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How an engineering student pics his wheels. Any thoughts?(21 posts)

How an engineering student pics his wheels. Any thoughts?niteschaos
Jun 8, 2003 12:48 PM
I started with summing the moments to equal zero around the bottom bracket of my frame to find the loading on each wheel. I used my mass centered over the bottom bracket to make the model simpler. That showed 110 pounds over the rear wheel and 75 pounds over the front (I weigh 185 pounds so that checks out). I know that an OpenPro rim laced with 32 DT 14/15 double butted spokes is strong enough to withstand my riding and used that as the model to base the front wheel on. I took the total cross sectional area of the rear spokes and came up with the static strain of 1.35pounds/mm^2. I then used this loading to determine the number and minimum diameter of the spokes for the front wheel. I came to the conclusion that I could use 28 14/15 spokes on the front for a loading of 1.33pounds/mm^2, or I could use 32 spokes of 14/17 and achieve the loading of 1.32pounds/mm^2 and save 50 grams of rotating weight. I am now planning to get a 32front/32rear Ultegra/Open Pro wheelset made with 14/15DB DT Swiss on the rear and 14/17DB DT Swiss on the front for use in Road races and crits. All numbers were pulled from the DT Swiss website. Any thoughts? Is this model to simple to be used to find the optimum wheel build?
re: How an engineering student pics his wheels. Any thoughts?PaulNYC
Jun 8, 2003 1:56 PM
not all the spokes are under tension. ie. the ones on the bottom are under compression and cannot hold a compressive force.
I was under the impression...niteschaos
Jun 8, 2003 2:03 PM
I was under the impression that on a well built wheel the spokes were always under tension or else the elbows would fatigue very rapidly. Either way, how do you think I should change my model based on your comment?
I was under the impression...eddie m
Jun 8, 2003 2:28 PM
You are right that in a properly tensioned wheel, the spokes are always in tension, but your model missed a couple of points.
First, you ignored dynamic loads. Under maximum braking, as much as 100% of your weight(or more, as acceleration increases the riders weight)may be on the front wheel. But the greatest loads on a bicycle wheel occur when the wheel hits bumps in the road. The dead weight of the rider is simply not an accurate estimate of the loads on either wheel, although those loads should be proportionate to the riders weight, i.e. a bike and rider of a combind weight of 200 lbs should creat twice the load a rider with a combined weight of 100 lbs.
Also, you ignored the difference between the left and right side spokes in the rear wheel. The right side carry significantly more tension than the left side. I use 1 gauge lighter spokes on the left side. The lighter gauge spoke stretches more to create the same tension, and is much less likely to go slack, and therefore lasts longer.
Any single spoke is more than strong enough to hold the weight of the rider. The way the wheel structure works is that spoke tension creates compression in the rim, and that compression holds the rim round. The limiting factor on the number of spokes is the amount of tension each spoke can hold. Often the limit is the failure of the rim by bulging or cracking at the spoke hole.
I think the right way to design a wheel is to decide how heavy the rim needs to be for acceptable reliability, determine how much total compression is needed to keep the rim stable, and determine the minimum number of spokes by dividing the total required compression by the maximum safe tension of each spoke.
Compressive tensionKerry
Jun 8, 2003 4:24 PM
The only way a spoke in a properly built wheel is ever under "compression" is when certain anally rentive mechanical engineers insist that reduced tension = compression. This happens when you assume that the static condition of the wheel is the "zero reference" and since the tension then goes negative, the spoke must be "under compression." An interesting way of looking at it, and mathematically correct, but completely meaninless for the purposes of discussing a bicycle wheel. For the rest of humanity, who cling to the foolish notion that if the spoke has not gone slack, it must be under tension, this mind set is annoying at best. PaulNYC, if you are one of the "zero reference" guys, then yes, you are annoying. If you are simply repeating what you heard someplace, then no offense intended.
I was under the impression.....niteschaos
Jun 8, 2003 4:55 PM
that a well built wheel will always have spokes in tension at all times.
What happens when you hit a pothole at 40 mph on a tight corner?Continental
Jun 8, 2003 3:15 PM
Your model is way too simple. There are so many dynamic forces on the wheel in actual use that you would need a supercomputer to model the wheel to any degree of value. So, the best thing to do is revert to actual experience of wheelbuilders. The knowledge of the wheel builder is of much greater value than any simple static model. The bottom line for an engineering student is DON'T RE-INVENT THE WHEEL!
Did you even read my original post?niteschaos
Jun 8, 2003 5:04 PM
You must not be an engineer. You'd be surprised just how many events can be modeled without the need for a "supercomputer," mainly due to the invention of calculus.

