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It was an innocent age... when I could expect that my readers would be amused by the idea of a drive-by shooting and would also be willing to sit through 5,200 words on frame construction and geometry. A lot of it still rings true today... and I was the first person to warn BMX riders that aluminum breaks while Cr-Mo tends to bend. A decade later, people are still ignoring that advice, at their own peril...
"Yo, man, hit the lights. Turn the music down." Slowly, I rounded the corner in the town of H. It was one in the morning, and there was no moon out to alert our target. Mark rolled down the window and pulled back the slide on his Chinese SKS semiautomatic rifle. "There he goes. Don't shoot unless I tell ya." I eased up alongside a blue-clad figure on the street. Mark leaned out the window and screamed at him.
"Ayay, man, this is Mark Baruth! Get in the car!" He grabbed the cowering dude and pitched him in the back seat of my VeeDub. I flipped the light on and pointed a S&W 10mm pistol in his face.
"Found you again, Source X. Now you're gonna fess up bigtime." Source X stuffed his face in his hands.
"No way, man! You already got me in enough trouble last time! I'm not talking! I'd rather be shot!"
"We can do that," I growled. Source X must have seen that I was serious, cause he began crying like a baby.
"Okay, I'll tell you anything you want to know. Anything! Mike King's home phone number! What the '10 Beats Stronger' on the Dirt Bike means! Who's responsible for the Triple Triangle!" I jabbed him with a seatpost.
"Just shut up for a second. Now listen hard. I'm sick of all the junk being written about frame geometry. What's it all really mean? How do you make your bike handle well? Fess up, Source X, 'cause my brother's got an itchy trigger finger."
"Oh no, man. Go ahead and shoot me. BMX companies guard that stuff like the Holy Grail! I wouldn't make it out of my front door the day after you publish. I'm begging you, ask me something else. I know what brand of vitamins Erik Abadessa takes! I've seen the '95 Mongooses!"
"I'm going to count to ten. Onetwothreefourfivesixseveneight..."
"Okay! Okay!" The things I do for you, loyal readers. Thanks to Source X, I've finally found out everything there is to know about frame geometry and design. And, I'm going to share it all with you. Gosh, this is scary. Tomorrow morning, every bike manufacturer in the world is going to have to face a thoroughly educated consumer group. It could change the world! Or at least lose me my column. So what.
Let's begin with the basic construction of a BMX frame. By and large, frames are "double-diamond" designs. You've got a seat tube which forms the base for the two "triangles," which reach out to the head tube and the rear dropout. Even though this design has been around for nearly 100 years in road bikes, it's still the best because it allows for the use of shorter tubes than any other design.The length of the tubes is very important, because tubes flex in geometric proportion to their length. So a tube that's twice as long doesn't flex twice as muchtry between four and ten times as much.
So in order to build a strong, flex-free frame, you've got to have short tubes.What the tubes are constructed of makes a lot of difference. BMX companies are faced with a nearly limitless choice, but only a few different kinds of material actually make any sense for a pro-size BMX bike. Mild steel, the stuff used in most departmentstore bikes, has an extremely low strength-to-weight ratio. High-tensile steel, which is featured on bikes like the Dyno VFR and its ilk, is a little better.
Considerably better than both these pretenders to the crown is what we call "cr-mo," which is steel tempered with small amounts of chromium and molybdenum. When a frame has some cr-mo tubes and some hightensile tubes, it's called "Trimoly." Trimoly frames used to be more common than they are today, but their cost closely approaches that of a full crmo frame, and consumers have gotten wise.Now check this out. Probably your frame says "4130" on it. That's a reference to the SAE guidelines for mixing metals. You see, nobody wants to get cheated, and the guidelines are useful to ensure that manufacturers don't play games.
