[J-Rod]

I am feeling really lazy and don't feel like doing the calcs but here is the deal. I am working on my final paper for my senior project

I need to know a weight that would need to be hung from a brake locked wheel to recreate the weight of a rider applying the brakes.

I did static testing and hung weights 10-60 pounds from the o.d. of a 26 in wheel with a hookworm on it with the brakes locked. I need to prove the validity of my testing. I did not do anymore then 60 pounds because the it was kind of unsafe


if this doesn't make sense let me know, i will try to clarify.

and thanks for any help

Jarad

 

[Matt D]

So you're hanging weights tangentially from the wheel, like this?

How did you determine the amount of force to apply to the brake lever to make a "brake locked" wheel?

 

[J-Rod]

that is what i did, i put the brake lever in a clamp and cranked on it until the wheel wouldn't move, I was using disc brakes if it matters

 

[- seb]

60 sounds about right.

Suppose an even weight distrbution, going to have 200lb or so (for a light rider) spread between the wheels, or to put it another way 100lb per wheel.

The frictional force is 100lb * coefficient of friction, I believe the coefficient between a car tyre and asphalt is reckoned to be 0.8 or so. This is the most force you can brake with without breaking traction, so 80lb or so.

I presume you've done sums saying the coefficient is 0.6.

- seb

 

[- seb]

Of course under hard front-wheel braking you will see a trasfer of weight off the back wheel. The most braking force you can really apply is going to be double what I just said, since all the weight will be on the front wheel.

 

[Kornphlake]

Originally posted by - seb
60 sounds about right.

Suppose an even weight distrbution, going to have 200lb or so (for a light rider) spread between the wheels, or to put it another way 100lb per wheel.

The frictional force is 100lb * coefficient of friction, I believe the coefficient between a car tyre and asphalt is reckoned to be 0.8 or so. This is the most force you can brake with without breaking traction, so 80lb or so.

I presume you've done sums saying the coefficient is 0.6.

- seb

I usually shift my wieght back when braking, I'd say more than 50 % of my weight is over the rear wheel. However the coefficient of a smooth tire (hookworm) on dirt would be considerably less than that of a tire on asphalt so maybe the two cancel each other out.

 

[slcpunk21]

Originally posted by Kornphlake
I usually shift my wieght back when braking, I'd say more than 50 % of my weight is over the rear wheel. However the coefficient of a smooth tire (hookworm) on dirt would be considerably less than that of a tire on asphalt so maybe the two cancel each other out.

Even if you shift you weight back the momentum is still pushing most of your weight on the front wheel when breaking, you usually end up with about 70% of your weight on the front wheel no matter what you do. Because if you had it equally ballanced 50/50 you'd loose traction of the front tire.

 

[J-Rod]

I used not coefficient of friction b/c i was doing static testing. The brake was locked, i just want to know if the the weight i used was legit or not.

 

[Kornphlake]

probablly not.

I'm having trouble visualizing exactly what you're trying to recreate by doing this test. First off clamping the lever will potentially put way more force on the lever than you would be able to apply with your hands, second what is hanging weight from the wheel supposed to show. Did you slowly unscrew the clamp and note when the wheel started to turn???

Posting your project on the internet and having a bunch of bike geeks do the work for you won't help you graduate and I commend you for being vague, but if you want any real help you may need to reveal a little bit more about the project.

 

[Matt D]

Originally posted by J-Rod
that is what i did, i put the brake lever in a clamp and cranked on it until the wheel wouldn't move, I was using disc brakes if it matters

I still don't get it. Unless you know exactly how much braking force you're applying (in other words, how hard you're pulling on the brake lever), what's the point?

You could barely be pulling the brakes and the 60lbs would move it, or you could pull on them so hard almost nothing would move it.

From your brief description, this doesn't sound very scientific IMO. Is this HS or college you're graduating from?

Not dissing you, I just want to make sure you know your variables.

 

[DßR]

I don't get it either. Taking the lever force out of the equation, the force you're re-creating with that weight hung off the wheel changes massively with variables such as rider weight, speed, rate of accelleration ("decelleration"), tire compound/traction/riding surface, wheel size, etc.

I don't see how one weight could represent all those variables, but if I were guessing, a 2.5 slick, soft-compound tire, on dry concrete, slowing a 200lb rider down from 20mph by locking the wheel, would be several hundred pounds at the front wheel.

 

[J-Rod]

DBR, that is what i am asking. Does the sixty pound weight compare at all to a rider on a bike.

Think of it like this, Someone doing a trials hop, how much weight could I hang from the wheel that would compare the force the rider is putting on the wheel


And this is a great idea, i am lazy and just curious if I could get any opinions from other people, I was hoping for more math based answers from people. I always see posts were someone asks a questions and people come out of the woodwork with calculations and other info. I might as well use this online community to my benefit.

this question isn't life or death, as long as I get a passing grade that is all that matters. So I will just leave this out if it is too much trouble

 

[Kornphlake]

striving for mediocrity will get you a passing grade. Stating you are too lazy to do anything more and expecting others to contribute their time to make your report better will only make you a less productive member of society. If you don't know how to do the calculations you should leave them out of the report end of story. If you are willing to form a thoughtful hypothesis and do some research I'm sure people will chime in, at this point it sounds like you just wanted to hang some stuff off your wheel and see if your brakes could hold it, now since you have a report you are looking for a way of adding that little experiment to it. I really don't see any conclusion other than that your brakes can stop a wheel from spinning under a tangental load of 60#.

 

[shock]

Once in college, I had a classmate the was really fast in motocross, he could kick my ass in every race, so I was kinda impressed by him. But he would ask me insane questions about every homework assignement or test, until I realized he was dumb as a stump, and I lost respect, figured out ways to avoid him, and hoped I never had to fly on a plane or depend on a project designed by him.

Don't be lazy! Figure it out! Work hard at your chosen career! Your calculation might save or cost somebody their life!

Oh, by the way, on the Hayes website you can find a design force for disc brakes that estimates with perfect traction (let's say landing from a drop in loam with a meaty tire) a brake can produce 1000lbs of force.

Brian