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PostPosted: Thu Jul 02, 2020 10:59 am 
Dirt Disciple

Joined: Tue Apr 28, 2020 9:38 am
Posts: 21
Hi All,

I am planning to build myself a frankengravel (or sixty-niner monstercross if you like) bike and I would appreciate your thoughts on whether it's even worth building something like this and if it is, which frame to pick.

Build option 1:
- Take my (very) old Mongoose NX 7.1 26er frame that's about a size smaller than I need; Frame weight is 1.9kg.
- put a 29er carbon fork up front with a 700c wheel & disk brakes;
- Change rear wheel from 26" to 27.5" (the frame has enough clearance); disk brakes.
- Install flared drop bars (on one midge) and brifters (I have a spare set of 3x9 Micronew).

The brifters have road brake pull, but I have Avid BB7 road calipers to match.

Build Option 2:
As above, but use my mate's old noname frame that's similar size and geometry as my NX7.1, but can only comfortably clear a 26" rear wheel and only allows v-brakes. On the plus side, the frame is 500g lighter and lack of rear disk brake will probably save another 200g. I'll need to get something like a travel agent pulley to be able to run rear v-brakes with brifters.

Which frame would you use? Would you have any preference of 27.5x29 vs sixty-niner? Is ~700g of weight worth the saving? Anything obvious I am missing in the build that I need to think about?

Thanks.


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 Post subject: Re:
PostPosted: Thu Jul 02, 2020 11:21 am 
Retro Guru

Joined: Tue Dec 21, 2010 6:31 pm
Posts: 1164
Hey,

I love this kind of stuff :-) I've done a few of these types of conversions with varying degrees of success.

A few comments:

For the 27.5/29 bike: why not just go 27.5 front and back? A 27.5 up front might give you a bit more room for a fatter tyre
than 29 would.

For the Midge bars: take care that you can get the Midges up high enough. I tried a pair of Midges and while they ride nicely
in the drops they were horrible for me in the hoods and ramps. If only the drops are usable, you'll want to get the bars up really high so they are comfortable. Getting up high is hard with an old school MTB frame with a low front end. On One does a good cheap gooseneck stem that can help.

For your mate's 26er frame, you could still go 27.5 if you get a set of BMX v brakes with long slots for the pads. I've used Promax P-1 brakes successfully for this. They give you room to move the pads up to match the 27.5 rim. For some reason,
short pull brake levers seem to work better than v brake levers for this. Something to do with cable pull. So you might even be able to use your road levers. Be careful that the brakes still work. If you succeed with the 27.5 conversion it opens up a world of nice gravel bike tyres, I can get up to a 50mm tyre in my 26" mtb frame. It does raise the BB up a bit, but not excessively.
The BB on my 27.5 conversion is about the same height as my cyclocross bike.

Some people use certain types of cantilever brakes that also allow for pad adjustment, but it doesn't work for me. I took a bike set up like that for a test ride in the forest. I missed a turn and when I grabbed my brakes I only stopped when I ws about 20m further down the road. Not good.

Finally, if you want to stay 26", Panaracer have just put out a 26" version of their gravel king tyre. It is 54mm wide, so you get the same diameter as a 27.5 x 42mm tyre, but with more width and volume. They are really nice tyres and make the kind of conversion you want to do pretty straightforward.

Good luck,

Johnny


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 Post subject: Re:
PostPosted: Thu Jul 02, 2020 11:32 am 
Retro Guru

Joined: Tue Dec 21, 2010 6:31 pm
Posts: 1164
FWIW, here is my latest effort (see link). It's a 1980s Koga Miyata Terrarunner, 26" wheel frame converted to 27.5 with Magura hydraulic brakes w/road levers.

Frame is actually exactly my size: 58cm seat tube. The whole thing about sizing down for drop bars on an mtb is a bit of a myth IMO. You want to raise up the bars a lot, which brings them closer to you and so shortens the top tube by a couple of cm. It all depends on the specific frame, so decide what sort of fit and position you want and measure carefully.

https://drive.google.com/drive/folders/ ... sp=sharing


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PostPosted: Thu Jul 02, 2020 12:13 pm 
Dirt Disciple

Joined: Tue Apr 28, 2020 9:38 am
Posts: 21
Thanks for the tips.

I'm not married to the niner wheel up front, but I've got a niner fork and a spare 700c hoop already. Would it still be "franken" if the wheels are of the same size? The bigger wheel should raise the bars a bit. Plus I never had a sixty niner and thought I'd try it out.
I do have an adjustable angle stem that should give me some wiggle room for placing the bars. Haven't tried the midges yet.

I think the lighter frame has less clearance (haven't measured it yet), but it might just about take a 27.5. I might turn it into an ultralight single speed if my mate doesn't find a use for it.


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PostPosted: Thu Jul 02, 2020 2:15 pm 
Retro Guru

Joined: Tue Dec 21, 2010 6:31 pm
Posts: 1164
If you've got the bits anyway then give it a go.

Adjustable stems are a bit weird for positioning. You lose a lot of reach as you raise them up. Probably worth leaving the steerer on your 9er
fork as long as you can at first and borrow some different stems till you find a good setup.

For the light frame you can probably get 42mm or smaller slick or semi slick 27,5 tyres in there since those are about the same diameter as 2,1" mtb tyres anyway. Then you'd have a nice singlespeed for road and for not too rough gravel.


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PostPosted: Thu Jul 02, 2020 5:25 pm 
GOLD | PoTM | Rider | rBOTM
GOLD | PoTM | Rider | rBOTM
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Joined: Tue Feb 26, 2013 6:26 pm
Posts: 22197
Location: Northest North Yorkshire. whippet real good...
The weirder the better. Do it bad.

I have pics. But t won’t post unless you ask. You’ve been warned.


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 Post subject: Re:
PostPosted: Thu Jul 02, 2020 7:52 pm 
Dirt Disciple

Joined: Tue Apr 28, 2020 9:38 am
Posts: 21
Well, go on, inspire me!

Johnsqual - sounds like you would much prefer a 27.5 over 26 on your bike? Is it purely for the tyre selection?


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PostPosted: Thu Jul 02, 2020 8:08 pm 
Retro Guru

Joined: Tue Dec 21, 2010 6:31 pm
Posts: 1164
Yep, tyre choice is a big part of it. Not much choice in semi slick tyres in 26" for example.
27.5 tends to look a bit more in proportion on a bigger bike.


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 Post subject: Re:
PostPosted: Thu Jul 02, 2020 10:04 pm 
retrobike rider
retrobike rider
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Joined: Wed Nov 18, 2009 2:37 pm
Posts: 2167
I'm testing my Franken-Gringo tomorrow...
Orange Gringo - Orange RX9 disk forks - 700c disc wheels with cyclocross tires
Pics if you ask nicely ;)


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 Post subject: Re:
PostPosted: Fri Jul 03, 2020 9:32 am 
GOLD | PoTM | Rider | rBOTM
GOLD | PoTM | Rider | rBOTM
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Joined: Tue Feb 26, 2013 6:26 pm
Posts: 22197
Location: Northest North Yorkshire. whippet real good...
Well.


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