mount with a similar product from Drag Specialties (DS-243515), which developed cracks in less than a year.

Excess vibration from the engine to the frame has been a very serious problem, and not just because it's hammered my spine: I’ve had my upper front motor mount bolts fall out multiple times in one year, even though they were torqued down properly with copious amounts of thread locker applied. Over the last year, I've had many other fasteners come loose with some falling out completely.

To be fair to Harley-Davidson and Drag Specialties here, I will add that I’ve installed S&S heads on my bike with a Vance & Hines two-into-one exhaust system. The heads in particular may have significantly altered the vibration patterns of the engine, thus leading to accelerated failure of the motor mount and bolts. However, even if this is the case, it’s an indictment on the quality of the Drag Specialty motor mount in particular that it should deteriorate so quickly.

Given how many other fasteners on my bike came loose over the last 12-18 months, and as I wrote last July, my horn bracket cracked for no apparent reason, I’m convinced the worn motor mount(s) have been at least part of the problem.

Here’s my logic: (1) the rubber dampening pad in the old motor mount cracked so badly that it no longer provided the vibrational damping for which it was designed between the engine and frame; (2) a resonant frequency was achieved — at least at intervals — between the engine and the upper motor mount bolts (see two photos), which caused the nuts to either vibrate all the way off and the bolts to fall out, or the bolts to shear apart and fall out; and (3) the loss of the upper motor mount bolts then permitted excessive vibration to transfer from the engine to the frame and my spine over thousands of miles last year.

Aluminum versus Steel

The Harley-Davidson specification for these upper mounting bolts calls for a 3/8” diameter, inserted from the bottom up with the nuts fastened topside. The holes in the engine block are smooth, unthreaded. When my friend Mike Russo and I inspected the remaining temporary bolt Kevin had inserted, we found that it was a loose fit. I managed to twist in a 7/16” instead. On further reflection however, we concluded that just because we could fit a 7/16” didn’t mean we should, even though I am desperate to get a snug fit to avoid losing anymore of these fasteners.

What we figured, and many of our colleagues agree with this assessment, is that the bolts must have some play in the fitment because they are steel in an aluminum block. These metals have very different coefficients of thermal expansion, and when they cool, the aluminum cools faster, and thus contracts faster than the steel. If the steel bolts fit tightly at room temperature, then they will be too large when hot in a rapidly cooling aluminum engine block. They could thus crack the block.

The problem is, of course, a loose fit for these bolts—which will be that way at least until the engine gets hot—invites excess lateral vibration (shear) across the diameter of these bolts. I suspect that rather than the nuts coming loose and the bolt(s) falling out, the heads of the bolds sheared off and the rest of the fastener simply fell out. In the photo at right, you can see the new, correct, Harley-Davidson bolts inserted bottom-to-top. However, two of the three bolts that went missing since last July were the ones mounted underneath the oil filter, and they were inserted top-to-bottom. The bolts in that position fit so tightly under the oil filter, it’s hard to imagine how they could have come out; there’s simply not enough space between the bolt head and the oil filter.

Regardless, we have learned to keep an eye on these motor mount bolts. I also learned that it’s no big deal to remove the oil filter—especially the K&N filter with the hex nut welded on the end—and reuse it in order to get at the right-side motor mount bolt. Just be sure to put an oil pan underneath first and add a little bit of oil at the end of the job. The voltage regulator comes off easily enough as well, especially if you don't have to remove a chrome cover first. (Until we had all of this resolved, I left off the chrome voltage regulator cover.)

Glide Pro Motor Mount

In order to address the root cause of this vibration problem—as I see it—I purchased a Glide Pro front motor mount for my bike. It cost me a $100+, which I’ll admit seems excessive for such a simple thing as a motor mount. However, given my experience over the last year, motor mounts don’t seem so simple to me anymore.

As you can see in the accompanying photos, the Drag Specialties motor mount I bought new and installed just last July had already developed significant cracks, top and bottom, in the rubber damping pad. Were these cracks caused by excessive vibration?

Glide Pro uses an entirely different material and a different design for its motor mount. I have already noticed a huge reduction in vibration in the footboards at most RPMs, especially in the range (2,800 - 3,000 rpm) I achieve when riding on the highway at 70+ mph. Kevin also said he noticed a significant reduction in vibration when he took the bike for a test ride. (We finally found the source of the oil leak and he and the guys at Jockeys successfully repaired it…a topic for another blog post.) As always, your mileage may vary.

Keep the shiny side up and have a great summer!

-The Long Rider

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