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Old 09-16-08, 10:12 AM   #1
itsajustme
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Downtube Cruzbike (FS or Mini?)

I'd like to make a Cruzbike that can fold without removing the seat and so that the seat can be sat on while folded.

What do you people think would make a better donor, a Downtube FS or a Downtube Mini? Both seem to have the level top tube necessary to fold under the seat.

Mini Pros:
-lighter stock bike
-longer seat tube and stem riser which will save even more weight when discarded for the conversion
-nonsuspension fork won't interfere with Cruzbike attachment
-shorter and more aerodynamic
-smaller folded bike which will be easier to sit on top of like a normal chair and also easier to conceal

Full Suspension Pros:
-longer wheelbase for better handling
-larger wheels for greater efficiency and comfort
-front suspension for greater comfort
-wheel/fork size is closer to the 26" size for which the cruzbike kit was designed
-derailleur is more likely to work with cruzbike kit (I've read some reports of the cruzbike kit not fitting the OLD of the SA 8-speed and the DT Mini capreo doesn't seem to be available anymore).

I know some of you have done Downtube Cruzbike conversions, so what are your thoughts?

Last edited by itsajustme; 09-16-08 at 10:20 AM.
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Old 09-16-08, 02:54 PM   #2
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To make the bike foldable with the seat, you have to construct your own modification for the seat pan bracket. It needs to be elevated a bit. This applies equally to both the Mini and the FS. There's a guy that's done one for a Downtube 8H model. Check the Cruzbike Yahoo Group site. His name is defjack, I believe.

There are some additional considerations for the internal hub. Again, defjack has done that so you should consult the Yahoo Group.

I think the Mini would work fine. If I had to do it over again, I'd probably choose the Mini. Smaller footprint all around. Lighter too. The FS is a heavy bike and the Cruzbike kit will add around 10 lbs.
You may find that you'll want the Big Apple tires for a little smoother ride.
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Old 09-16-08, 03:46 PM   #3
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Here's the photos of the guy who did the 8H conversion with the elevated seatpan so it folds without taking the seat off.

http://sports.ph.groups.yahoo.com/gr...os/browse/e9a9
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Old 09-16-08, 04:19 PM   #4
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I did a IXNS I think Alans was a better bike with the longer wheelbase and some suspension.I dont know about the mini Jack
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Old 09-16-08, 04:51 PM   #5
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Quote:
Originally Posted by SesameCrunch View Post
To make the bike foldable with the seat, you have to construct your own modification for the seat pan bracket. It needs to be elevated a bit. This applies equally to both the Mini and the FS.
Why does the seat pan need to be elevated?

You can see in this picture of the Mini that the front and rear halves of the frame are parallel on the folded bike.

So if the seat is mounted to the rear frame half then the front frame half will also be underneath the seat and, thus, the frame will clear the seat.

Moreover, you can see in this picture of the Mini that there is a good 6 plus inches of space between the frame halves.

So there should be plenty of clearance for the narrower part of the seat pan which hugs the rear frame half.

Honestly if it requires an elevated seat pan then I'm not going to bother. The appeal of the Cruzbike kit is that I'd be able to get a relatively light bike cheaply and easily and a custom bracket will ruin that.

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Originally Posted by SesameCrunch View Post
I think the Mini would work fine. If I had to do it over again, I'd probably choose the Mini. Smaller footprint all around. Lighter too. The FS is a heavy bike and the Cruzbike kit will add around 10 lbs.
You may find that you'll want the Big Apple tires for a little smoother ride.
My main concern with the Mini is that the Cruzbike kit boom will be too long to keep the seat on the rear frame half, which will be essential to retain the possibility of sitting on the seat of the folded bike. From the pictures on the yahoo groups it seems that even the 20" bikes suffer from this problem to some degree as evidenced by most of the seats being mounted on front frame half.

You see I want to setup the bike something like this:
downtube_cruz.jpg
So the wheels and handlebars go behind the seat and the chainwheel/boom under the seat. Then I can add a double legged kickstand to keep the seat in a sitable position.
Do you see this being a problem with the Mini?

Last edited by itsajustme; 09-16-08 at 05:24 PM.
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Old 09-16-08, 05:29 PM   #6
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Ok. I see that you want to put the seat pan on the rear half. You'll still have a problem with the fold, as evidenced in this picture:



The seat bottom will still hit the frame unless it's elevated a little. You could just put a block of wood in to elevate it, doesn't have to be too complicated. Check the photo link I provided earlier.

You don't need to be worried about the boom being too long. It will shorten enough for you to do what you want. This photo is the first one I took of my conversion. The seat pan was in the back half of the frame. The boom was not in the shortest position, and this frame is much longer than the Mini's. I'm 6'0 tall with an X-seam of 43", as a reference.



