Folding Bikes - Fall project: Downtube Mini Rebuild

OK, so a few weeks ago my Downtube Mini frame cracked at the seatpost. Yan was kind enough to send me a replacement frame - that's what I call service! :thumb: Glad to see Downtube stands behind their products.

Meanwhile I bought a used Dahon D7 so I'm still rolling while my Mini is out of commission.

For the moment, I'm assuming that I'm rebuilding this Mini to keep not trade. I can do some of the work myself, but I don't have tools or experience dealing with the bottom bracket or headset.

A few questions to start with:

1. The paint on the Mini chips off if you look at it funny. Now that I have a new frame, is there a cheap and reliable way to protect the finish, i.e., some kind of clearcoat that I can spray on? Sandblast + powdercoat would do it, but costs a lot in NYC.

2. Old frame has a 27.2mm x 500mm seatpost. New frame accepts a 30.4mm OD seatpost. I will have to get a shim, or a new seatpost. Hard to find long enough seatposts (Downtube does sell them.) I think I'll try to find a long shim and use it. Any recommendations?

3. I plan to move as many parts as possible over from the original Mini. I can do a lot of the work myself, but I do not have the tools or experience to deal with the bottom bracket or headset. Are those parts worth the cost (mainly LBS labor) to move over, or should I buy new parts? They seemed to work perfectly well on my Mini, but I do not know the quality of the OEM bottom bracket or headset. If I would get noticeably better quality for not a lot of money, I'd upgrade.

Thanks!

Dan

A few questions to start with:

1. The paint on the Mini chips off if you look at it funny. Now that I have a new frame, is there a cheap and reliable way to protect the finish, i.e., some kind of clearcoat that I can spray on? Sandblast + powdercoat would do it, but costs a lot in NYC.

Can't help you here. Although I would not do it myself.


2. Old frame has a 27.2mm x 500mm seatpost. New frame accepts a 30.4mm OD seatpost. I will have to get a shim, or a new seatpost. Hard to find long enough seatposts (Downtube does sell them.) I think I'll try to find a long shim and use it. Any recommendations?

What model year do you have?

I would say that it is impossible to find an alternative other than some unicycle/BMX seat posts and they generally have a different saddle mount.


3. I plan to move as many parts as possible over from the original Mini. I can do a lot of the work myself, but I do not have the tools or experience to deal with the bottom bracket or headset. Are those parts worth the cost (mainly LBS labor) to move over, or should I buy new parts? They seemed to work perfectly well on my Mini, but I do not know the quality of the OEM bottom bracket or headset. If I would get noticeably better quality for not a lot of money, I'd upgrade.

Thanks!

Dan

The BB is a standard sealed bearing square taper BB. You will have to get the appropriate tool to remove/install one which is generally inexpensive. A new Shimano UN54 BB -- I think that the crank takes a JIS square taper BB; but you would want to double check with Yan to make sure -- will be about $20-30. I would keep the BB unless it feels crunchy. Oh ... if you call Yan about the ISO/JIS type of bottom bracket, you might as well ask about the axle length too. Although the old BB is probably labeled appropriately.

Moving the headset will be more of a pain. You will have to knock out the pressed cups from the head tube -- tool #1 -- but my guess is that you will not have to move the crown race over by using the same fork. Anecdotally, a lot of people simply buy a new one. If you get a tool to install the cups, it is expensive. There are do-it-yourself tools, that apparently work well. If you are not going to install and remove headsets regularly, I would take it to the shop.

invisiblehand, thanks for the info. I have a 2009 model Mini with a 2008 model handlepost. This thread (http://www.downtube.com/Folding_Bike_Forum/read.php?3,461) tells me that the Mini uses a 114x68 bottom bracket. Not sure how common that size is. The Shimano UN54 BB is available in 113 or 115mm, not 114. Given that the chainline on the Mini is not perfect, perhaps a different width would be an improvement. As for the headset, I won't be doing this frequently, so good advice to take it to the LBS.

My memory is fuzzy, but I think that my 2007 Mini has a 30.4 mm diameter seatpost.

The Shimano UN 54 BBs with 113 and 115 axles have the same drive side length. The 113 is asymmetric whereas the 115 is symmetric.

If I remember correctly, given the stock setup, you would want to bring the crank in a few mm for a better chainline.

Old frame has a 27.2mm x 500mm seatpost. New frame accepts a 30.4mm OD seatpost. I will have to get a shim, or a new seatpost. Hard to find long enough seatposts (Downtube does sell them.) I think I'll try to find a long shim and use it. Any recommendations?

Don't you hate that? Seems to be a typical design evolution for folders, but as you noted there's much more selection for 27.2mm posts.

Interesting how yours cracked at the same place as mine. Yan included a 30.4 seatpost with my new frame, but no updated seatpost collar. Double check the packaging to see if a seatpost is buried in there (mine was wrapped so well I almost threw it away with the packaging and the box)

No seatpost, nor seatpost collar, in the box.

Another Bikeforums member who had a DT 8H in need of a seatpost contacted me and we've worked out a deal where he buys my 27.2 seatpost, and I buy a 30.4 seatpost from Yan. I don't know where else one would get a 27.2 x 500mm seatpost, so this is a win-win all around.

Are you saying I'll need to buy a 30.4 seatpost collar, or will my 27.2 seatpost collar work OK? I was hoping to reuse. I have no idea where to source a 30.4mm seatpost collar.

Hmm, looks like the OD of my new seatpost tube is 34.9 or 35mm, and seatpost collars are available in that size.