26" longjohn mk8

[surlypud]

So I am working on a design for a (you guessed it) 26" longjohn. I'm interested in using a top push rod, instead of the more common "below keel" method. But I'm not sure how to achieve this top push rod/heim joint method. Specifically the headset/steerer. With the push rod on top, I don't need to use a full headset (or do I?), I could just use the top half along with a length of steerer to accommodate the headset, stem, heim joint and any spacers I might need. But with that setup, there would be nothing to prevent the whole thing from separating from the bike if I pull up. What am I missing?

It seems like I have seen other cargo bikes with just the upper headset cup, but I've never examined a bike like that in person. stubby front loader | Flickr - Photo Sharing!

Bicycle Forge longjohn mk8.jpg

[artfrank]

I am not sure exactly what you mean by only part of the headset, but here are a couple of examples of top steer cargo bikes that sound like what yer getting at.
Cam Falconers: This. #rad #detail #fabrication #falconercycles @coffeeandeggs | Flickr - Photo Sharing! and here Flickr: coffee&eggs' Photostream
Frances mixtehaul (cable steer): mixtehaul - a set on Flickr

[surlypud]

On further reflection, I think I may have answered my own question.

I wanted to see if I could use just the top cup of a headset and a short steer tube to serve as the bearing upon which the handlebars would turn. The problem I was having was that I couldn't think of a way to secure the steer tube to the bike like on a traditional setup where the fork crown race serves as the anchor for the whole steering mechanism. But then I remembered about star nuts. Normally, the star nut goes into the steer tube to set the preload for the headset. Well, why not just get a bigger star nut that would fit a 31.7 mm tube, have it below the top cup of the headset and then thread a long bolt through the top cap to hold everything together? So that is my answer to my not very well explained question.

And really, if anyone benefits from this thread, kudos to you.

[Andrew R Stewart]

First thing I thought about looking at this design is whether there will be sufficient frame flex to effect the steering/links. AS the link spans the open side of the frame any vertical flex could cause the top to change distances between the steerer and the handle bars. Andy.

[surlypud]

So bumps in the road could cause unwanted steering input. Good point. I've already got the top blocked with the steer rods, I suppose I could just put in a top tube.

[cp43]

On further reflection, I think I may have answered my own question.

I wanted to see if I could use just the top cup of a headset and a short steer tube to serve as the bearing upon which the handlebars would turn. The problem I was having was that I couldn't think of a way to secure the steer tube to the bike like on a traditional setup where the fork crown race serves as the anchor for the whole steering mechanism. But then I remembered about star nuts. Normally, the star nut goes into the steer tube to set the preload for the headset. Well, why not just get a bigger star nut that would fit a 31.7 mm tube, have it below the top cup of the headset and then thread a long bolt through the top cap to hold everything together? So that is my answer to my not very well explained question.

And really, if anyone benefits from this thread, kudos to you.

I'm not sure I follow what you're saying. But, I don't see how a star nut will work here. Normally the star nut turns with the steerer, in your case it won't, right? So, when you steer, either the starnut will try to turn in the tube, or the bolt in it will turn (loosen or tighten), or the head of that bolt will slip on the top cap. None of these seem like good things to me.

Could be that you've thought of this, and I missed it.

Chris

[zetroc]

The other reason I'd be hesitant to use a top steering method like this is in case your load was a tall one. Wouldn't want to have to get a car just because the steering linkage wouldn't allow you to fit your load. Admittedly this isn't the most likely scenario.

[surlypud]

Well, crude. You're right Chris. It won't work to have the star nut fixed in the head tube. Looks like I need to rethink my design to incorporate a short head tube. Or just use a long steer tube that will allow me to use a full headset. If I do that I may rethink having the top tube over the cargo platform.

Thanks for everyone's input.