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Need advice on why my spokes are breaking climbing stairs.

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I have a light weight custom dirtbike that uses 11 gauge spokes.

 

I have a question that I was hoping to get some insight on.

 

My spokes seem to break when, for example, I am going to climb a steep set of stairs. I give it full throttle and come and the stairs with a little bit of speed and as soon as the rear tire impacts the first step I will sometimes break about 6 spokes in the same location. These are J bend spokes and they all break at the elbow.

 

I can do jumps, drops, riding down stairs and mostly anything else without any trouble.

 

There was another time I was climbing a steep hill that had a concrete curb at the bottom, as soon as that rear tire slammed the curb I heard a POP and then had 6 spokes broken.

 

 

Does anyone know why they seem to be breaking only when I'm climbing and impact a stair or a curb with the rear tire? All times they break seem to be when I have the front lifted up high do to the climb ,and the rear tire makes impact and I am full throttling bike.

 

Does it have to do with the impact, or does it have to do with the torque from the motor pulling on the spokes and then the impact of the rear tire suddenly stops the spinning of the tire which jolts the spokes?

 

One other issue is, may I have over tensioned the spokes which causes them to break too easily?

Edited by Question Man

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Um, why are you climbing stairs with a dirt bike?

Nevertheless, i recommend trying a higher tire pressure, and trying the rim a little better

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Um, why are you climbing stairs with a dirt bike?

 

 

Maybe people were complaining when he used the elevator?

 

I'd say try the elevator again, but this time kill the engine??!!!??

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Think of it this way, when you land on dirt, you have a large patch of contact area spreading the weight over a distance, for S&G let's say 10psi. Now, when your tire lands on a corner of a stair step, the contact patch is GREATLY reduced, and confines the weight to a single area, let's say 100psi.

 

It's the same reason you can easily do pushups with both of your palms on the floor, but can't do them with just one finger on the tip of the finger.

 

When I had my DRZ-SM, I never had a problem with stairs. But it also had wide street tires, and a lot more rubber than a dirt tire.

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Spokes break because:

Not tensioned correctly

Poor quality/incorrect  (insufficient number of 'crosses') construction for use

Spoke bend/hub incorrect and over stresses spoke head

Spokes too small/overloaded.

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I don't see how having a smaller point of impact makes much of a difference. The impact point must get threat first through contact with the tire, then the air pushes up against the rim and distributes the load through the rim, which then distributes through the spokes.

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One other issue is, may I have over tensioned the spokes which causes them to break too easily?

 

This is the answer.The tension usually expressed as a torque of a fastener is determined by the diameter and tensile strength of the fastener. When you exceed that value any extra pull on the fastener either from mechanical shock or heat expansion will break the fastener.

 

If a bolt breaks its almost always because it was to tight.The argument could also be made that the diameter was to small or the material was inferior,but engineers usually have those factors taken into account.

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I don't see how having a smaller point of impact makes much of a difference. The impact point must get threat first through contact with the tire, then the air pushes up against the rim and distributes the load through the rim, which then distributes through the spokes.

 

It's simple physics. Tractors use wide tires so as to not sink into the dirt. The wider, the less pressure. The narrower, the more pressure.

 

In your case, a stair step corner is sharp, and while the tire may absorb some of the force, it might not have enough time to dissipate the force evenly through the wheel.

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Just wanted to let you guys know I did some stupid things.

 

I probably over tightened my spokes way too tight. The tension was also not uniform. They were either tight or very tight, I just didn't want them to come loose so I cranked down any that came loose and didn't back off any that were already tight.

 

The rim is also not perfectly round, there is a pretty bad hop in the wheel. When spinning the wheel with a zip tie on the sidewall of the rim, the zip tie will fall below the sidewall, and then move all the way up to half or more of the sidewall. I'm talking 3/8 to 1/2 inch in radial movement.

 

I wonder if I bent the rim by over tightening the spokes.

 

What is interesting is that after I broke 6 spokes one time and heard the pop, I didn't realize it was spokes until later. I then proceeded to beat the rim on the same climb and no additional spokes broke. How could a wheel that broke 6 spokes on the first hit, not be able to break any more spokes on more of the same hit when I have 6 broken spokes? It must be because of my high tension causing them to burst on the first hit.

 

Do you guys think I should buy a spoke torque wrench? It is hard for me to determine how tight the spokes should be.

Edited by Question Man

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Just wanted to let you guys know I did some stupid things.

 

I probably over tightened my spokes way too tight. The tension was also not uniform. They were either tight or very tight, I just didn't want them to come loose so I cranked down any that came loose and didn't back off any that were already tight.

 

The rim is also not perfectly round, there is a pretty bad hop in the wheel. When spinning the wheel with a zip tie on the sidewall of the rim, the zip tie will fall below the sidewall, and then move all the way up to half or more of the sidewall. I'm talking 3/8 to 1/2 inch in radial movement.

 

I wonder if I bent the rim by over tightening the spokes.

 

Do you guys think I should buy a spoke torque wrench? It is hard for me to determine how tight the spokes should be.

If you have your wheels in that bad of shape, you should #1 learn how to adjust spokes- there's more to it than you think apparently- and #2 get thicker spokes. Even a terribly tensioned wheeel doesn't break spokes like yours do.

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