Bicycle Mechanics - Why huge holes? Rack mounting question....

Ok, I give up. This is the first bike I've seen that has huge holes, about half-inch diameter at the rear triangle ends.

I would like to mount a rack, but these holes were made for something else. I asked the mechanic on duty at the local bike shop, he says they are for the disc brake setup.

I looked at other GT brand bikes and noticed the ones that have disc brakes are using the two smaller holes shown, so the guy had no explanation afterwards.

So, can anyone edumacate me on the purpose of having big-arse holes at these locations instead of the normal smaller holes?

Thanks....



http://green427.smugmug.com/Miscellaneous/Bicycling/i-MpTKVqC/0/XL/P1020412-XL.jpg


http://green427.smugmug.com/Miscellaneous/Bicycling/i-T6fqPnv/0/XL/P1020413-XL.jpg

Post a link to the bike model.

Maybe this frame uses the same dropouts as a full suspension model? The big holes would be used for the pivot bushings. They look kinda kool on a hardtail...

Obviously to save weight. Drill a few more yourself and you'd be surprised how light your bike can get.

Those are not functional rack holes.

http://www.gtbicycles.com/nomad-sport

http://www.gtbicycles.com/nomad-sport

Thanks for the link.
No clue.
Guess you should use the contact link and ask the GT folks.

That looks like a job for Wellnuts.
http://www.spaenaur.com/pdf/sectionC/C147.pdf

I think the two smaller holes on the drive side might be for disc brake (not positive though). But the lack eyelets on the drive side tells me the bike is not equipped for rear rack mounting, unless you use the skewer type of mounting like the one available here (http://www.thetouringstore.com/TUBUS/Fit%20Solutions/FIT%20SOLUTIONS%20PAGE.htm). Seems odd since on the page for the bike that is linked to it says "all utility braze-ons."

I wouldn't trust the published specs. they say 7000 series aluminum but the decals on the seatstay say 6061.

I don't think these holes provide any funtionality.

I wouldn't trust the published specs. they say 7000 series aluminum but the decals on the seatstay say 6061.

I don't think these holes provide any functionality.

Well, they do, but only in a structural sense. They make the welding area larger, strengthening the joint between the dropout and the chainstay/ seatstay. Great place to accumulate mud, too.

They make the welding area larger, strengthening the joint between the dropout and the chainstay/ seatstay.

I also think that this is the reason for the small sections of tubes (the two bigger holes on each side) in the dropout area - larger area for the welding.

The two smaller holes present only on the left side (above the seatstay) are clearly IS disc brake caliper mounts.

Jeff has it - it's obviously just a lightweight way of providing a decent amount of surface area for welding that's amenable to various frame geometries.

You should be able to use those holes to mount a rack with a bit of ingenuity. Maybe just a couple of large washers and some innertube wrapped around the mounting bolt.

Jeff has it - it's obviously just a lightweight way of providing a decent amount of surface area for welding that's amenable to various frame geometries.

Notice the small holes to vent welding gases? With these dropouts, the manufacturer wouldn't need to drill/punch holes into the stay tubes. I also suspect that the large holes position the dropouts onto a welding jig...

That too, but I'd say the former is a secondary benefit.

From the pictures, the holes on the frame are cosmetic / design, and the frame as shown is not designed for using racks.

The OP needs a different frame if he wants to use a rack; or use a QR system as noted in post #9.

Looking closely at the photos on the GT website, some frames with these dropouts seem to have a insert in at least one of those holes where it looks like a rack could possibly be attached.

203055

http://www.bikeforums.net/attachment.php?attachmentid=203055&d=1306162113

The OP needs a different frame if he wants to use a rack; or use a QR system as noted in post #9.

Bah! Sif get another frame or marry your wheel to the rack when you have these holes waiting to be filled.

It's so you can mount the derailer upsidedown,to keep from bashing it on rocks...

I'm guessing it's for production,set up in a jig and being able to use 1 forging for many frames angles.

Just put a bushing in it and use it for rack mounts.Alot stronger than a brazeon,you'd have to tear the whole dropout off for it to fail.

Perhaps you can use a bit of ingenuity, if you want to use it as a rack mount.

How about a rubber bushing with a smaller bolt thru it?

How about a nylon bushing with a hole in it?

it's so you can mount the derailer upsidedown,to keep from bashing it on rocks...

pmsl : D

Perhaps you can use a bit of ingenuity, if you want to use it as a rack mount.

How about a rubber bushing with a smaller bolt thru it?
Well yeah. Kinda like the dodads suggested in post #8. I've seen them used in similar applications.

a length of rubber hose and some nuts bolts and washers.. a variation on a motor mount in your car.
sleeve just the right length around the bolt would expand the rubber tubing enough to fit tight
Nylock nut will keep it there

Dan's widget maybe, but the simpler pieces come locally.

A nylon plastic bushing may work.
If you want to be really tight and secure, take one drilled out tube of nylon. Cut the tube in two at an angle so that compression will cause expansion (like and old style quill stem).
You will need a washer that contacts the frame tube.

Thanks to all for the replies & suggestions. There are bolts on the seat tube for a rack, but nothing down there. I have several ideas, will go to the hardware store and check out my options.

I agree that these holes are there for ease of manufacturing.

wood dowel with a hole drilled in it

Update:

I found some leftover aluminum stock brackets that I made for my motorcycle, but no longer using. They are lightweight, and the rack I had does not fit the bike well, so I improvised. Commuted to work today, they seem to be working out well:

http://green427.smugmug.com/Miscellaneous/Bicycling/i-zR686n3/0/L/P1020424-L.jpg

I feel like those will pivot downward with any weight on the rack. I made some in a triangle shape for one of my bikes.

I feel like those will pivot downward with any weight on the rack. I made some in a triangle shape for one of my bikes.

That is one of my concerns, however, I tried to see if excessive weight would make them move, but it would require some bending of the rack's parts. Will give it time and see what happens.