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Old 09-10-10, 03:22 AM   #1
Chris_W
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OK to trim frame's seat tube by 5mm (incl. pics)?

I'm trying to use a seat-post mounted rack (a Topeak MTX BeamRack) on a Specialized Tricross Comp frame. The problem is that I'm also using a Cane Creek Thudbuster ST suspension seatpost and so there is not much spare seatpost showing; I would have to move the seatpost up by 5mm to make enough space to clamp the rack to, as I've done for the picture below, but the seat is then too high for me. So I was considering trimming off the top 5mm of the frame's seat tube. The seat tube is aluminum (as is the seat post). Is this a completely stupid idea?



It would seem that the only effect trimming the seat tube would have would be to shorten the slot in the top of the seat tube that allows the seat-post clamp to work (sorry, I don't know the proper name for this, see pic below). This slot is currently 41mm long; on other aluminum frames that I've measured this can be 30-35mm long, so it seems that cutting off 5mm from this would still keep it at a decent length (approx. 36mm). The seat post has loads of extra length extending into the seat tube (I'm about 10cm above the min. insert mark), so this is not a concern. I understand that I'll invalidate the frame's warranty by making this modification. I have a cutting guide tool and I've cut many fork steerers, seat-posts, and handlebars before without any problems.



Before doing anything stupid, I wanted to ask some more knowledgeable people - so please let me know what you think. Thanks.



(BTW, I know this frame has mounts for a proper rack, which I often use, but I'd like to have the simplicity of the seatpost mounted rack for small loads and for the ease of fitting and removing it).

Last edited by Chris_W; 09-10-10 at 04:46 AM.
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Old 09-10-10, 08:24 AM   #2
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It depends upon how you feel about having a frame with a warranty, which you mentioned. However, if there was any one place on a frame, other than the steerer tube, that I had to trim, trimming that location would make me the least nervous. Just make sure the cut is clean and properly finished. I would also put duct or masking tape on nearby areas of the frame to avoid unwanted damage.

Pete

Last edited by mudboy; 09-10-10 at 08:26 AM. Reason: thought it was a CF frame, it's Al.
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Old 09-10-10, 09:08 AM   #3
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the issue you might run into is seat post retention. If there is any hit of slipping, cutting 5mm off is going to exacerbate it. It almost looks like your clamp isn't all the way down
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Old 09-10-10, 09:28 AM   #4
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I've had a few different 27.2mm seatposts in this frame, and have had no problems with any of them slipping despite never tightening the clamp bolt all that much - the seatpost fit is very good.

The clamp is certainly all the way down, and you can see a light ring marked on the seat tube in the 2nd photo showing where the bottom of the clamp always lies. The clamp is not exactly low-profile, but even with a different clamp I don't think I would gain any more than 1mm, maybe 2mm, but that would not be enough, I need at least 5mm.
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Old 09-10-10, 01:44 PM   #5
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could you take 5 mill off the rack 2.5 on each end?
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Old 09-10-10, 04:11 PM   #6
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Doesn't the Tricross have rack mounts on the frame?
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Old 09-12-10, 12:49 AM   #7
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Thanks for all the responses.

squirtdad - Taking the extra off the rack is worth thinking about. There is a lot of material at the top and bottom of the rack - it would be a huge amount of work and not easy to make it clean and look nice (I don't have any power tools). Plus, I'm worried that it could compromise the clamp's strength, so I'd rather not do it.

jmichael - Yes, it does have rack mounts, but read the last line of my original post.

I've thought of using cranks that are 5mm shorter, which would allow me to raise the seat by that much. However, I've decided that I don't want to change that without a good reason (and it would be the most costly option). Plus, since it is a cyclocross bike, the BB is reasonably high and so the seat is already quite far above the ground and I'd prefer to not make the reach to the ground any longer when I stop the bike.

I've been measuring a few more of these slots on other aluminum bikes, and haven't found one that is more than 35mm yet, some are as short as 28mm.

Since no-one has told me that it is a stupid idea, I think I'm going to go for it and make it as clean a job as possible.

Last edited by Chris_W; 09-12-10 at 12:53 AM.
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Old 09-16-10, 03:06 PM   #8
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"Taking the extra off the rack is worth thinking about. There is a lot of material at the top and bottom of the rack - it would be a huge amount of work and not easy to make it clean and look nice (I don't have any power tools). Plus, I'm worried that it could compromise the clamp's strength, so I'd rather not do it."

Better for the rack to break than the frame, surely?

Could you find a saddle with lower rails so you could raise the post the extra 5mm without increasing the height of the top of the saddle? That way, nothing gets cut.
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