The only consideration now will be that you may have a little poorer traction on the front drive wheels, with the seat at the back. But unless you're climbing big hills on wet conditions, you won't have too much problems. Oh, and the seat may not recline in an aggressive position with the seat pan way far back, but again, it sounds like this will be a cruiser bike, not a racing rig.

Go for it! Make sure you post some pictures when you're done!
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Old 09-17-08, 02:03 PM   #7
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Originally Posted by SesameCrunch View Post
Ok. I see that you want to put the seat pan on the rear half. You'll still have a problem with the fold, as evidenced in this picture:



The seat bottom will still hit the frame unless it's elevated a little. You could just put a block of wood in to elevate it, doesn't have to be too complicated. Check the photo link I provided earlier.
I still don't see why the seat would need to be elevated above the rear frame half. The rear frame halves are already elevated on the Mini and FullS by their rear suspensions. That's why the Mini and FullS frames don't slope upwards towards the front (and also why the Cruzbike seat should be about the same height when mounted to the Mini as it is when mounted on the FrontS despite the difference in wheel size).

Quote:
Originally Posted by SesameCrunch View Post
You don't need to be worried about the boom being too long. It will shorten enough for you to do what you want. This photo is the first one I took of my conversion. The seat pan was in the back half of the frame. The boom was not in the shortest position, and this frame is much longer than the Mini's. I'm 6'0 tall with an X-seam of 43", as a reference.



The only consideration now will be that you may have a little poorer traction on the front drive wheels, with the seat at the back. But unless you're climbing big hills on wet conditions, you won't have too much problems. Oh, and the seat may not recline in an aggressive position with the seat pan way far back, but again, it sounds like this will be a cruiser bike, not a racing rig.
The problem is that since the boom adjustment follows the ~26" arc shortening it horizontally entails raising it vertically. In the photo of your first conversion the crank bracket is already probably too high for a cruiser bike with a nonaggressive seat recline. Shortening it more will not only raise it even higher, but also push the weight distribution further rearward.

I had an idea that perhaps I could take up some of the slack in the Cruz fork by mounting the wheel behind the regular Mini fork instead of in front of it. However, with the wheel behind the Mini fork and the crank bracket in front of the Mini fork, the Cruz fork would have to somehow clear the Mini fork to cross, which may not be possible. The trail would also be increased a lot, which might not be a bad idea for stabalizing the small wheels (besides I've read comments from John Tolhurst that the weight of the legs on PBFWD bikes lends itself well to high trail designs).

However, with all these "what if"s floating around I might be better off just getting a steel front fork to spread and hacking the rear end off a junk yard BMX bike. I mean, the Cruzbike kit would be worth it if I knew it were going to work. But once I start messing with the geometry and praying for clearance what is the point of spending the extra money on the kit? Besides a new fork might be in order anyway because it seems like a stiffer threadless steerer would work better on a PBFWD than the Mini's threaded steerer.

What do you think?
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Old 09-17-08, 05:13 PM   #8
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Originally Posted by itsajustme View Post

The problem is that since the boom adjustment follows the ~26" arc shortening it horizontally entails raising it vertically. In the photo of your first conversion the crank bracket is already probably too high for a cruiser bike with a nonaggressive seat recline. Shortening it more will not only raise it even higher, but also push the weight distribution further rearward.
The Mini's frame is shorter than that of the Front Suspension. You'd be lengthening the boom, not shortening it, compared to the setup in the picture.

I can't hazard a guess at your idea of messing with where to install the wheel. That's above my pay grade, to use a currently popular expression. You may want to ask John Tolhurst or Doug Burton on the Yahoo Groups about that.
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Old 09-17-08, 07:05 PM   #9
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The Mini's frame is shorter than that of the Front Suspension. You'd be lengthening the boom, not shortening it, compared to the setup in the picture.

I can't hazard a guess at your idea of messing with where to install the wheel. That's above my pay grade, to use a currently popular expression. You may want to ask John Tolhurst or Doug Burton on the Yahoo Groups about that.
Thanks. I think I'm going to try it, but don't hold your breath because it may take me a while to funnel a few dollars a week into an offshore bank account to get it past the wife.
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Old 06-27-10, 08:01 AM   #10
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Sesame, just curious, what did your Downtube with the conversion ultimately weigh? I've ordered a conversion and am thinking of putting it on the DT.
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Old 06-27-10, 09:00 AM   #11
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It was over 30 lbs. Something like 32, 33. I don't have the bike anymore so I can't weigh it.

It was a nice marriage. An affordable, foldable recumbent. Good cruiser, not for racing tho.
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