Lugged BB Angles
Happy New Year everyone!
Very novice framebuilder here, I built one at a course last year, and I'm looking to build one on my own this year. I'm fairly short (5'3"), and I'd like to build a lugged 700C racing frame.
So, I'm running into probably the normal challenges of designing a small frame with large wheels. Even after drawing out several options, I found that my ST/DT angle is very acute - 56 degrees. Most of the BB shells I've seen online are about 60 degrees.
Is that too big of a difference?
Or is it alright to pull the DT port downwards when fitting things up? I recall doing this for the chainstay ports, with the instructor saying "they never fit right, just pull them downwards until they fit right".
A cast bottom bracket might be a problem, unless you can find one cast at the needed angle. Bulge-formed shells can be adjusted several degrees, but might not be the look you want.
Any reason you've ruled out smaller wheels? They solve a lot of build problems with small frames.
Hi John, thanks for the response.
I'd like to use this frame for racing / training, so for tire availability, I'd like to go 700c if I can. I suppose I can use 650c.
I've never heard of bulge-forming before, I'll look into it. Thanks for the lead.
I guess I could fillet braze the BB...
is the angle so acute because your seat tube angle is very steep? I don't think that's a very good idea. Slack head tubes are fairly benign, steep seat tubes put you in the wrong place.
I'm curious what dimensions this frame will have. I have built a number of small 700c frames that needed very little dt/st adjustments, not unless the angles are really steep like unterhausen suggested.
Here's the dimensions I'm using:
Seat Tube Length: 500
Top Tube Length: 510 (horizontal, no slope)
BB Drop: 68
Seat Tube Angle: 74
Head Tube Angle: 73
I guess if I use a 72 degree seat tube angle, I'd get halfway there! Perhaps if I stretched out the TT a little, it would open it up too? Unfortunately, I don't have bikeCAD, I'm doing this all by hand, so there's not much to see.
there is the web based bikecad. You also might want to look at rattlecad, it's free
I assume your okay with a little toe overlap, I would need to draw it up to get a real idea, but that sounds fine. I built one recently that was almost identical with exception of 1/2 a degree off the seat tube angle. I did add 1cm to the natural no slope head tube length to give it a very slight positive slope to artificially decrease the angles while providing the rider with a little more upright comfort. Any way, it seems like that should be close
A few questions and comments. What crank arm length are you using? The 68mm BB drop is not very much, especially if you're using 165mm arms. But a lower drop makes the ST/DT angle even tighter... The TT is pretty short for 700C wheels. especially with a 73* HT. What's the rake? Are you OK with toe/wheel overlap? I'd also wonder how much post is above the ST, or more to the point what is the crotch clearance when straddling the bike?
I just finished a frame that needed about 4* of tightening of the ST/DT angle. It was a lot given the cast shell. I did a lot of bending with solid bars first and then a bunch of socket manipulation with pliers and crescent wrench, a little bit of die grinder action (in the socket walls) finished off the changes. The whole time i wondered why i didn't do a fillet frame (I was matching another frame's style, which was lugged).
I agree with the suggestion to consider 650C wheels. In your racing have you ever had a wheel change during the race? This about the only reason to use what every other one is using, the chance of a neutral support wheel change.
Maybe 2/3s of the bikes I've built have been for riders under 5'7". Things get tight and compromises abound with 700c wheels. The free version of Bike Cad is worth the effort to download if you can. Andy.
For the fork, I'm using the Surly Pacer (I wasn't planning on building one... maybe that's an option). 45mm rake for 57mm trail, 371mm axle-to-crown.
There's a little overlap with tire radius at 28mm, but I plan on running 23mm. I could stretch out the TT to 520-530 (I noticed many commercial frames go about this), and just use a shorter stem? That would buy me about 1-2 degrees, I think.
The standover is about as high as I want - with shoes and 23mm tires, about 1" clearance. I tried making the ST shorter (I think I'm nominally a 48), but then the TT and DT overlap on the HT. Ugh.
If I built a fork, perhaps I could minimize the tire clearance, thus extending the HT downwards, and opening up that angle?
Thanks for your story about your experience modifying the ST/DT angle. I'm definitely leaning towards not doing that. Small frame compromises indeed!
jjames2b- I just ran your givens through Bikecad. As you see your front center is about 30-35mm less then what 170 arms would want for no overlap. This is a lot of overlap in most books. The TT and DT do have some overlap, the HT C-C is only 12mm, a HT C-C of about 30mm (with traditional tube diameters) is about the minimum and still have the lug points not completely gone. Your straddle height is just over 30", real tall for my 29" inseam. I got about 54.7* for the DT/ST angle, worse then your 56* as far as needing to change a shell goes.
I did make some assumptions to fully calculate the geometry. The tires are 674 in diameter. The chain stay is 430mm , but these don't come into play where we're talking about. Tubing diameters are larger then classic ones.
I'll redo the design with some changes and post that in a few minutes. Andy.
Attachment 357630jjames2b- Here's what 650C wheels can get you. I changed the drop, ST length, chainstay length, tube diameters to ones that make more sense with a bike this small. Note the resulting straddle height, HT length, DT/ST angle (59.1*), DT/HT angle (61.1*), toe overlap (now 27mm better then before) changes.
I have no idea as to your seat set back, seat/bar distance or drop, crank arm length. But you should see the much greater range of adaptability and fit possibilities the 650C wheels get you. Of course this means a different fork. The one that i patterned the specs after is a Profile BRS (1" steerer Al and carbon mix, probably no longer available)
I'd be happy to push through more design specs for you after you ponder all this. At worst it would take a day or so if I'm on the shop schedule that day. I can also work directly with you and we can bypass this forum. Andy.
Oh Wow, thank you so much for these Andrew. So on the drawings, the angle next to the DT is from the ground to DT? Therefore, the ST/DT angle would be 180 - 74 - that number? So, on the 700C frame, it'd be 180-74-51.3 = 54.7*, which is even worse than I thought (at 56*).
I was thinking of fillet brazing the BB, and lugging the rest (because I like lugs...).
Unterhausen pointed me to the free version of BikeCAD online. I'm going to play with a few more options there, but thank you so much for these drawings and the offer, Andrew.
jjames2b- You're welcome. I am curious about your experience and such. Have you built a frame(s) already? Are you self taught or have/planning to attend a class? A luggless BB shell is, IME, the hardest brazing to do and also finish well. I would think that if you're Ok with that a fork would be NBD. I do understand the desire to retain 700C wheels if you race. Even though I stand by my earlier comment about wheel changes being the only reason for size conformity in reality the mind is our strongest "muscle". The need for a positive self view is very important for best performance. Again, if I can be of more help please ask. Andy.
Hi Andy, super valid question. I'm very inexperienced. I took a brazing class last summer in Portland (UBI, great experience, highly recommended) and built one lugged frame. It is my commuter with 26" wheels, and the proportions and angles are much more normal!
Half the class did fillet brazing, so I learned the theory and watched the instructor demos, but didn't actually do any myself. Luckily, one of my classmates who did do fillet brazing is from the same city as me (Vancouver), so I can hit him up for advice.
In any case, I'll get some spare tubing and practice first. But it looks like I won't be running out to buy that lugset just yet.
Thanks again for all your advice and help,