Angle on Angles

I'm thinking about tweaking the angles on my Cruiser frame. The front triangle is done, now it's just a matter of the chainstays. I'm hoping for a Head angle of 72.5, with a bottom bracket height 12 inchs or above. It's amazing how much 1/4" inch of bottom bracket height can change the geometry of a bike. A 1/4" adjust the head angle a little over 1/2 a degree. It's something to think about when buying tires, since you an buy 24" x 1.5, 1.75, 1.85, 1.95, 2.0 and 2.125 and people like to mix them up.

Nova BMX Chainstays

I got my Chainstay in from Nova to finish up on my 24" Cruiser. Why is it everytime I hear the word "Nova", I always think about that story where Chevy goes to sell the Nova to Mexicans and it doesn't do too well because the name "No va" in Spanish means "No Go".

Just in case your wondering here is the link to Snopes where they say the rumor is false.

I've been "No Va ing" my feet on my cruiser because now I want to change to geometry. Since I don't have the chainstays on I can still tweek it a little (Post to Come).

Where the supertherm were round these are oval. Oval works well for me because now I don't have to dimple or bend the stays.

32mm this a way

. . . and 18mm that away.

The next frame I'm going to build will be using the Nova BMX tubset . These tubes are very stiff and light . . . what more do you want?

. . . a Mutant Trumpet? Check this guy out Ben Neill and his Mutant Trumpet. If you think you might like a Ambient/Techno/Jazz search around a bit for this guy. He does some cool music.

Ohhh those chainstays

, 24" CrBack to the 24" BMX frame. Got the chainstays built.

They turned out to be a little more flexible than I thought, so I decided to add a gusset. I should have clamped it all together . The heat distorterd it all (Mostly the yoke/plate), and I tried to manipulate it back into position . . . didn't work. Can't say I feel sad. Supertherm Chainstays are thick and round. Nova makes a set of oval BMX chainstays.

I should have went with these in the first place.

5 songs
The Ghost Who Walks - Karen Elson
She Gives Me Love - The Godfathers
summertime cowboy - Husky Rescue
Dear Diary - Luna
struck a nerve - Bad Religion

More drive by blogging

Up late again.
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Once a friend of mine hipped me to knotting up my extension cords like this, I'm never going back. It flys apart in seconds, and it doesn't take long to knot back up, and you also don;t have to unravel the whole thing when you use it.

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Ahhh the chainstays. I have 22mm round chainstays, and a 17mm wide dropout. With a vice, some wooden shim to craddle the tube, and give taper to the squeeze, you too can do this in a matter of hours to do both sides so they match.

It's a matter of going little by little by little, once you go too far, you can't go back. crimp a tube, and it's game over . . . not that I would feel that bad. These chainstay are thick, they don't like to bend.

Success!
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On the other end it's going to look like this.

There you have it .

Now time to make the miters a little tighter, and on to the brazing.

Chainstays again

First you have to get your stuff laid out. I've got all the centerlines, and clearances marked. Now I'm going to trace my yoke and the dropouts on . . . Yes the dropouts are 90 off. It just gives it something to line up to.

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I need a 6 degree bend not to far, Like before I'm using a conduit bender. These chainstays are a lot tougher than 4130 to bend. At one point I didn't think I was going to get it done. I found the chainstay was too short to bend, so I have to put a piece of 1" 4130 over the othe end to get leverage.

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It worked perfectly.



How about Moby doing, Mission of Burma's, "That's when I reach for my Revolver"?

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OK what about the next race?
mwi's Gentlemens Race

It's a 55 mile, 6 man Team Time Trial, on farm roads in south-east, MO. Instead of re-inventing the wheel, just click on the link about and find out more if your interested.
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I was going to ride the course today, but I shyed away from the ice pellets coming from the sky. I've had to ride in them a couple times. Once they start bead blasting your face, you tend to slow down. A visor helps. Well it would have been epic.

