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  1. #1
    No one carries the DogBoy
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    Help with Math...Front wheel sutibility question

    I purchased a tandem bike for my family, a co-motion periscope. We tend to ride in the evenings and have been caught out late sometimes so I'd like to put some "to see with" lighting on it. I have a 700c dyno wheel built up with a SON hub with 36 spokes and a mavic open pro rim, and I'm considering its suitability on a tandem. The stoker will alternate between 7 year old and 10 year old, 50 - 70 lbs. I had the wheel built when I was 30 lbs heavier, so I was thinking total weight may still be in the "okay" range for the wheel. I'll be running 700/28 tires. What I can't figure out is the split on the math for single bike vs tandem. Will the tandem put a lot more weight on the wheel than my 30 lbs heavier self did on a solo bike?

    My instinct is telling me this is probably not a good idea, but I need help with the numbers to substantiate the cost of a new rim/spokes and a wheel build. I have an IQ fly light and taillight, so its really a comparison between new light, a wheel build, or go with what I have and maybe worry about wheel integrity.

  2. #2
    Nigel nfmisso's Avatar
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    It is easiest solve your question graphically.

    You need to know:
    * the wheelbase for the tandem and the single.
    * the positions of the center of mass of the riders in each case.
    * the mass of each rider.

    Do one rider at a time, and add the results for the tandem.

    For example:
    Let's say the tandem's wheelbase is 8 feet, the center of mass of the captain is two feet behind the front axle (six feet in front of the rear axle).
    The captain's mass is 220lbs.
    6/8ths of the 220lbs will be on the front wheel, 165lbs.
    2/8ths will be on the rear wheel, 55lbs.

    During heavy braking, the force on the front axle will increase dramatically, to the point where under maximum braking, ALL of the weight (bike and riders) will be supported by the front wheel.
    Nigel
    Mechanical Design Engineer

  3. #3
    Senior Member waynesulak's Avatar
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    Quote Originally Posted by DogBoy View Post
    I purchased a tandem bike for my family, a co-motion periscope. We tend to ride in the evenings and have been caught out late sometimes so I'd like to put some "to see with" lighting on it. I have a 700c dyno wheel built up with a SON hub with 36 spokes and a mavic open pro rim, and I'm considering its suitability on a tandem. The stoker will alternate between 7 year old and 10 year old, 50 - 70 lbs. I had the wheel built when I was 30 lbs heavier, so I was thinking total weight may still be in the "okay" range for the wheel. I'll be running 700/28 tires. What I can't figure out is the split on the math for single bike vs tandem. Will the tandem put a lot more weight on the wheel than my 30 lbs heavier self did on a solo bike?

    My instinct is telling me this is probably not a good idea, but I need help with the numbers to substantiate the cost of a new rim/spokes and a wheel build. I have an IQ fly light and taillight, so its really a comparison between new light, a wheel build, or go with what I have and maybe worry about wheel integrity.
    I have often heard that 40% of team weight is on the front and 60% on the back but with such a light stoker it is probably best to use a bathroom scale and put something under the other wheel to level the bike then measure. I doubt the 50 lb stoker would put much weight on the front wheel and there is probably less weight on the front of the tandem than on your single without the stoker. The scale will tell you the answer.

    In any case weight alone is not all important. Tandem front wheels generally take more hits because you cannot avoid as many road hazards and you can never lift the front wheel.

  4. #4
    No one carries the DogBoy
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    Quote Originally Posted by nfmisso View Post
    It is easiest solve your question graphically.

    You need to know:
    * the wheelbase for the tandem and the single.
    * the positions of the center of mass of the riders in each case.
    * the mass of each rider.

    Do one rider at a time, and add the results for the tandem.

    For example:
    Let's say the tandem's wheelbase is 8 feet, the center of mass of the captain is two feet behind the front axle (six feet in front of the rear axle).
    The captain's mass is 220lbs.
    6/8ths of the 220lbs will be on the front wheel, 165lbs.
    2/8ths will be on the rear wheel, 55lbs.

    During heavy braking, the force on the front axle will increase dramatically, to the point where under maximum braking, ALL of the weight (bike and riders) will be supported by the front wheel.
    2 years of undergrad physics and it never even occured to me to graph it out. Consider me properly shamed. Thanks, I'll give it a go if the bathroom scale thing doesn't work.