Unfortunately, the "4130" guidelines are nearly nonexistent. Nearly anything can (and does) pass for chromoly. The "aircraft" label doesn't mean diddly-squat either. That's like saying "hospital" food. Some of it's pretty terrible. So manufacturers are able to use cheap metal and take us for a ride sometimes. At this point, Bonzai and P.K. Ripper owners are going to giggle and point at us. Sorry buds, you're in for it too. The term "6061T6" aluminum means two things: "6061" for the type of aluminum alloy used, and "T6" for the heattreating process. In the words of Bicycle Guide magazine, which researches these things with a vengeance, "Sixty-sixty-one is not a terribly strong alloy." That means you've got to use more of it to make a frame as strong as one built of crmo. Luckily, it's light. Which allows Cannondale, Trek, and other road bike manufacturers to use very big tubes. Big tubes don't flex. However, small 6061 tubes, and floval 6061 tubes, flex like plastic spoons.
In many ways, BMX is like tandem cycling; there are heavy loads on the frame tubes, and those tubes have to have a long fatigue cycle. Steel bends and aluminum breaks. Which is why Santana, builder of what many consider to be the best tandems in the world, sticks with steel. (Six years later, Santana finally introduced the aluminum 'Sovereign', which is now their most popular bike - JB) Don't get me wrong. I ride a Cannondale, and I have enjoyed riding many a P.K. But if all other things are equal, steel is the best choice.
Carbon fiber and titanium are not relevant choices for a BMX frame due to the staggering cost of assembling a frame made of either material. (Don't forget, this was written in 1992 - JB) Now you say, "But Jack, you just said that '4130' doesn't mean anything, and that all the metals are different. So how do I know which metal to pick?" Well, it goes like this. Columbus Nivacrom is the best steel tubing in the world, but it's too expensive to construct a BMX frameset with. After that, you might consider a number of Reynolds and Tange blends, particularly Tange Prestige. BMX frames built with those metals would be lighter and stronger than any of the frames on the market. But, they'd be expensive. How expensive? Call your Schwinn dealer and ask him how much a Paramount Prologue or similar frame is. That expensive. (Of course, a few years later Rick Moliterno had the people who made the Paramount Prologue, Waterford Cycles, build the Standards. HE STOLE MY IDEA! :) - JB )
Rather far down the ladder of tubing radness, we come upon True Temper standard blends. Finally, a name that we recognize. GT Bicycles, in their infinite wisdom, has elected to construct a number of their frames from this stuff. It should be a measure of just how behind the times we in BMX are that we get excited about the use of name-brand steel, no matter what that name brand is. Again, don't get me wrong. True Temper makes good steel, some of it absolutely world-class. (And fine shovels, I might add, good for building jumps and other things). But the stuff that we get is not their best. Then again, who wants to pay five hundred bucks for a frame made with Columbus SL/SLP?
Below the "GT Elite" stuff is the rest of the motley crew of BMX tubing. Most of it is foreign-brewed to uncertain standards, even if the frames are made in America. And, yeah, it's all "aircraft-quality." So was the DC10. (Kids, ask your parents about the DC10).So how do we separate fair from foul? Buzzwords help.
Two buzzwords to look out for are "double butted." Think of a frame tube as a big hot dog with a toothpick through it at each end. When you pull on the toothpicks, what happens? More often than not, and this is the result of scientific testing, the ends rip out. So the ideal hot dog for this kind of use would be thicker at the ends.Tubing is the same way. You want tubing that's thicker or bigger at each end. Otherwise, when you land real hard the ends will break. I see bikes on which this has happened rather often."Well, then," you reply, "make the whole tube thicker. Don't take any chances." Don't take any trophies home either, because the resulting tube is really, really, heavy. Listen to me now and believe me later; tubing must be double-butted in order to be at all useful for a BMX bike. "Triple-butted" means much the same thing, and is equally acceptable. Don't fool yourself into believing that any big factory pro has ever shown up on the gate with straight-gauge (nonbutted, or the same thickness all the way through) tubing.
So our best possible BMX frame has doublebutted tubing. It also has big, and I mean big, tubes. You see, when you expand the diameter of a tube, you increase your strength exponentially, or something like that. And if you use lighter-gauge (thinner) tubing to keep the weight the same, you still get a stiffer tube. It's a no-lose situation. If you look at a Free Agent Limo, the seminal bike design of the 80's, you'll see that it has big tubes. If you look at a Badd & Company Stretch, you'll see that it has oversize tubing both front and rear. Anything less, and you are, as my brother's football coach says, "cheating yourself! You're cheating yourself, son!"
