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Thread: physics lesson

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    physics lesson

    I've searched around and seen some nice back and forth on trailers vs. panniers/racks for carrying gear. The consensus as I can determine it: to each their own.

    I'm in the process of selecting some gear-carrying equipment and can't quite figure out if I'm misssing something in the discussion--wouldn't having a trailer behind you create all kinds of weird/bad physical forces during the ride? And wouldn't having all that weight spread out over such a large area (bike and trailer) be MUCH less effecient than having it loaded up on the bike?

    I've never used either, so I'm genuinely wondering about the differences and what might contribute to those differences (drag, sway, gravity, weight distribution, etc.).

    Thanks in advance.

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    Senior Member ken cummings's Avatar
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    I have gone both ways. I'll stink with bags as they are easier to remove when I need to go into a store or shop. I carry a U-bolt lock even on tour for bike security.
    This space open

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    Senior Member teamcompi's Avatar
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    You will never get a consensus on this one but IMHO trailers have a place, just not in my life for on road touring. A good touring bike with panniers seems to ride better especially up hill, downhill and around corners, trailers have an upside in the wind on the flat. Trailers can indeed behave strange under speed or when the road gets tough. That said trailers are great in situations where you need or want to carry a ton of stuff, enjoy the organization of a sac, and/or you ride a bike which is hard to rack up.

    Like many things though once you are used to it the trailer just seems to disappear, and touring with one is better than staying home, by far.

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    Thanks. I understand it's a matter of preference, I guess I'm trying to understand why one would prefer bags to trailers. To my uninformed mind it seems like trailers are less effecient given how they're attached, where the weight of the gear is carried, and the drag. Anyone who prefers trailers disagree?

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    Senior Member greenstork's Avatar
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    The only physics in play here is the fact that the trailer itself is slightly heavier than racks and the trailer wheel does add some extra rolling resistance. But trailers don't add drag other than rolling resistance and as other have pointed out, and may actually decrease wind resistance compared to racks and panniers.

    But you're still moving a given amount of mass up and down a slope using different gears. Take wind and rolling resistance out of the equation -- how that weight is positioned fore and aft, on the bike or off the back of the bike -- makes no difference on how much effort is needed.

    I think the decision to tour with racks/panniers vs a trailer is more about ride characteristics, terrain, transportability, adaptability and personal preference than it is about physics. The bike will ride differently with a rack, cornering will be different for instance, and balance. The amount of force required to push the bike up a hill will be roughly the same however.

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    Mad bike riding scientist cyccommute's Avatar
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    Quote Originally Posted by greenstork
    The only physics in play here is the fact that the trailer itself is slightly heavier than racks and the trailer wheel does add some extra rolling resistance. But trailers don't add drag other than rolling resistance and as other have pointed out, and may actually decrease wind resistance compared to racks and panniers.

    But you're still moving a given amount of mass up and down a slope using different gears. Take wind and rolling resistance out of the equation -- how that weight is positioned fore and aft, on the bike or off the back of the bike -- makes no difference on how much effort is needed.

    I think the decision to tour with racks/panniers vs a trailer is more about ride characteristics, terrain, transportability, adaptability and personal preference than it is about physics. The bike will ride differently with a rack, cornering will be different for instance, and balance. The amount of force required to push the bike up a hill will be roughly the same however.
    Actually, racks and bags aren't really lighter than a trailer. My bags (Ortliebs) and racks weigh in at about the same weight as a trailer.

    The place where physics and a trailer really come into play is on corners. The bike, bags and rider are one unit with the forces of cornering applied only to the wheels of the bike. All forces are acting on this single unit and the two contact patches of the tires.

    When a trailer used instead of bags, you now have a separate unit which the forces of cornering act on independently (or nearly ) of the towing vehicle. Because the trailer is typically attached low on the bike, it can effect the center of gravity and how the bike maintains contact with the road. As you enter the corner, the trailer has it's own momentum and a vector that it wants to follow off the road which the bike isn't following at the same time. This tends to push the back of the bike off line towards the outside of the corner. Additionally, the trailer can 'lift' the back of the bike and reduce the contact patch area a little, leading to more force towards maintaining the straight line and less force to continuing around the corner.

    Other forces on the bike are frame flexure as the trailer wants to keep going straight. The frame flex can translate all the way to the front of the bike and change the steering characteristics of the bike. If you have a stiff frame, this flexure is less but still noticeable and, the higher the load, the worse this flexure would be.