Fresh Air

Got out and about for 2 hours today. Not too bad 37 degrees. Actually I like the country roads out in south eat KCMO.

I have been feeling stale, so I decided to inject the body with some clean country air for a couple minutes. Sort of an oxygen transfusion.

and the bike. Hows that yoke coming along? I decided to go another way. That first yoke weighted 12oz. This one is significantly lighter. Instead of coming off the BB with a round 1-1/2" tube. I'm ovalizing it this time. The things you can do with a vice, 4 c-clamps, wood blocks, and 2 pieces of 1/2" barstock. I took the 38.1mm round tube, and made it 45mm x28mm. The trick is to pad the vice with the wood. The wood cradles the tube as you press it, rather then crushing it in steel jaws. As your doing that you need to re-enforce the sides so they don't collapes by clamping the 1/2" pieces of barstock to the sides. It also give something for the tube to form around as it bends.

90 degrees and it still feels cold outside

Not too bad. A little clean up, and a very little tweaking from heat distortion and we're ready to go . . . Oh and one more small detail . . . The bottom bracket is 90 degrees off.

. . . but I'm not totally put out. It's heavy than I thought . . . right before I discovered the 90 degree detail, I was looking for ways to possibly drill to the plate to relieve some weight. I'm beginning to think . . . well I'm not sure what to think. I know after failure simplicity looks real good. Simply order a longer set of chainstays, or use some other dropouts. but if your building a custom bicycle what's the point of convention?

You got to be Yoking me...again

After 4 different designs I settled on this one. I still have to braze it together, but here is a sneak peak.

Where were we?

Chainstays....That's where we were. The only way to start out is with a layout. I knew going in that I'm going to have to manipulate the chainstays. Marked on the board is the bottom bracket, Chainring clearance for a 45. Crank arm clearance for 185mm cranks, 1.95 wide BMX tire. Now I was going to say 120mm rear dropout spacing, but I've just noticed that I centered the chainstays at 120mm. And as you all know 120mm is the inside spacing. So it looks like I'm going to have to re draw it all over . . . not that I already didn't have to. Supertherm chainstays are 14-1/4" long. Add 3/4" for 1/2 the bottom bracket shell, and 1/2" for the breeze style bottom bracket. That's 15-1/2" total and I haven't even mitered the ends of the tube. The plan was for 15-1/2". Well back to the drawing board.

Bra-assssss

Oppps, deleted the picutes I had of the brazing pre filing, but here are some after a couple rounds. After not brazing for a while, it didn't take long to re-learn everything, and gain some more experience. Can't wait till the next frame. Anybody can braze, but if your really good, you can braze, and not leave yourself a lot of cleanup work with the files and sand paper.
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Here is the headtube cut and ready for some facing and chasing. Next time I'm going to give myself 12mm between the top of the top tube, and top of the head tube. instead of 10mm

Usually when I get to the filing part, I ask myself why I got myself into this mess. This time it took me about 1/2 as long, and I think next time I can even cut more time. The toptube/seat tube I did last. And I almost didn't need to file it.

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For some reason I started with the Bottom Bracket. I think a normal person would have chose a less complicated joint to make a comeback on, but if you know me, I'm not one to back down from a challenge. Not too bad. I'd even say it's one of my best.

Like I said before. I think frames spec'd with 1-1/2" downtubes are a little over built. We'll see. I think you will get less frame flex with the 1-1/2" tube vs. a 1-3/8". That's just physics. But before any of those tube flex. I most your measurable flex is going to come from your tires and wheels. This Supertherm tubing is some tough stuff.

Pesky Downtube

The thing about the down tube is, you have to miter the tube 3 times. once at the head tube, and 2 times at the bottom bracket. At the bottom bracket you have to miter the downtube to contour the bottom racket, as well as the seat tube.

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If it's done right. It might look something like this. This still needs a little touching up, but not too much. This is kind of a pain. Once I get this done, I can relax . . . a little.

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At the other end you have the headtube.

