· Premium Member
I think your Siemens engineer friend has the right idea, but he (and I) didn't go far enough. More below.I have a friend who is an Engineer with Siemens rail automation and i had a long chat with him about this problem (yeah you scared me a bit) He did some digging for me and came back with his idea, that the loading is more about stacking too much weight behind the rear wheel. With an amount of weight in the box, rider + passenger the risk is NOT the rack breaking but more that you might be going up an incline, give it some juice and end up with the bike on top of you. Structurally speaking he thinks 20-30kgs without a passenger would not be an issue at all but without knowing more he would NOT guarantee it, only his opinion. Having said this, i will carefully watch the frame and rack for any signs of failure. When i have a passenger i will be using the box for the 2 helmets more than anything else, when i travel i plan to put anything with weight in a bag strapped between the box and myself and use it for some back support.
Yes, I agree, this is important, but the specifications for the bolts holding the rack vastly exceed the static loading I postulated. Key word, static. More below.It's not just the loading (inside the top box) that matters, but rather the lever effect which may break the motorcycle's tail.
✋ Using the diagram above (rear frame): the further the load is from the screws 3 / 4 the higher the risk of breaking the tail is.
See moments & levers... to understand how it actually works.
Yes, I saw that, my manual discusses a "topbox" with a 5kg max payload and a "topbox light" with a 3kg max payload. Since both are allowed, then the heavier of the two is the best choice to use as the rack's max payload (i.e., the above 3.12 kg + 5 kg = 8.12 kg). Also, I checked that all model years from 2016 to present have the same part number for both the luggage rack and the rear frame, so model year isn't an issue.The user manual for my ym. G310GS says maximum 3 kg in the topcase.
From the page 108 "Topbox" section of my manual...
From the page 109 "Topbox Light" section of my manual...
...continued on page 110...
Now, the rest of the story...
After reading the scenario postulated by D1v1ine's Seimens engineer, it occurred to me that the most strenuous scenario isn't necessarily flipping the bike backwards. The most strenuous scenario is hitting a pothole at high speed, the kind of pothole that might bend a rim. Such an impact would greatly magnify the effect of the combined weight/moment of the topbox and contents on the rack and rear frame as a lever arm. In this scenario, particularly after repeated potholes over time, I think stress fractures followed by failure is entirely possible.
So, summarizing, do watch the frame and rack for signs, but be especially diligent after a significant pothole or pothole like event. If I were you, periodically, but especially after a hard pothole like event, I would expose the rear frame by removing the plastic panels covering it; have someone hold the bike upright by sitting on it; and I would put my full body weight on the rear of the rack while looking for cracks. Of course, as you friend postulated, be cautious that the weight might make the bike more likely to wheelie or, in the extreme, flip over backward. Last, as mentioned by @ARBOLMANO above, but also worth repeating, be cautious as you ride with it the first few times, that the airflow and top heaviness may cause changes in how the bike handles, especially during abrupt changes in speed and direction and/or windy conditions. If I were you, I would load up the topbox, find a safe place, and practice maneuvers to see how the bike behaves with the extra weight in that location, slowly and incrementally working your way up to full speed emergency maneuvers (e.g., braking in a straight line, braking on a curve, accelerating, swerving). Watch out for hard swerves, significant topbox weight might make the front tire skip. I've done this a few times; I haven't gone down because of it, but it's truly a heart stopping moment if you do it yourself or if you have to watch as one of your students does it (I've done both).
I think I've reached my limits on this; I hope it helped. JerryG