  5. #5
    Senior Member ahultin's Avatar
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    Forget the math, as long as the build is solid, you should be fine. We run a dynamo front wheel. 36h SP Disc dyno hub (so a mere 50mm Flange to flange) and velocity deep V rim. I have three "regular" stokers, wife, daughter and son, which produce team weights from 395-454. I also have ridden a few times a year with a blind stoker where our team weight is 532. While I did use DT Alpine III spoke, I was told by the builder that they where overkill. We have had no issue with the front wheel and turned 2000 miles on the bike and about 1500 on the wheel today.

  6. #6
    Senior Member waynesulak's Avatar
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    Having used both Open Pro and Deep V I have found the Open Pro to be much weaker rim. I do agree that it could be fine but since the question did not mention the captain's weight (only that he has lost 30 lbs) then there is no way to know the load on the front wheel. In the end the captain has to decide what he is comfortable with.

  7. #7
    No one carries the DogBoy
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    Ah...neglected that. My weight is 215.

  8. #8
    just another gosling Carbonfiberboy's Avatar
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    We are a 295 lb. team and have bent an Open Pro rim. I had put a single wheel on there temporarily. Deep V rims are very reliable.

  9. #9
    Senior Member waynesulak's Avatar
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    A properly built symmetric Deep V 36 hole is a very strong wheel.

  10. #10
    Senior Member SJX426's Avatar
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    My mechanical engineering instinct says it is not a problem. The weakest factor of safety componant is likely the rim, but I am not sure one data point of failure for the Open Pro is valid for making a general decision. I don't think you will be racing or taking a lot of risks with a child as a stoker, so the service is not likely to be severe. If the wheel is built well, it should be fine. JMHO.

  11. #11
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    Oh bag it and go pick up new lights. I highly recommend Niterider 650 or 700
    Rechargeable, small, and bright. And if you find you want/need more get another to mount on your helmet.

    Probably cheaper than building up a new rim with the advantage of no wires.

    You can scale the tail light depending on your needs. I run a German-made Topline which is big and part reflector and part tail light. The Germans do a lot of riding at night and have official standards on what is acceptable to put on your bike. Means it is good quality stuff in my experience.

  12. #12
    No one carries the DogBoy
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    Quote Originally Posted by vrooom3440 View Post
    Oh bag it and go pick up new lights.
    This is what I decided to do. $60 got me a cygolite metro 350 and a Hotshot 2W taillight that are USB chargable. Just a new rim would have run me that much, not to mention the spokes and build cost. I think the open pro would have been fine, but since I was worried at all, I figured I should just not use it and be worry free. Thanks for the advice all.

  13. #13
    Clipless in Coeur d'Alene twocicle's Avatar
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    Quote Originally Posted by waynesulak View Post
    In any case weight alone is not all important. Tandem front wheels generally take more hits because you cannot avoid as many road hazards and you can never lift the front wheel.
    That is just not true.

    We bunny hop our road tandem all the time, just enough to clear small curbs and other obstacles. More lead time is needed in order to setup for the move, and a verbal queue to the stoker for timing. As long as we stop pedaling, I can always get the front wheel up off the ground far enough to be effective. My little stoker's results vary with maybe half the attempts being successful clearance. Taking the time to practice skills has its benefits.

  14. #14
    Senior Member waynesulak's Avatar
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    Quote Originally Posted by twocicle View Post
    That is just not true.

    We bunny hop our road tandem all the time, just enough to clear small curbs and other obstacles. More lead time is needed in order to setup for the move, and a verbal queue to the stoker for timing. As long as we stop pedaling, I can always get the front wheel up off the ground far enough to be effective. My little stoker's results vary with maybe half the attempts being successful clearance. Taking the time to practice skills has its benefits.
    I have no trouble bunny hopping my single but since my stoker is only 25-30lbs lighter than I am and she doesn't try to hop at all I have no luck on the tandem. Maybe I will try again to convince her to hop too. Our 75psi tires should help give us a little more spring to get us started.

    I agree skill practicing is fun.

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