    I've ridden a flexible framed bike with a trailer and a stiff framed bike with a trailer and both have issues with corners. The flexible framed bike couldn't hold the line on a corner with as little as a 40 lb load. To be fair, this bike doesn't do well with much more than that in panniers. The other bike still had some problems but not as much but it also has a front shock which seems to help with the wandering on corners issue.

    I do think that trailers have their place...like off-road touring or bikes that can't have racks mounted. I use one for off-road tours because the benefits of being able to loft the front wheel over obstacles outweigh the cornering issues. If I have to walk sections, I found that a trailer makes the bike a bit easier to handle than panniers, too.
    Stuart Black
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  7. #7
    cyclopath vik's Avatar
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    I have both setups and frankly they both work well. I would generally go with panniers/racks because I try to carry as little as possible and two rear panniers are easier to deal with than a trailer. But, if I need to carry a ton of stuff my Bob trailer hauls heavy loads well.

    The reality is that which solution will work for you is highly dependent on you, your bike and your load. If you have the possibility of trying some of the gear you are interested in before buying that would be a smart idea.
    safe riding - Vik
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    Thanks all.

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    Senior Member Gotte's Avatar
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    I've carried my daughter in a trailer and on my bike, and on my bike is by far the easiest. Not a scientific test, but that's my only exerience.

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    People always say their trailer weighs X relative to their racks and bags, which seems to leave out the weight of the bags used in the trailer. Obviously those bags aren't as technical as the panniers, no boards required to hang them on racks, but at one extreme they might end up as internal frame packs or waterproof canoe bags. I thnk we do at least have to consider that weight.

    I think there is more resistance than rolling resistance in a trailer, but I already blabbed at length on that subject this month.

    If I had the free use of all the best gear out there. I would probably use the paniers for ultralight and some kinds of technical terrain, and add the trailer when the load got above a certain point. There is probably some situation when any of us would switch for other than what we have, or mainly use.

  11. #11
    Mad bike riding scientist cyccommute's Avatar
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    Quote Originally Posted by Peterpan1
    People always say their trailer weighs X relative to their racks and bags, which seems to leave out the weight of the bags used in the trailer. Obviously those bags aren't as technical as the panniers, no boards required to hang them on racks, but at one extreme they might end up as internal frame packs or waterproof canoe bags. I thnk we do at least have to consider that weight.

    I think there is more resistance than rolling resistance in a trailer, but I already blabbed at length on that subject this month.

    If I had the free use of all the best gear out there. I would probably use the paniers for ultralight and some kinds of technical terrain, and add the trailer when the load got above a certain point. There is probably some situation when any of us would switch for other than what we have, or mainly use.
    I'll admit that I didn't think about the weight of the bag on the trailer. If you wanted to be a real weight weenie about it, you could probably carry your gear in garbage bags.

    The one other place that I think trailers have a use would be in tandem touring. We never could figure out how to do tandem touring and carry all the stuff you need for 2 people in 4 bags, considering that, for the most part, two people on 2 bikes carry at least 6 bags...usually
    Stuart Black
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  12. #12
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    Jackknife is a possibility with a trailer also it gives you less options in portability and where you can take it
    The Ferrari ('05 Bianchi Forza) had a flat (Stupid Glass) the Japanese wagon ('77 Nishiki with Arkel Utility Basket) was in the body shop (On my bench being repainted...repent ye rust)
    so I took the SUV ( Cannondale V2000 Active 100SL)

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    For a newbee, its hard to imagine that there is no meaningful cost, weight, handling, or capacity difference between trailers and panniers. But it's true. If your bike can handle racks and panniers and you will be touring on the road, there is no one reason or cummulative reasons to choose one over the other. But we will be doomed to see this thread over and over again....

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    Quote Originally Posted by Cyclesafe
    For a newbee, its hard to imagine that there is no meaningful cost, weight, handling, or capacity difference between trailers and panniers. But it's true. If your bike can handle racks and panniers and you will be touring on the road, there is no one reason or cummulative reasons to choose one over the other. But we will be doomed to see this thread over and over again....
    Well, just to clarify, I was looking not for what each of the thousands of members prefer but WHY they prefer it, specifically looking at the performance characteristics of each method which hasn't been addressed in other threads as far as I can tell. And really hasn't yet been addressed here. There are clearly performance differences--carrying a trailer just isn't the same as carrying bags no matter how much one thinks the rider won't notice it. That's what I was trying to find out so I can make my own determination. So sorry to doom you to that fate. There's always the option of either (1) contributing to the thread or (2) skipping it.

