4Rings6Stars

I've got a torch and I've got space, now I'm stumbling with finalizing my frame design as I keep designing it over and over on paper (and just started using BikeCAD) and I can't get a long enough head tube to allow me to have a horizontal top tube AND ample standover height.

After riding lots of different bikes, reading a few framebuilding manuals and researching the internets, I've decided on what I want my frame to be. I've got a long torso and short legs and usually am comfortable on frames that are "too big" for me but that means I have no standover. This is usually fine but for this bike, which will be doing Cyclocross/off road duty as well as commuting, standover is a must.

This is what I require:
50-52cm (c-c) seat tube
55.5cm (c-c) top tube
77-78cm Standover
Relatively high BB
Clearance for 700x35 tires

Here is my problem: I am somewhat limited by the lugs I have and also by the pre-bent fork tubes I will build my fork from. I could buy different lugs but $ is tight right now and I don't want to spend too much on this first frame as it might turn out junk.
Head lug: 74*
Seat tube: 74* (not 100% sure about this. It isn't marked and I'm measuring with an adjustable set square. Short of using a laser is there any way to tell definitively?).
DT lug: 60*
Fork Rake: 56mm

Here is a screenshot of my design. This design almost works but the heat tube is my absolute minimum length and the standover (with 700x32 touring tires) ends up at about 32". Though this is hard to tell as I can't figure out how to have the program tell me the standover and this is my calculation.



If I give in and make the top tube slope 2*, I think it would work and isn't too bad for aesthetics but I'm worried I wouldn't be able to manipulate my lugs enough to accommodate it.
Here is the design with a sloping top tube and a shorter (50.5) st.



Any suggestions? Grow a few inches, maybe learn to fillet braze?

Scooper

For standover height in BikeCAD, go to "View", then "Dimensions", then "Frame", then "Standover height"...

There's always Dazza's slant six lugs if you want to go with a sloped top tube. Kirk Pacenti sells them.

4Rings6Stars

It is greyed out for me. I'm thinking it might be only available as part of the "Pro" version of bikecad. I'm trying to see if I can pull of the geometry I want (or close to it) with the bits I already have.

Scooper

You can usually tweak most lugs a couple of degrees...

unterhausen

I'm not expert, but my understanding is that many cyclocross bikes are using road-like bb heights nowadays. Although you are really limited by your fork clearance, head tube and desire to keep the tt level.

It seems to me that having less room in the front triangle to put your shoulder is more of a handicap than standover height.

NoReg

You can build on regular lugs and have a sloped top tube. The only required adjustment would be in the chainstays and the head tube angle. It would depend on what head tube angle you could tolerate. The CS adjustment has a lot of potential fixes.

Or as you say you could braze it.

If you seriously want to learn custom frame building then my advice would be to learn never to make compromises. The absolute requirement to success is to fit yourself, and the best geometry. For all the time and money you will still put into this thing, a buck or two for the right fit will repay you many fold.

Dave Kirk 

I'm struck by a few things and have a few brief thoughts.

* I think that with the tire size you are using you could increase the BB drop by a good bit. Unless you have some special need (using long cranks or toe clips) then I'd be looking at using a drop of 75-80 mm. This will increase standover in a big way.

* given the wheelbase of the bike and your size I'm be leaning toward a more shallow head angle. 74° is pretty darn steep for even a track bike let alone a mixed use cross/commuter. You might also consider adjusting the fork rake so you end up with more trail. A trail of 41 mm is very short and will certainly make the bike a bit busy. I'd be looking at a trail of 55- 60 mm for your use. If you are running a handlebar bag or front rack you might go as low as 50 mm.

* it doesn't take great skill to tweak the top tube lugs a few degrees so I wouldn't shy away from 1.5° - 2.0°. The lug that is hard to tweak is the lower head tube lug. BikeCad will give you the downtube/headtube miter angle and you are going to want to look at it. Most DT lugs will be in the 59° to 60° range and it's a bit tricky to move far outside this range.

* I see from your image that your fork is 400 mm long. There is no reason you can't go a good bit shorter............You can make it as short as 380 mm and still have plenty of room for a cross tire and a fender if you wish. In your case you might end up with a pretty short head tube (makes for more stress on the headset and makes it much hard to squeeze the lugs onto a short head tube) and you can make it longer by making the fork shorter.

* so in the end I would consider lowering the BB, giving the top tube a bit of slope and shortening the fork and your life should be easier.


I hope that helps,

dave

Scooper

OP, you just got some great advice from one of the best in the business.

mudboy

Yeah, that fork looks super long...

schooner

Obviously, I agree with everything that Dave says. I would also suggest that you put the rest of your personal dimensions in there so you can get an idea of what the bike will look like built up. A bike with a high bb and a short headtube might be pitching you more forward than you might be comfortable with. Stick your seatpost height, saddle offset, cranks length, and stem length on that frame as is and take a look at your saddle to bb drop. Compare your 3 points (pedal, saddle, bars) as they would be on that bike to what you are currently riding and see how your weight will be distributed.

Also, if this is your first frame, I would follow Dave's suggestion on the angles. 73 is kind of the perfect balance or starting off point for every frame, then you adjust from there for different handling characteristics. Move your ht/st to parallel 73s, and maybe move your fork rake out to 43-45, and that will give you a nice stable bike. Plus it will be fairly neutral, so you can get an idea of what going steeper or shallower will feel like on your next bike.