Let's take a moment to note that square tubes are not as efficient as round tubes, not by a long shot. And that's all I have to say on the matter. When Ferrari designed their F40, they elected to provide a round-tubed integral rollcage made entirely of 4130 crmo round tubes, even though the body is mostly Kevlar. Take the hint from Ferrari. Round tubes are the way to go.A while ago, I bought a Redline RL20 frame that featured a floval down tube. It was pretty cool, but was it necessary, or even helpful? Let's return to our pals Santana Cycles. They say that a flovalized (for those of you who are scratching your heads, floval tubing is ovalized tubing with flat sides, hence the name flatoval, or floval.) tube sacrifices torsional rigidity (resistance against twisting) for strength along the line of the flovalization. But wait! My RL20 was flovalized along an axis that was relatively unstressed. It should have been flovalized horizontally to prevent the bottom bracket from swinging. But, the RL20 was a freestyle bike. So bottom bracket swing was relatively unimportant. What was important was stiffness upon reentry to a quarterpipe. So maybe that was right after all.
We've decided that the bigger the tube, the better. Why, then, aren't all tubes big and beefy? Two reasons. Road bikers held out for a long time against big tubes because they were "ugly" and "indelicate." But look at Greg Lemond's bike now. Big tubes, homeboy. Eventually, science won out against aesthetics. I always like to see that happen.BMX companies don't consider small tubes to be aesthetically sound, but they sure are cheap to buy and weld. Big, thin tubes, such as the ones we'd like to have, cost more because the tubing quality has to be more even. Otherwise, you'd break the tube in a weak spot. That's why department-store bikes have got such small tubes. It hides a multitude of sins, as we say in the biz.In addition to the higher cost of tubing, big tubes require a greater degree of precision to weld. That costs money. It's why Cannondale and Klein, the big dudes of big tubes, have all their frames meticulously handwelded and sanded. It's also why a lot of frames on the market aren't as fat as they should be.
So we've got our tubing. What about the dropouts? Isn't bigger better? The answer is yes, but using better steel in a standard thickness is mo'better. Most frame companies figure you're too stupid to pick up on that, so they double-thicken the dropouts. My Tange Switchblade forks, by contrast, have Infinity heat-treated dropouts in a nearly standard thickness. Lighter and stronger. Too bad they cost four times as much as a set of standard forks. Don't be ashamed to buy double-thick stuff, though. It's better than cheap steel in a cheap gauge, which is usually the alternative.
So, we've determined that fat steel tubes are the way to go. It's less than half the battle when it comes to building a better frameset. You've still got to arrange the tubes in a way that makes sense. And that's known as geometry.I hated geometry in high school. But I like it when it comes to BMX. It's easy to understand and you can feel the results after a lap on the track. So dive in. You only have two basic variables, the length of the tubes and the angle at which they join. As we've already determined, you can't change the doublediamond format without running into big trouble. Forget extra tubes unless you have a specific purpose in mind. My "Prometheus SLP" frameset, (Oh boy. Another side project.- JB) which I designed a couple of months ago and featured in BT, was incorrect in this respect. I would have been better off including a true 2 1/2" inch down tube and forgetting the Profile-style reinforcing stuff. So consider it done.The length of the front and rear ends are important both independently and in the ratio by which they relate.