I have talked to several wheelbuilders, but they don't agreeing on the direction I should go. I have had one guy refer to me as "a heavy rider" and should be using 36-spoke deep-v wheels (strong but very heavy) while another wheelbuilder said that a standard Open Pro build with 32-spokes would be best.

Being a college student I don't have money to spend on mistakes. My last set of wheels were machine built and didn't make 3000 miles, even after having the rear rebuilt 2 times (CXP21's with 14g 3-cross to 105 hubs). I am not trying to "invent" anything, just trying to justify why some builders are saying one thing and another will say something else. I have budget of 300 bucks, so my only real choices seem to be standard build wheels. At the same time I don't want wheels so heavy as to hamper the group rides I do in the mountains of South Carolina (riders in Colorado keep your snickering to yourselves).
For $200.00 CC will build on your hubs...MrDan
Jun 8, 2003 5:17 PM
Colorado cyclist will build wheels on a set of hubs you supply. I'm thus far impressed with my cxp33's. 36 spokes, 14/15DT 3x.

I am in no way affiliated with CC.
-D
Did you even read my original post?torquecal
Jun 8, 2003 6:41 PM
I doubt you need a supercomputer but you do need one heck of a basket of variables you didn't mention in your original post.

First of all I think you probably overgeneralize by putting your moment cg at the BB. Because of the seatstay there is probably an exageration of moment to the rear.

Second, you disregard spoke lacing patterns. While I haven't done the math, the difference between 3x and 2x is probably fairly significant in the compression/decompression forces each spoke experiences. Mind you, since this is tangenital to gravity and rotational speed, you also have to figure in average rpm.

Third, probably the biggest problem, your initial model assumes a predictable road surface. Perhaps you might make a mental adjustment for harsh roads but the real problem (as another poster pointed out) is the sudden change from nice road to harsh road... i.e. pothole.

Bottom line - find a wheelbuilder you trust implicitly and then trust his experience. Better yet - take it up as a hobby and take notes on actual experience. Either way you'll eventually end up with a better set of wheels than the calculus will give you.
Budget limited? Go for the stronger wheels.Continental
Jun 9, 2003 1:51 PM
Both wheel builders are offering a strong wheelset. One is being more conservative. Personally, if I had your budget limitations I'd go for strength and not try to save a few hundred grams. As you can probably calculate, the few hundered grams of weight, even on wheels, makes very little difference in performance.

Wheel strength is not defined by spoke cross sectional area. There are many other factors, as other posters have pointed out. Your model is not helping your decision making process.

I am an engineer and do understand calculus. I also understand finite element modeling, differential equations, and even tensor equations. It's as important to learn when not to attempt to apply these tools as it is to learn how to apply these tools.
Yeah, Mechanical Desktop wouldn't load right....niteschaos
Jun 8, 2003 7:54 PM
for me to do finite element analysis. Due to budget cuts and therefore fewer classes offered for summer semester, I find myself with a lot of extra time. I've read all the reviews on what I thought I should get and feel pretty confident that the wheels I bought should last.
I respect your attempt at greatness.......but...usna00
Jun 8, 2003 8:55 PM
You really should listen to what everyone else has to say on this.

My guess is you probably just finished up a semester with a Dynamics class mixed around with a little Strength of Materials class. Figure, hey, what the heck, I know how to calculate stress and strain, poof, a little Poisson's ratio here, and tada, ultra cool wheelset. Outsmart the system eh?

