Pimp Your Kids Ride
Intro: Pimp Your Kids Ride
With summer about over, give your kid something that they'll be proud to swing a leg around for that fall ride to school. With a quick fork extension you can quickly build a great looking and fun to ride chopper.
STEP 1: Start With Your Kids Bike
STEP 2: Take Off the Handle Bars So That They Aren't in the Way
It's also a good idea to take the entire fork out of the headset. unfortunately, the reflector mount sometimes becomes loose and strips the threads on the stem of the fork. If this happens you might want to leave the fork and frame together.
STEP 3: Draw a Line on the Forks Before Cutting
Drawing a line on the forks makes it easier to line up later when you weld them back together. This is easier to see in the later steps.
STEP 4: Cut the Forks
You can cut the forks using a variety of different metal saws. Here I'm using a pneumatic auto body saw. This saw works great, doesn't require that much air, and only costs about $20 from Harbor Freight.
STEP 5: Forks Cut Apart and Grinding Them Down to Remove Paint for Welding
STEP 6: Test Assembly of Forks
The assembly of these forks was especially easy. I used 1" .040 steel for the new fork tubing. it turned out that the inner diameter of the original forks was just slightly bigger than the 1" steel. This allowed for the extensions to fit inside the original fork.
STEP 7: Draw Alignment Lines on the Fork Extensions
we'll use these makes as an alignment aid when we assembly the final forks.
29 Comments
wyldestyle 10 years ago
oggryn 10 years ago
HammE 10 years ago
Psychonautilus 12 years ago
jvetterick 13 years ago
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knexman123 14 years ago
callmeshane 16 years ago
I applaud the idea of doing cool stuff for your kids, friends and favourite squeeze.
But this kind of workmanship - including the lack of forethought is legally negligent.
Ok without getting into maths that I have mostly long forgotten, such as triangulation etc...
We shall go probably into incorrect vectors and simple comparisons.
Probably the more mathematically adept can clarify it all later.
Please do.
OK - why this alteration is a BAD idea.
The average bike (like so), with average quality welding, average strength and wall thickness tubing, under the weight of an average rider, doing average riding - will with a very high probability - give excellent service with NO structural failure.
This is why the average bike has labels on it such as "Rider weight must not exceed X Kg. - Do not use for stunt riding. Do not use for off road racing or sporting events" etc.
The whole unit is designed and built to a price and a purpose.
Ok now to implement what is left of my retarded mathematical skills.
The stock forks extend from the center of the head stem, at a rake of say 15*.
This means that if you draw a vertical line from the center point in the head stem, to the axle height, that the axle is about 150mm ahead of this head stem center.
This design, with the riders weight, bumps and pot holes and up and down gutters and the forces under braking.... means that the whole package holds together nicely.
It's strong, it steers well and gives reliable service.
Now my for what is left of my retarded maths.....
OK so by extending the forks so that the axle center is now about 700mm ahead of the center point in the head stem, this now gives a "LEVERAGE RATIO" of about 5 X that of the stock setup.
So with the stock bike, the center point is say 5 X the distance of the vertical line, to the axle.
Now if one were to put a 50kg force onto the stock handle bars... that would through the resultant forces put about a 10Kg force to push the front axle forward.
Because I don't know all the lengths and it's a long time since I qualified as an engineer and then moved into another trade...
Assuming that the fork is 1/2 a meter in length, his would give a TWISTING force of about 10Kg meters (actually about 5 newton meters of torque).
With the extended forks, extending the vertical distance from the center point of the head stem, horizontally along to the axle, now being about 5 X that of what they previously were, this means that with a 50Kg weight on the handle bars, one has about 50Kg of horizontal force, pulling or pushing the front axle AWAY from the bike frame.
Assuming the extended forks are about 1 meter in length, this force multiplies through the application of the extended forks, from the original to about 50 Nm or Newton Meter (or is that Kilo Newtons Kn?).
This feeds a stress from the axle, up the fork legs, into the head stem bearings, and the tubes of the frame, (50 / 5 =) about 1000% greater than the original bike "composition" was designed for.
Given that most engineers are not idiots, and that the people and companies avoid public liability suites (being sued) and getting done for criminal negligence; and they usually calculate in about a 500% safety factor... this means that if the bolt can take 1000Kg, before it begins to stretch and does not return to it’s original size when the load is released, that one has exceeded the UTS or Ultimate Tensile Strength.
So they toss in bolts that take 5000Kg.
You have the STATIC loads, then you have the shock loads of pot holes and gutters etc.. that greatly multiply and add to the weight or force being thrust down on the bike frame.
And worsera – you have cyclic stressing – the loading and unloading of stress in the parts.
Cyclic stress will seek out any weakness in any part or assembly and usually cause catastrophic or sudden failure in parts, that could have held 10X the load if it was a static load.
Then you have an AVERAGE frame with AVERAGE welds, and these can act as stress concentrators.
The welds and joints are taking loads that they were NEVER EVER designed to take, in ways that they were never ever designed to take them.
The tubes are apparently not the appropriate wall thickness, or quality for a high stress component.
You have changed the fork tubes to a SMALLER outside diameter, - this concentrates the stress of compression on the upside and tension on the underside of the tubes.
The tubes are set like a rat trap – one decent knock and they will buckle.
In your exhuberance to create a "fashion accessoryâ€, what you really have created is a rolling death trap for your kid.
I was talking with a cop friend of mine one day... about motor cycle crashes, and in one scenario we discussed, the phrase she made went like this; â€Landing in the middle of the road ain’t good, but it’s the car following behind you that is the bad thing – "Badooomp - Badoomp"
I am OK with tricked frames, but I suggest that you go and see some professional frame builders who do build "choppers" and ask for their opinions and expertise on how to make a GOOD strong and safe design.
While your on your way there, make an appointment to see an qualified engineer who works in public liability insurance - and ask for their opinion of your current design – and ask them is your modification marketable.
If I were you, I'd take them forks off the bike immediately, I'd check the head stem and welds for cracks or signs of stressing in the paint or metal, and then if the frame looked OK, I'd go buy a decent set of brand new ordinary forks and head stem bearings.
If the frame showed signs of distress, such as cracking or stretching of the paint or metal in the joints around the head stem etc.. I'd either get a NEW bike or a NEW frame.
minerug 14 years ago
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KoolKat 17 years ago
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