In order to keep us all sane, we'll examine them one at a time, beginning with front end length. A bigger rider needs a longer front end, but not for purposes of reach. On a BMX bike, the reach is variable anyway. Having a long front end allows the taller rider to make better balance adjustments. Consider it this way. You've got two sticks connected to a common axis. One stick is short, the other one is twice as long as the first. If you move the end of each stick one inch around the axis, which stick moves a greater number of degrees? The shorter stick, of course. With a longer front end, you can move the bars more without unbalancing the bike. Taller riders need this extra measure of security because they are more likely to become unbalanced. Back in the days, Stu and the other pros rode small bikes and did well because they were very talented. Today's XL and XXL framesets reduce the amount of talent that a taller guy needs to stay on the bike. Short squids also benefit from the use of an XL ride.When you approach a National-style "wall," (Brief digression here. When this column was written, in 1992, nearly every National track featured, as the first jump, a very steep but lipless tabletop as the first jump. It was designed to slow you down and produce random airing-out. The best way to handle it was to pull like heck and pedal up the front. When doubles returned to fashion, the National Wall went away - JB) you can pull up hard on an XL bike without hurting yourself. With a standard-size bike, you need incredible balance. Consider this. When did the "manual" become a recognized skill, particularly among non-factory riders? After bigger frames became available. Try to manual an old Motomag and you'll see what I mean. Can't be done without endless practice. But where I live, we have a guy named Chad Radford who can manual for a quarter mile or more. Though he's only five eight or so, he rides a Limo. If I stole his bike and made him ride a standard Free Agent, would he fare as well? I doubt it.
The bad thing about a long front end is that it increases the wheelbase of the bike. More on that later. It also makes jumping more difficult. Most XL bikes like to be jumped in a "Hannah" position. Nose-dives become a risky endeavor. My old Redline 600C had to be jumped with the nose down a bit. If I tried the same thing on my Hyper, I wouldn't be alive to write this. (Times change -JB) The rear end serves an entirely different purpose. That's obvious, but hear me out. Bigger riders actually benefit from a shorter rear end. Not only does it keep the overall length of the bike within reasonable bounds, it moves the center of gravity forward and makes a long front end more useful. Bikes with short rear ends are easy to pick up as you approach a jump.
My brother, who rides an Auburn, praises it for its balance. He's got a point. The bike has a short front end and relatively long rear, which keeps you from getting into trouble in the air. And it's very "forgiving," meaning that Todd Corbitt can drop the rear of his bike off and to the side on big jumps and realign himself for landing more easily than he could on, say, a TNT Longfella. But as far as manuals... forget it.
Much has been written about the importance of having a short rear end when it comes to getting a good start. Not really true. The advantage is mainly due to poor design criteria that are prevalent in the industry. A short rear end reduces flex because the bottom bracket doesn't have as long of an axis on which to "swing." Remember that short tubes are much better when it comes to cutting down on flex. Short rear ends also allow for the use of a shorter chain. That's good but not really important. Back in the days when bikes were all really crummy steel and welded poorly (read 1992), a short rear end mattered. Now, use of oversize tubing should cure most rearend flex problems. Look at Mark's Auburn. It has a long rear end, but there's a lot of steel (but square, arrgh!) back there to keep things honest. If it had proper round oversize tubes, it would be equally strong and a pound lighter. Either way, there's little flex.
In the air, a short rear end can unbalance a bike. Good ramp and street bikes feature long chainstays. Why risk your life for a little flex? But with an XL front end, you need short stays.The reason has to do with handling. All other things being equal, a short bike turns in a smaller circle than a longer one. So someone with a Dirt Bike should be able to undercut an identical Holmes rider in a turn while leaning the same amount. Combine a long front end with an equally long rear end, and you've got a bike that won't turn from a straight line. Since many BMX tracks continue to feature turns, sweepers and Europeans even, this simply won't do.
So you shorten the wheelbase a little by taking an inch out of the rear. But wait a cotton-pickin' minute! Now you've got an Air Deathtrap that pitches its rider every which way over a jump. What's a frame maker to do? Simple. You play with the angles. You've got two to change, head tube and seat tube. Seat tube angles have a rather esoteric effect on handling and are best left out of any reasonable discussion, save to note that they should generally mimic the head tube angle and should never fall below seventy-one degrees. But everybody knows about head tube angles.Why don't we take some imaginary bikes out of the shed and run them around the track? One bike is something like the old CW and Mongoose rides. It has a sixty-nine degree head tube. The other imaginary bike is a Wicked Bobmobile. (Fat Chance mountain bikes come in Wicked versions with steeper head tubes, so this is a Wicked Bobmobile). It's got a seventy-six degree head tube.