Well, if you really want to, go ahead and try. That should be a good lesson in practical experience. If it works, pat yourself on the back for a job well done. But just think about your wheels once you start taking a Materials class.

Listen, I'm not trying to be a tool about this, but modeling a bicycle wheel would be hard enough if you were talking about a trispoke (or something similar in geometric simplicity). Now, consider the person riding those wheels.....similarly difficult to model. And one more thing, dynamic loading could scale in weird and wondrous ways, user beware.

Again, I respect your desire to actually use your education, lord knows most people don't, but I would simply caution you that it might be beneficial to weight practical knowledge (your friendly wheel builder) more heavily than your engineering prowess.

Best of luck! By the way, I'm not really an ass, I just play one on TV.
so far this year, I've ridden over 5500 miles, and I don't thinkBarnyard
Jun 8, 2003 4:12 PM
that much about my wheels. I just pray that I don't hit something really hard and put a big flat spot on them. Personally, I pick the wheels without big flat spots on the rims.
wouldn't dynamic forces cancel out?niteschaos
Jun 8, 2003 5:46 PM
If the dynamic forces are proportional to the weigh pushing down on each wheel, then if you know one wheel to be strong enough, then if there is less force on the other wheel, you could calculate the percentage of force on the wheel, but not the ACTUAL force.
re: How an engineering student pics his wheels. Any thoughts?jamesau
Jun 9, 2003 4:55 AM
Sound engineering is often more about application of accepted standards, heuristics, and correlation (based on extensive experimentation), than it is on analytic or numerical solution to rigorous mathematical models; modelling is more often very useful in design refinement.

The beauty of this forum is the wealth of experience from which you can customize your build specification to suit your cycling needs and goals. In the application of engineering, there is no substitute for experience; there are too often uncertainties and unmodelled factors that can cause a seemingly sound approach to fail.

Good luck and keep up with your studies.

(One opinion from a 15 year practicing engineer with BS and MS degrees in Chem E).

Jim
Great, now you can learn how to spell "picks".SnowBlind
Jun 9, 2003 7:49 AM
God I hope my ankle heals up quickly, I am getting so cranky...
going about this all wrongDougSloan
Jun 9, 2003 8:23 AM
When you decide what gasoline to purchase, do you run predetonation tests on your engine?

You cannot model sufficiently the dynamics of wheels in use, at least without much more data than you have. Plus, many times assumed dynamic behavior is far different than reality.

Some others noted some missing data/assumptions, but I didn't see rim stiffness, for example. A deep section wheel is much stiffer, at least vertically, so it can use fewer spokes. A small box section rim, like an Open Pro, flexes more, allowing the the lower spokes to "compress" (detension) more. We are talking about miniscule numbers here, but important when thinking about fatigue over time, something else your model doesn't address.

Wheels have been built by trial and error and tested by others for over 100 years. Why not just ask/look around and do what works for others?

Doug
Because I haven't found someoneniteschaos
Jun 9, 2003 10:04 AM
who is only 185 pounds that has broken 2 cranks, 3 chains, and 3 rear wheels in just 3000 miles. You find me a guy that breaks as much stuff and I'll gladly accept what he rides. I went to Velomax.com and they have a guy that does distructive testing and he says their wheels last longest, but even the Circuits are out of my 300 dollar budget.
what the heck are you doing?DougSloan
Jun 9, 2003 11:53 AM
Is your name really Marty Nothstein (a little light, though), or Hulk? That's bizzare. How are these things breaking? Sounds more like you need 40 spoke tandem wheels.

Doug
how an experienced engineer picks a wheel...C-40
Jun 9, 2003 8:58 AM
At your weight a 32H 14/15 rear should be more than adequate. Use a 28H front with 14/15 spokes if you want to play it safe and 14/17 spokes on the front and left rear if you really think a 2-3 ounce weight savings is significant.

No model, no calculations, just experience, BSME 1981.