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Aluminum Mount Points? Really?

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Aluminum Mount Points? Really?

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Old 03-08-18, 10:01 PM
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raywood
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Aluminum Mount Points? Really?

I am thinking of building a rear rack capable of hauling a heavy and/or wide load. Another thread has more information on what may go on the rack.

In a different thread, I looked at the option of installing a through axle, and basing the rack on that. That's still a possibility; it's just that the axle plus installation will cost $40-50.

A couple of people suggested just using the existing mount points. That would save that $40-50. I would probably have to drill a 5mm hole in the strut material shown in the photos. That approach would still have the structural drawbacks debated in that through-axle discussion.

What I'm seeking advice on, here, is another way of using the mount points. The strut (i.e., vertical rack support) material that I want to use has 3/8" holes. The mount point has a diameter of roughly 1/2" and sticks out maybe 3/16". I've thought about drilling out the strut (i.e., making it a 1/2" hole) so that it will slide over the mount point, then using 1/2" washers as spacers, followed by a fender washer, and a 5mm screw to hold it on.

When that idea came to me, I assumed the mount point must be made of steel or some other sturdy material. But a magnet doesn't stick to it, so it's not steel. I contacted Trek today, to ask about it. Their rep said the frame on this bike is entirely aluminum. It wouldn't seem wise to have a relatively thin steel strut pushing down on an aluminum mount point. It seems like the steel would cut into it.

But could the Trek guy be wrong? Would they really be running steel 5mm screws into aluminum threads? It doesn't seem possible. Surely the mount point is made of something tougher than aluminum.
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Old 03-08-18, 10:57 PM
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Doug64
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They are aluminum on my Trek and my Cannondale T2 touring bike. The T2 has been on several tours and carries a load quite well.

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Old 03-09-18, 12:01 AM
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The steel strut wouldn't be pushing down on aluminum, it would be bolted to it. How could that possibly cut the aluminum?
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Old 03-09-18, 12:18 AM
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if you are concerned about mounting the rack to just one
eyelet, you can consider using both eyelets.

get a chunk of aluminum or steel stock, cut and shape to
fit the dropouts, drill a couple holes to match the two
dropout eyelets. then pick a suitable spot on the metal
stock for your rack mounting hole.

this was using a pair of aluminum supports from a scooter
shop junk bin. configuration of the dropouts made it
difficult to mount the rack. total cost a couple bucks
for bolts and washers and locknuts.

could prolly do something similar with a pair of stainless
steel mending plates from the home depot.
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Old 03-09-18, 10:47 AM
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I think the only Trek that is steel is the 520, at least that was the situation several years ago. I assume it is still the case.

If you really are concerned about the strength of the frame where the rack bolts go, install longer rack bolts from the inside, pan head probably has less material in the head to make sure you do not get in the way of the cassette or chain. Then put your mounts on the bolts and use nylock nuts on the bolts. This way you are relying on steel nuts on steel bolts. And if a bolt breaks, you can unthread it from the frame.
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Old 03-09-18, 11:48 AM
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Originally Posted by raywood View Post
I am thinking of building a rear rack capable of hauling a heavy and/or wide load.
Looks to me like a safety hazard - a sharp edge waiting to cut your ankle or shin.

Why don't you simply use an inexpensive Al rack - many are cheaper than the materials you'd likely use to DIY. Your stays and dropouts are made of Al. You could spend more on something like a Tubus Cargo rack, but ultimately your Trek 7.3 frame is the weak point in this scenario. Tubus sells a QR mount adapter, which I doubt makes any difference over simply using the provided rack mounts. The QR nount adapter is compatible only with Classic Tubus racks, which appear to no longer be sold in USA. You'd have to purchase from a Euro IBS.

How much weight do you intend to carry? How much do you weigh - bear in mind your bike is probably rated for a total load of 300 lbs or less. Some of that load should be distributed to the front wheel if you wish to avoid hastening failure of the rear wheel, and having bicycle handling issues. Do you intend to fit a rack to the fork?

Have you considered a trailer, like the Burley Nomad or BoB Yak?