Let's bone up on the basics for a moment. The head tube angle determines how much the front wheel actually steers when you turn the bars. With a relaxed (numerically lower) head tube, the wheel kind of "flops." Like a big chopper motorcycle. You're changing the angle of the tire more than the direction in which it's pointing. A steep (numerically higher) head tube makes the wheel steer more, but keeps the tire flat on the ground. Okay? Off we go to the track, just you and me.When we ride our bikes into a turn, the sixty-nine degree bike is very slowsteering and rather reluctant. So we've got to lean on it hard just to turn at all. Pretty bogus. Let's put that thing away and go to the second bike. It turns in really sharply. In no time at all, we've swooped Bike #1. But wait! Not only is the bike more squirrely in a straight line, if we turn too sharply during conditions in which we're excited or unable to judge things properly, like a race, we flip right over the bars. Meanwhile, the bike we swooped has passed us over the jumps because with a chopper head tube you can forget about squirreliness over jumps. We've got to pedal harder to catch up on our Wicked Bobmobile, which means we're swinging the bike more, which means we're more likely to wreck by flipping over the bars. But as soon as a turn comes up, we swoop right under the other guy.
Clearly, both setups have their advantages and pitfalls. Do you remember the JMC Darrell Young? It was a big (for the day) bike with a very steep head tube. Riding one required a fair amount of skill and recklessness, but it was almost unbeatable when ridden properly. Some people still have them, and I hear that three hundred dollars is not out of the question as an asking price. If I were under six feet tall, I would spend the money. (You'll need more than $300 now... JB)
Free Agent's Limo showed us the way to the future. It was a long front end coupled with a pretty short and narrow rear. The head tube was quite steep, I believe 74 degrees or something very close to that. It was a handful in the air, and liable to pitch you right over the bars if you made a mistake. But when I got mine, I found that I could use parts of the berm I'd never seen before. Before you could say "winning breeds imitation," there were fifteen XL framesets on the market. A lot of these framesets were simply big bikes with little attention paid to their angles and design. I can think of three or four XL bikes in particular that were (and are) so poor, I would rather race an old Motomag.
Recently I put my Badd Stretch out to pasture and picked up a Hyper HPR20. I loved the Stretch, but it wasn't much of a street bike. Since I'm not much of a racer, I thought I'd try something a bit smaller and maneuverable so I could learn wallrides and stuff. I would have gotten a Holmes, but those S&M guys keep making fun of me, so I got one of Clay and Erick's creations.Let's compare the two. The Badd had a steeper head tube and is 3/8" longer in the top tube. Kind of like a super-Limo. The Hyper, though no slack chopper, proved to be more of a chore to steer through turns until I got used to it, at which point I found out that I could countersteer a bit (wait a few months, and I'll cover that technique) and swoop, particularly at the Grands. Everybody caught me over the jumps, but so what. In the turn, I ruled.
The shorter the bike, the easier it is to steer. A steeper head tube makes the turn-in more sharp, but it's still wheelbase that makes the turn go 'round. Fork rake is the last part of our equation. The smaller the fork rake, the lower the steering effort because you're not moving the wheel laterally as much. Imagine a fork with a two-foot rake. The wheel would move side-to-side more than it would actually turn. And that takes power to do. So a small rake is better, almost without a doubt.
Five years or so, there were forks on the market with two wheel positions. To the eternal credit of my fellow riders, I never saw anybody with the axle in the position farther from the fork. Low fork rake is like a free bonus. Road bikes have a lot of rake because road racers frequently find themselves heading down hills at sixty or so mph, and they don't want the front wheel to turn easily. That could hurt. But we don't need to be bound by road bike ideas. We've got real helmets. And we know how to handle our bikes.
