Bmx top tube too long. What can i do?
10-21-23, 07:17 PM
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Bmx top tube too long. What can i do?
Hi Guys,
I bought my scooter mad son a bmx as he was asking for one and having looked at a few online posts i thought that Chromoly frame and double walled wheels were the most important things to look for and completely overlooked the size of the top tube. My son is 11 and 5ft tall and I've bought a bike with a 20.75 top tube. Is there anything i can do with the seat or handlebars to make this bike more suitable for him? I can't believe I've made such a basic error.
Thanks.
I bought my scooter mad son a bmx as he was asking for one and having looked at a few online posts i thought that Chromoly frame and double walled wheels were the most important things to look for and completely overlooked the size of the top tube. My son is 11 and 5ft tall and I've bought a bike with a 20.75 top tube. Is there anything i can do with the seat or handlebars to make this bike more suitable for him? I can't believe I've made such a basic error.
Thanks.
10-22-23, 06:49 AM
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The seat is usually "slammed" and not used except for sitting while stopped. The bars can be tilted back slightly but the bike will still be too big if he intends on learning park/street tricks.
Unless your 5ft 11yo son is a heavyweight a full chromoly frame and dbl walled rims is not that important. Get him another bike that fits (even used) to learn on and keep the 20.75 for yourself (for now) so you can ride with him.....
Unless your 5ft 11yo son is a heavyweight a full chromoly frame and dbl walled rims is not that important. Get him another bike that fits (even used) to learn on and keep the 20.75 for yourself (for now) so you can ride with him.....
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10-22-23, 12:00 PM
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Get a bike that fits him.
Around here anyway there are always piles of used bikes cheap. They aren't great, but they aren't Canadian Tire garbage either. I got a Stolen brand in the colour my kid loves for $40. It needed minor repairs that I could do from my parts bin.
Around here anyway there are always piles of used bikes cheap. They aren't great, but they aren't Canadian Tire garbage either. I got a Stolen brand in the colour my kid loves for $40. It needed minor repairs that I could do from my parts bin.
10-23-23, 04:27 PM
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When you said you bought a cro-mo bmx bike that tells us you got a "real" bmx bike intended for racing where riders usually stand up to pedal for the whole race. (30 seconds). A long top tube is the norm for cro-mo racing bmx bikes. A longer top tube combined with longer cranks and a higher crank shell (for ground clearance) means the rider isn't hitting his knees on the handle bars. And the longer wheel base means its more stable as the rider is thrashing around while standing and pedaling as fast as they can for those 30 seconds.
Most boxmart bmx bikes have way overbuilt and very heavy frames that are nearly indestructible with wheels and brakes that won't hold up for any abuse. Cheap imitations but not built for racing or stunts. Many of the cheap gas pipe bmx bikes are intended for just kicking around on and I see much more variation in crank height and top tube length on those.
Fit really depends on how you ride a BMX bike.
For general riding around, the seat needs to go way up so the rider isn't over bending their knees. You get most power out of your leg muscles when they are nearly straight, not bent at 90 degrees. Being able to put your feet flat on the ground while sitting on the seat is bad news for your knees and really limits your power.
Crank arm length matters much more. Many bmx cranks are 170 to 180 mm long for rapid acceleration out of the gate. But that means shorter kids are hitting their chins with their knees when the seat is slammed and they are trying to sit on the seat. Many kids bikes come with shorter cranks, relative to their leg length. 170 to 175 is the norm for regular adults. ~150mm is common on non-bmx kids bikes.
There are all sizes and angles of stems that will fit on the bike to move the bars up/down/forward/back. I've seen bikes with the stems installed backwards to move the bars closer to the seat. Handle bars are made in many heights and widths and angles. An experienced coach/mechanic/bike shop worker can sure help get it setup to fit well. They would need to see the kid and bike together to make it better. Adjustments for leg lengths, arms, torso, etc are all dialed in for pro racers to get max power and efficiency so why not for kids.
General fit recommendation for ages 10 to 14 is 24" wheel bikes. My guess is he is about the tallest kid in class. At 5' tall, he is ready for a 26" bike in a small frame (15"). The make 24" and 26" wheel bmx bikes and mountain bikes. It really depends on what kind of riding he wants to do.
Most boxmart bmx bikes have way overbuilt and very heavy frames that are nearly indestructible with wheels and brakes that won't hold up for any abuse. Cheap imitations but not built for racing or stunts. Many of the cheap gas pipe bmx bikes are intended for just kicking around on and I see much more variation in crank height and top tube length on those.
Fit really depends on how you ride a BMX bike.
For general riding around, the seat needs to go way up so the rider isn't over bending their knees. You get most power out of your leg muscles when they are nearly straight, not bent at 90 degrees. Being able to put your feet flat on the ground while sitting on the seat is bad news for your knees and really limits your power.
Crank arm length matters much more. Many bmx cranks are 170 to 180 mm long for rapid acceleration out of the gate. But that means shorter kids are hitting their chins with their knees when the seat is slammed and they are trying to sit on the seat. Many kids bikes come with shorter cranks, relative to their leg length. 170 to 175 is the norm for regular adults. ~150mm is common on non-bmx kids bikes.