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    Mad bike riding scientist cyccommute's Avatar
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    Quote Originally Posted by russian fighter
    Well, just to clarify, I was looking not for what each of the thousands of members prefer but WHY they prefer it, specifically looking at the performance characteristics of each method which hasn't been addressed in other threads as far as I can tell. And really hasn't yet been addressed here. There are clearly performance differences--carrying a trailer just isn't the same as carrying bags no matter how much one thinks the rider won't notice it. That's what I was trying to find out so I can make my own determination. So sorry to doom you to that fate. There's always the option of either (1) contributing to the thread or (2) skipping it.
    From your original post, I think I pretty much addressed your questions with the exception of drag. But air drag just isn't that big an issue with touring bikes. You don't often move fast enough for it to be something to consider. Additionally, if you look at bags on a bike, they might even offer some aerodynamic advantage on their own. Most bags mounted on the front of a bike aren't big sails but are more like airfoils mounted on either side of the wheel. The air is going to flow around them and not catch the big animal sitting on the saddle. Add in a handlebar bag and you might get ever more advantage. (I have noticed you can tuck behind a handlebar bag on a down hill and get a little more speed out of the bike. )

    With a trailer, you have the drag of an additional wheel and the wind hits the rider full force. I don't know about you but I'm no too airfoil in shape I'm more like a square sail going backwards into the wind

    Do I have data to back this up? Nope. Is it all conjecture? Yep. Is it wrong? Maybe or maybe not. Is a trailer better than bags? Depends. What do I use? Bags. Why? It's just easier to deal with, unless it's rough. What should you use? What ever you want.
    Stuart Black
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  16. #16
    You know you want to. Eatadonut's Avatar
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    Quote Originally Posted by ken cummings
    I'll stink with bags
    Might want to add one of these to your bags:

    Weather today: Hot. Humid. Potholes.

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    Thanks--sorry for sounding snippy. Having a bad day here at my desk...

    Do I have data to back this up? Nope. Is it all conjecture? Yep. Is it wrong? Maybe or maybe not. Is a trailer better than bags? Depends. What do I use? Bags. Why? It's just easier to deal with, unless it's rough. What should you use? What ever you want.[/QUOTE]

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    Mad bike riding scientist cyccommute's Avatar
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    Quote Originally Posted by russian fighter
    Thanks--sorry for sounding snippy. Having a bad day here at my desk...
    'sokay. We all have days like that. That's why we tour
    Stuart Black
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  19. #19
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    I suppose one, if inclined, could do a vector analysis at the three versus two points of contact with the ground while in a turn.

    What I can contribute, however, because it measured, it is that 1/3 of the weight of a loaded BOB trailer is exerted on the rear hub. If the loaded trailer weighs 60 lbs, the 20 lbs extra on the rear wheel is not much different than the weight of rear racks and loaded rear panniers. What you don't have with the BOB is panniers on the front, which all would agree add stability.

  20. #20
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    I will try to rephrase the questions and arguments of Cyclesafe and Russian Fighter, and add my own experienve with panniers and a 2-wheel cargo trailer.


    I have lots of touring experience with panniers, even from the days when we had fairly flexible frames and only rear panniers. I have also used a child trailer both for humans and cargo.

    The trailer – especially the 2-wheel variety – definitely fits on more bikes, but even if you have the right kind of bike that works well with panniers, there are differences between using panniers or a trailer. Apart from personal preferences, I would say that:

    – There is a learing curve in riding with panniers. Not a huge one, but still, one notices some differences in bike handling. Even on a good touring bike that I know well, it takes 30-60 minutes to get used to the different handling and about a couple of hours to really feel totally in tune with the bike. However, after that, I feel that I can do whatever tricky manoeuvres I want to do, within limits of course.

    – The learning curve with a 2-wheel trailer is nil. With 30-40 lb of gear, the only difference is that it starts slower. However zigzagging, high speed manoeuvres... are not as easy to do. It doesn't make a difference on easy terrain, but if I decide to tour through the mountains (steep hills and winding roads) or on recreational paths (more zigzags), I'm about 1-2 km/h slower with the trailer.

    – If you enjoy recreational paths, beware. In many places, they have the very unsafe habit of installing non-breakeable bollards at all intersections. If you can go through in a straight line, a trailer has a slight advantage, but if there is an offset or a chicane and you have to zigzag, then a trailer becomes problematic.
    Michel Gagnon
    Montréal (Québec, Canada)

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    Thank you!

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