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The trick is to get the toptube, and the down tube to equally cradle the head tube at a 73.5 degree angle.

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Something like this.

Now I'm ready to weld up the front triangle!

The Top Tube

When you get a True Temper, Supertherm top tube; you get a tube that is 1-1/4" in diameter, and 675mm long. It is also double butted, that means it is thicker at the ends, than it is in the middle. In this case it's 1.0mm at the ends, and tapers to 0.7mm in the middle. They design it that way to save weight. When you get the spec sheet, they will tell you the dia, Length,thickness, then the butt lengths. example; 100/75/225/75/200. So it's 1.0 for the first 100mm, then tapers from 1.0 to 0.7 in the next 75mm, then is 0.7mm for 225mm, then tapers from 0.7 to 1.0 for 75mm, then is 1.0 for 200mm.
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You will notice it is not symetrical. The designer now has to determine where to place the butts. I need to cut the tube down to 520mm long. If I want the tube symetrical, I'd cut it so I'd have 72.5mm of 1.0mm thickness on both ends. So it would be 72.5/75/225/75/72.5.
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So now you have a symetrical tube, but the stress on the tube is not symetrical. Most of the stress will be at the toptube/headtube joint. So you want to cut the tube so the butt at the top/headtube joint is longer than the one at the top/seattube joint. I'm sure there is a long list of engineers out there who can come up with a formula, but I'm not one of them. Looking at the numbers, I made the long butt 10% (72.5x 0.20) longer than the short butt. 87/75/225/75/58.
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You might even go longer than 20% on a BMX frame. I say that because there is a lot more stress put on the handlebars on a BMX frame vs. a road frame.
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If you notice the top tube has a larger diameter than the seat tube. So now if you cut a hole for the seat tube, you will get something that looks like this.

Some people will cut a hole throught the top tube, and do it like the old GT, and there is nothing wrong with that, I just didn't design my frame like that
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With a vise and a couple wood wedges the problem is quickly fixed. This way you can squeeze the tube down 1/8", and give it a gentle taper.

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If you don't have one of these, you got to get one of these. It measures as accurate as 0.1 degrees. You can't see it, but at the other end is a level. The level is sensitive to 0.1 degrees. It will get you close enough.

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Now we are jigged up.

The Seat Tube

The True Temper Supertherm seat post tube is 290mm/11.4"in length, and is a straight guage .7mm thick. I was surprise is was not butted 1.0/0.7, but it's only about 11" long.
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The only thing you have to do to the seat post is notch one end 38.1 (1-1/2"), at 90 degrees. So set the tube notcher to 90 degrees and lett'r rip.

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With a little file work and...Viola!

One tube down.

The Cruiser has arrived

There it is. Minor assembly required.

Geometry So far

Well . . . There you have it.
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The top-tube length doesn't really mean anything it seems. First they are all measured by their true length, and not horizontal from center of head tube to center of seat tube . . . aka, Center to Center. . . . not that that perfect either, but if one BMX frame has a 11" seat tube, and another a 13" seat tube, with identical top tube lengths, then the one with 11" seat tube would have more reach (Cockpit) than the 13". I looked at a lot of geometrys. If the seat-tube was more laid back, then top tube was longer; if the seat angle was steeper, the top tube was shorter. Either way the downtube would be the same length. The down-tube length on a BMX frame would be more helpful in determining the reach of the handlebars.

Also Bottom Bracket height is really interesting on a 24". I found most 24" Bottom Bracket heights between 11-1/2" to 12". A 24x1.75" wheel is not really 24" tall, it's 23.625" or 23-5/8", Half of which is 11.81". So if you have a bottom bracket height of 12", that puts the bottom bracket above the wheel axle, and if you have 11.5, that puts it below the bottom bracket. so this brings another element into play. I chose 11.75, or right about level.
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I'm not sure how much this little nuance will make in manualing the bicycle in reality. Mechanically there is a big difference. If the bottom bracket is below the axle of the rear wheel, it is harder to manual, than when it's above.

