Some five months ago, I recommended that riders dissatisfied with their frames try switching to a standard bar-seat-pedal setup and seeing if that made a positive difference. I also suggested some less expensive ways to improve your bike, such as stickier pedals and a thinner chain, as well as a couple tricks that are free---proper brake adjustment, intelligent tire inflation strategies. And I'm willing to bet those changes, taken together, probably worked better than trying a new frame.
You see, most BMX frames will work for most people. There just aren't a lot of lemons on the market. And it's a common bad habit for racers to agonize over their frame and fork selection while running crooked bars and loose cranks. Setting up your existing frame properly will often cure imagined frame-related ills. There remains, however, a certain possibility that you might need a different frame in order to do your best. Let's figure out how to find the right one.
Extremely detailed discussions of frame geometry are available elsewhere, and have been published in BMX Today as recently as, oh, say, 1992. (An insider's reference to my "One Racer's Perspective" frame article---JB) For the non-technically inclined, though, a detailed discussion doesn't help much. After all, a competent frame designer will take into account more than twenty different dimensions when building a frame. But you only really need to worry about three, and here they are.
Head tube angle determines how fast the front end of a bike will track through a turn. A higher number is faster. BMX bikes of the late Seventies and early Eighties generally featured head tube angles in the sixty-eight to seventy"two degree range. Currently built frames run from seventy-one to seventy-six and a half or so.
If you read the August 1995 "BMX Basics", you remember that a racer has three options when he or she enters a turn. And there isn't a lot of time to decide between those options. The faster-turning your frame is, then, the less time it takes to get into one of those three lines. That gives you more time to pick your line heading into the turn. So... a steeper (higher-numbered) head tube is always better. Right?
Well... As my boss told me back in my fast-food days, "Jim, there's no free lunch." He meant that I wasn't getting a free lunch. But what I mean here is that there's a trade-off for a steep head tube. It's the squirrel factor. A bike that turns quicker also wants to turn more often. Like when you want to be going straight. Like when you're coming down from a jump. This unwanted weaving around can wreck you when you least expect it.
A "slow" head tube bike, by contrast, likes going straight. You can jam on the pedals without swinging around, or land a bad jump without falling off. In short, decrease your medical bills. That's why street motorcycles have really relaxed head tubes. At 150 mph, you want that bike to go straight. Weaving around would be scary.
When you look for a new frame, check the various head tubes and pick one based on what you need. If you want flexibility during races and are willing to learn to control your bike a little better, go with a steep tube. If you're mainly into jumping and don't worry about turns and track strategy too much, get a relaxed rider. Halfpipe riders, by the way, should not even think about getting a steep head tube frame.
Your next concern is top tube length. This is easy. The taller you are, the longer a top tube you need. I'm 6'2" or so. I've got a bike with a 20.5" top tube, weld-to-weld. What do I mean, "weld-to-weld"? That's one way to measure the top tube. The other is "tube-to-tube". A weld-to-weld figure is about 1" shorter than an identical top tube, measured tube-to-tube.
Now here's one aspect of framebuilding that BMX manufacturers used to ignore. Most old BMX bikes have sixteen-inch top tubes. That's fine for a 5'4" rider, but way short for a tall pro. If you look at a photo of, say, Anthony Sewell or some other Golden Ager of Bicycle Motocross, you'll see that those old pros are cramped up on their short bikes. Contrast that with Charles Townsend's Amtrac or Barry McManus's Rap 42. Those guys are enjoying modern, longer frames. And it shows in the lower number of crashes and injuries in today's Pro racing.
Here's a rough top tube guideline. If you are 5'3", you should be looking at a fifteen-inch top tube, weld-to-weld. At 6'3", a twenty-one-inch top tube would be very reasonable. So, a rider in the middle should add one inch of top tube length for every two inches of height above 5'3". Your average older Expert is probably 5'10" or so and rides a Holmes/Limo/Auburn XL size frame. That's just right.
Manufacturers usually build "Pro" bikes with a 16"-17" top tube. "XL" bikes are 18"-20". "Stretch" scoots begin at 20" and go up from there. But don't be afraid to take a tape measure to the bike shop and check it out for yourself.
The advantages of proper top tube sizing include:
1) You can lift up your front end with just the right amount of force. You won't fail to speedjump properly because the frame is too long, or squirrel out because it's too short.
2) You won't have to use an "XXL" stem and goof up your steering geometry. I can't even begin to get into why you should not use too long of a stem. Please, trust me on this. Use a Pro or XL stem at most. You'll control your bike tons better.
3) More room "up front" on a bike lets you open up your shoulders and breathe more deeply, thus improving your power production. Despite the advantages listed above, I regularly see riders choosing frames that aren't anywhere close to the right size for them. Well, let those guys and girls handicap themselves. Doesn't bother us.
Last, but not least, in our three important factors is wheelbase. Measure it from axle to axle with the rear wheel set where you prefer. Wheelbase is important because you can't understand our first two factors, (head tube angle and top tube length) without relating them to wheelbase.
The longer the wheelbase of the bike, the longer it will take for it to turn. So Bike A, with a steep head tube and long wheelbase, will turn sort of like Bike B, which has a slack head tube and short wheelbase, despite the head tube difference.
In addition, two bikes with the same wheelbase can have different top tube lengths. The difference will be made up for in the rear end of the bike.
It would take ten pages to tell you precisely how and why wheelbase affects your riding. And you've got places to go and people to see. So here's a condensation. The longer the wheelbase, the more stable the bike. The shorter the wheelbase, the quicker the bike can do what you tell it to do. You can't have both good features in one bike. Remember, no free lunch.
Bigger riders who need longer top tubes will find that "XL" bikes tend to have longer wheelbases. Don't try to compare the wheelbase of an XL bike to a Pro size. Compare between bikes with approximately the same top tube lengths.
Have I confused you yet? Let's make it simple. Find out those three factors of headtube angle, toptube length, and wheelbase for each frame you're considering. Write them all down on a sheet of paper. Scratch off all the frames that don't have an appropriate toptube length. Then think about what you're going to do with the bike. Are you a cut-and-thrust racer who will do whatever it takes to win? Do you want to have total control of your bike, and are you willing to pay the price of extra twitchiness? If the answer to the above questions is "Yes," go with a short wheelbase and steep head tube.
If you answered "No," chances are you're a more laid-back rider who wants a stable bike to try new jumps with. Maybe you'll make up on the straights what you lose in the turns. Or you just like being able to concentrate on other riders during a race and letting the bike take care of itself. Then choose a long wheelbase with a slack (low-numbered) head tube and let the good times roll.
I made my decision a long time ago. I decided to go with quick-steering, twitchy bikes. It seemed like the better choice. Some of the most skilled BMX Pros feel the same way.
But when you look at #1 Pros of the past ten years, you'll see a lot of long wheelbases and relaxed head tube angles. So there's no right answer for everybody. There's just a right answer for you.
Readers who are interested in how other factors in framebuilding affect bike handling can write to me care of BT and receive a free twenty-page essay on frame dynamics. (Not any more, but the new version will be up on the Web sooner or later--JB) But the rest of you are ready to go out and swap for that new scoot. Get it wired together right... because next month, we're "taking it to the street."