I'm going to design the Ultimate BMX bike in just two paragraphs. It's so easy, I figure that by next week, everyone will be selling one. Pay attention now.For a rider between five eleven and six four, which is most of the pro class, we need fillet-brazed (beadless) Columbus Nivacrom tubing, with a short seat tube and a high bottom bracket. If we can't afford Nivacrom, we'll take a Columbus SLP frameset in oversize diameters.The wheelbase will be around forty to forty-one inches. The head tube will be seventy-five degrees, with a seventy-four degree seat tube. Make the rear end fifteen and a half inches long, and stick the rest on the front. All the tubing is big, very big, bigger than you've ever seen. The frame would weigh slightly less than four pounds, more than a pound less than leading pro bikes. It would be better than twice as stiff in most areas, and gate starts would be spectacular. It would be a handful in the air, but then again pros are able to handle nearly anything, so let them go to it.
The cost? I would guess it to be between eight hundred and a thousand two hundred dollars.Would it be worth it to pay that much? Certainly, if the frame made a significant difference and lasted more than a year. Top road frames can reach into the two thousand dollar range. The Tomac signature Raleigh ATB costs six and a half thousand bucks, and a lot of that is in the frame. It is my hope that some courageous framebuilder will offer something like this in the near future. I'd be first in line to purchase it, as soon as I sold my car.
So that's the future, but what about now? What can the pro-size rider buy today that will not hamper his quest for a firstplace trophy? I've ridden a lot of bikes over the past two years, and the following five are my best guesses as far as today's stuff. I am not endorsing these five, nor am I failing to endorse others. But these are the five guys closest to the Grail of good framebuilding, as it were.
1) Badd and Company Stretch. Fine long
bike with great turn-in. But the original forks are not as good as
they could be. Try a set of Landing Gear or Pitchforks.
2) Free Agent
Limo. The bike that started it all. I continually regret that I've
outgrown it. If Free Agent could find in their heart to use top-drawer
tubing, or if customers could find in theirs to pay for it, there
would be little reason to ride anything else.
3) Hyper HPR20. This
frame is a unique contradiction. Though it looks like the newest
piece of Trick Star machinery to hit the track, featuring a two-piece
design and whatnot, the feel of it is decidedly oldschool. I hope I'm
not jinxing myself when I say I haven't bailed hard on it yet, and
it's not because I'm getting smoother.
4) S&M Holmes. It's a lot
shorter than you think, and a bit more flexy out of the gate. The
turn-in is genuine, however, and it's the only XL bike that can be
truly considered a jumper, save for the Limo.
(I have to interrupt for a moment. Six months after I wrote this, I was racing at Brookville when a teenaged rider came up to tell me how much he liked the column. "In fact," he says, "I was going to buy a Holmes, but I read in your column that I should save for the Limo, so I saved up and bought it..." That was the last time I used that particular turn of phrase in print... JB)
5) GT Elite XL. Don't let the GT styling cues fool you. This bike is a lot closer to a Limo in design and conception than it is to the 1985 GT Pro. The tubing is decent for once, and perhaps best of all, it's the only of these five that is available as a no-brainer complete bike. Strip the chrome off, paint it brown and add "Base" stickers, and nobody will ever know you didn't assemble a hardcore bike from the ground up. WARNING: Not really an XL bike, particularly when compared to the above four.
Thanks for sticking with me through what has to be my longest column yet. (5,155 words - JB) We didn't even have a chance to discuss how handlebar and seat placement affect handling. Maybe in a few months. Next time you buy a frame and fork, take a hard look at it. Don't let shiny chrome or trick colors fool you into purchasing a sled. I've been fooled myself, and it hurts. Ride your pals' bikes, but don't listen to them too much. This is a decision you'll have to live with for anywhere from six months to two years, and those can be long years. Switching frames during a racing season can also mess you up, so it's worth it to take your time. And if you find an old Darrell Young, buy it, no questions asked. Then call me. I'm sure I could trade you something much better. No, it's not a Huffy - those are only the stickers.
Don't forget the BMX Fiction contest. The winner will get free lodging and some meals during the Christmas Nationals, and will get to shovel some snow off all of Columbus' hot jumping spots. If you can't make it to the race, I'll flow you some parts and stuff. The deadline is November 8. I will print not only the winner but any others that strike my fancy. Get out there and write.