Beyond the Pipes!

If I’m going to build a cruiser frame, I’m going to need some tubing . . . Lets Go Shopping!

From what I’ve seen Truetemper Superthem (TTST) tubing is a popular choice for BMX frames. http://www.truetemper.com/performance_tubing/bmx.asp

So for my first frame it’s only natural that I build it from TTST so when I experiment down the line I have something to reference.

So where do you go to get TTST?…Henry James!

http://www.henryjames.com/index.html

Henry can also tell you everything you need or what to know about tubing, as well as anything involved with metal working.

Below is my Parts List. TTST tubing is double-butted. That means it’s thicker are the ends, and thinner in the middle to save weight. For instance take the Top Tube for example. The tube they ship me is 675mm long or 26-1/2 inchs long. For the first 100mm its 1.0mm thick, then transitions for 75mm, from 1mm to 0.7mm thick, then for the 225mm mid section it’s 0.7mm thick, then transitions for 75mm from 0.7 to 1.0 thick, then for the last 200mm it’s 1.0mm thick.

If you notice the tube is not symmetric. One end/butt is 100mm longer than the other. This is where the magic comes in. If you spec a 22” (559mm) top tube for your frame, then you have to shorten the tube 116mm. If you take it all off the 200mm side, then (1.0x100mm)-75-(0.7x225mm)-75-(1.0x84mm) 559mm, -…or you can take 8mm off one side and 108 off together to make it symmetric, (1.0x 92mm)-75-(0.7x225mm)-75-(1.0x92mm) 559mm. It’s all up to the builder. Also not all tubing is the same. Some tubing manufactures have longer transitions. So some builder will blend different manufactures of tubing

Seat Tube (1-1/8”, 28.6mm)True Temper Supertherm, BMXST-ST01; Straight (0.7 x 290), 290mm

Top Tube (1-1/4”, 31.8mm)True Temper Supertherm; BMXST-DT01; Double Butted (1.0x100)-75-(0.7x225)-75-(1.0x200), 675mm

Down Tube (1-1/2”, 38.1mm)True Temper Supertherm; BMXST-DT03; (1.0x100)-50-(0.7x285)-50-(1.0x265), 750mm

Head Tube (37mm)True Temper's VERUS Heat Treated; MHT; (1.6x250), 250 mm

Seat Stay (16mm, 19mm, 12.4mm)True Temper OX; OX3SS; (0.66x537), 537mm

Chain Stay (22mm)True Temper Supertherm; BMXST-STAY04; (1.25x360); 360mm-Lugless Botom Bracket Shell (38.1mm)68mm; ISO THREAD

The rest are parts I have hanging around. DropoutsSurly Track Horizontal Dropouts

http://surlybikes.com/parts/frame_building_parts/

. . . Cantilever Bosses, and Cable Stops.

I thought about going with a 1.25mm thick Head tube instead of the 1.60mm, but with a 4” Head Tube what weight are you really saving? The tube they send you is about 9-1/5 inchs long, and the difference is about 90 grams between the two 9-1/2” tubes, so it’s about 45g or about 3 oz. It’s like my old saying. It doesn’t do you any good to save 45grams, but find 10 places to save 45g, now you’ve saved about 1Lbs.

I did change Seat Stays. I changed out the TTST 19mm stays in favor of the Platinum OX 19mm double taped for a couple of reasons.

First: Since I’m using Surly’s, Breezer style dropout, I’ll have to ovalize the end of the tube, so it can be mitered to mate to the dropout from 19mm wide to about 16mm.

Second: The cantilever bosses have to be spaced 92mm apart, plus don’t forget to add the offset. I’m going to have to bend them. The TTST Seat Stays are heat treated, where as the OX are not. So they will be easier to bend. I will note that TT does make heat treated seat stays (HOX), but since the stays on a BMX bikes are so short, I’m sure it’s not going to make a noticeable difference, other than ease of assembly.

What Now? We make a drawing.