Help me stop my motorcycle's number plate cracking
July 27, 2011 12:12 AM   Subscribe

Help me stop my motorcycle numberplate cracking.

For the second time in only a few years, my bike's numberplate is developing cracks near the bolts which hold it on. A picture of one of the new cracks is here, there's an identical one on the other side underneath the other bolt. The bolts hold the number plate onto a heavy aluminium plate on the rear fender.

It's a single-cylinder bike so there's substantial resonating vibration, and it shakes the plate visibly and violently between 1,100rpm (idle) and about 3000rpm. I'm willing to take suggestions about how I might be able to damp it or delay the cracking.
posted by Fiasco da Gama to Technology (7 answers total)
I can't access the pic, but do you have rubber grommets between plate and bolts? washers won't dampen enough.
posted by artdrectr at 12:27 AM on July 27, 2011

Response by poster: The image should be visible now.
posted by Fiasco da Gama at 12:35 AM on July 27, 2011

That kind of looks like a tear. Almost as if the bottom of the number plate has been pulled out. Does it catch the wind? Are your attaching bolts going in at an angle that might crease the plate?

I reckon your first step should be to cushion the vibration, keep the bolts off the number plate, and the number plate off the mounting plate. So seconding rubber or plastic grommets, o-rings or washers. Try and find something that fits snug around the bolt, and has enough depth to cushion stuff really well even after you've flattened it by tightening up your bolts.

Also, to cover the possibility of flex in the number plate - either from wind or the mounting plate flexing under the number plate, try attaching the number plate first to another flat rigid metal plate (at multiple points), then attaching that assembly via your plastic/rubber washers to your mounting plate.
posted by Ahab at 1:48 AM on July 27, 2011

If your plate is like most it is only attached by two bolt a at the top. Adding a couple of bolts at the bottom or getting a bracket that otherwise supports the bottom will help immensely. This is a common problem for those of us who ride 45 degree v- twins as well, so looking at aftermarket license plate brackets for Harleys might get you something more functional.
posted by TedW at 4:00 AM on July 27, 2011

Ahab has the best idea about mounting it to a less flexible piece of metal. It has all the tell tale signs of a stress fracture from the wind. It is very unlikely that it is from vibrations.
posted by JJ86 at 6:17 AM on July 27, 2011

I've had two different bikes with this problem. On the first, I could only mount it at the top, so I used beefy zip ties instead of bolts. Every couple of years one would break and I'd replace it, but the cracking stopped. On my other bike, I modded the mounting so it pulled the plate out of the wind, and securely rubber mounted with metal washer backup to the rear fender. It also stopped cracking.
posted by gofargogo at 9:24 AM on July 27, 2011

Response by poster: Thanks all. I've just ordered another new plate from the RTA, and I'm going to do a bit of shopping for some grommets and washers and/or an aftermarket bracket.
posted by Fiasco da Gama at 4:56 PM on July 31, 2011

« Older Help me catch trout   |   How to determine polarity of electric wires. Newer »
This thread is closed to new comments.