There are all sizes and angles of stems that will fit on the bike to move the bars up/down/forward/back. I've seen bikes with the stems installed backwards to move the bars closer to the seat. Handle bars are made in many heights and widths and angles. An experienced coach/mechanic/bike shop worker can sure help get it setup to fit well. They would need to see the kid and bike together to make it better. Adjustments for leg lengths, arms, torso, etc are all dialed in for pro racers to get max power and efficiency so why not for kids.
General fit recommendation for ages 10 to 14 is 24" wheel bikes. My guess is he is about the tallest kid in class. At 5' tall, he is ready for a 26" bike in a small frame (15"). The make 24" and 26" wheel bmx bikes and mountain bikes. It really depends on what kind of riding he wants to do.
10-23-23, 06:05 PM
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When you said you bought a cro-mo bmx bike that tells us you got a "real" bmx bike intended for racing where riders usually stand up to pedal for the whole race. (30 seconds). A long top tube is the norm for cro-mo racing bmx bikes. A longer top tube combined with longer cranks and a higher crank shell (for ground clearance) means the rider isn't hitting his knees on the handle bars. And the longer wheel base means its more stable as the rider is thrashing around while standing and pedaling as fast as they can for those 30 seconds.
Most boxmart bmx bikes have way overbuilt and very heavy frames that are nearly indestructible with wheels and brakes that won't hold up for any abuse. Cheap imitations but not built for racing or stunts. Many of the cheap gas pipe bmx bikes are intended for just kicking around on and I see much more variation in crank height and top tube length on those.
Fit really depends on how you ride a BMX bike.
For general riding around, the seat needs to go way up so the rider isn't over bending their knees. You get most power out of your leg muscles when they are nearly straight, not bent at 90 degrees. Being able to put your feet flat on the ground while sitting on the seat is bad news for your knees and really limits your power.
Crank arm length matters much more. Many bmx cranks are 170 to 180 mm long for rapid acceleration out of the gate. But that means shorter kids are hitting their chins with their knees when the seat is slammed and they are trying to sit on the seat. Many kids bikes come with shorter cranks, relative to their leg length. 170 to 175 is the norm for regular adults. ~150mm is common on non-bmx kids bikes.
There are all sizes and angles of stems that will fit on the bike to move the bars up/down/forward/back. I've seen bikes with the stems installed backwards to move the bars closer to the seat. Handle bars are made in many heights and widths and angles. An experienced coach/mechanic/bike shop worker can sure help get it setup to fit well. They would need to see the kid and bike together to make it better. Adjustments for leg lengths, arms, torso, etc are all dialed in for pro racers to get max power and efficiency so why not for kids.
General fit recommendation for ages 10 to 14 is 24" wheel bikes. My guess is he is about the tallest kid in class. At 5' tall, he is ready for a 26" bike in a small frame (15"). The make 24" and 26" wheel bmx bikes and mountain bikes. It really depends on what kind of riding he wants to do.
Most boxmart bmx bikes have way overbuilt and very heavy frames that are nearly indestructible with wheels and brakes that won't hold up for any abuse. Cheap imitations but not built for racing or stunts. Many of the cheap gas pipe bmx bikes are intended for just kicking around on and I see much more variation in crank height and top tube length on those.
Fit really depends on how you ride a BMX bike.
For general riding around, the seat needs to go way up so the rider isn't over bending their knees. You get most power out of your leg muscles when they are nearly straight, not bent at 90 degrees. Being able to put your feet flat on the ground while sitting on the seat is bad news for your knees and really limits your power.
Crank arm length matters much more. Many bmx cranks are 170 to 180 mm long for rapid acceleration out of the gate. But that means shorter kids are hitting their chins with their knees when the seat is slammed and they are trying to sit on the seat. Many kids bikes come with shorter cranks, relative to their leg length. 170 to 175 is the norm for regular adults. ~150mm is common on non-bmx kids bikes.
There are all sizes and angles of stems that will fit on the bike to move the bars up/down/forward/back. I've seen bikes with the stems installed backwards to move the bars closer to the seat. Handle bars are made in many heights and widths and angles. An experienced coach/mechanic/bike shop worker can sure help get it setup to fit well. They would need to see the kid and bike together to make it better. Adjustments for leg lengths, arms, torso, etc are all dialed in for pro racers to get max power and efficiency so why not for kids.
General fit recommendation for ages 10 to 14 is 24" wheel bikes. My guess is he is about the tallest kid in class. At 5' tall, he is ready for a 26" bike in a small frame (15"). The make 24" and 26" wheel bmx bikes and mountain bikes. It really depends on what kind of riding he wants to do.
The bike is a park bike. He actually has a GT speed series bmx for racing that is so lightweight it's ridiculous. He doesn't do any serious racing on it just round our local racetrack for fun. This bike is very heavy in comparison and is purely for use at the local skatepark which is indoor and has tons of bmx guys which is where he caught the bug. I was actually thinking of asking in the shop within the skatepark if the guys in there could help me set it up correctly for him because in the 80's when i grew up we just jumped on any bike and threw it down half pipes and over jumps and none of them where specialised. The odd guy had a Mongoose and everyone else had whatever Santa brought them that year.
I'll keep this thread updated and hopefully tilting the bars back will be all that's needed to get him started. He's not a well built boy just athletic. Boxes and plays football almost every night so strong but not heavy. Hopefully he'll manage. He's 12 in two weeks.