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Old 03-09-18, 11:48 AM
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Originally Posted by saddlesores View Post
if you are concerned about mounting the rack to just one
eyelet, you can consider using both eyelets.

get a chunk of aluminum or steel stock, cut and shape to
fit the dropouts, drill a couple holes to match the two
dropout eyelets. then pick a suitable spot on the metal
stock for your rack mounting hole.

this was using a pair of aluminum supports from a scooter
shop junk bin. configuration of the dropouts made it
difficult to mount the rack. total cost a couple bucks
for bolts and washers and locknuts.

could prolly do something similar with a pair of stainless
steel mending plates from the home depot.
I did something very similar to a bike that did not have rack mount. Fashioned a piece of flat aluminium.
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Old 03-09-18, 05:33 PM
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raywood
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Thanks for your replies. A few quick responses here:

Doug64 and Tourist in MSN: OK. I guess they really are aluminum.

Kontact: it was hard to explain well. The struts would be pushing down on the aluminum mount points, not bolted to it. The hole in the strut would be big enough to fit over the round mount point.

Saddlesores: I've thought about that approach. I was afraid I might be getting weird leverage, pulling out on the top mount while pushing against the bottom one. But your point remains: the load would be distributed. It would surely help. I have the additional problem that the two mount points stick out to different distances: I would need spacers on the lower one, and then the lower 5mm bolt would be carrying weight laterally. I've wondered whether a longer bar, P-clamped or U-bolted to the chainstay ahead of the axle, would help. With or without that, your suggestion would work if (a) I did go ahead and drill out the strut and mount it over (rather than screwing it into) the upper mount point and (b) the strut material, or some addition like you've suggested, could also be bolted onto the lower mount point.

Tourist in MSN: I think you were the one who mentioned that approach in one of the previous threads. It's a good one. I have to find 5mm bolts that are threaded their full length and long enough to go through the mount point and the strut. But that's not as much of a concern here, where they wouldn't go through the strut, but only through the fender washer.

Seeker in 333: thanks for your suggestions. They are more related to those previous discussions. I'm still watching those, too, in case any other possibilities emerge.

Zebede: that's a worthy alternative. I will have to look at the bike and see whether something like that will work for me.

To sum up, here's how it sounds. I guess the mount point really is aluminum. If I want to use it in lieu of an axle mount, there would be an option of sheathing it in relatively thin steel tubing, drilling an even larger hole through the strut, and sliding the strut over the tubing over the mount point. Otherwise, if I'm going to use this approach, I had better supplement it with a second mount point, using either the existing lower fender mount point with a bracket designed as Saddlesores suggests or a new supplementary mount point per Zebede.

I'll have to work on that. Thanks again.
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Old 03-09-18, 06:42 PM
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Originally Posted by raywood View Post
...
To sum up, here's how it sounds. I guess the mount point really is aluminum. If I want to use it in lieu of an axle mount, there would be an option of sheathing it in relatively thin steel tubing, drilling an even larger hole through the strut, and sliding the strut over the tubing over the mount point. Otherwise, if I'm going to use this approach, I had better supplement it with a second mount point, using either the existing lower fender mount point with a bracket designed as Saddlesores suggests or a new supplementary mount point per Zebede.
....
I am not exactly sure what your plan is, but I don't need to know the details. Sounds complicated. Are you sure you want to go through that much effort?

How much weight are you really talking about for what you put on your rack?

One manufacturer out there rates their heavy duty rack for 40 kg of weight with M5 bolts, but 60 kg with M6 bolts. My point is that they find that an M5 bolt is the limiting factor at about 40 kg. Even with an Aluminum frame, the way I suggested to run the bolts you would be taking stress off of the threading, the bolt and nut would be taking the stress.
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Old 03-10-18, 11:51 AM
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Had to make an adaptation to the front lowrider upper mount, being out of even their spec... Specialized - from Japan.

their fix was a piece w just 2 holes . Mine had 3.... 1 to fork 2 to rack so as to always be stable

not rotate around 1 loose bolt.
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Old 03-18-18, 02:16 PM
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raywood
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I went ahead with the thing I was trying to describe. Detailed writeup and pictures in another post.
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