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Old 08-06-14, 07:35 PM   #1
SkylarG
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Front-loads and low-trail vs. wheel flop.

I'm hoping someone can explain where trail vs. wheel flop comes into play with handling front loads. The discussions I find all seem to attribute this to trail, where high trail will give leaning more streering force and lower trail will be more about hands-on steering.

Wheel flop, according to Wikipedia, affects steering "Because wheel flop involves the lowering of the front end of a bicycle or motorcycle, the force due to gravity will tend to cause handlebar rotation to continue with increasing rotational velocity and without additional rider input on the handlebars."

In the mob of low-trail 650b bikes I see around, they all rely on heavy rake forks to achieve low trail figures. I'm wondering why frames are not being designed to use a steeper head tube and a standard 43-50mm offset fork.

E.g. the Soma Grand Randonneur uses a 72d head tube angle, and 69mm offset fork to get 34mm trail and 10mm flop.
By contrast, a frame could be built with a 75d head tube angle, and a 45mm offset fork to get 41mm trail and also have 10mm flop.

It seems to me that having a 15lb bag sitting off your bars or on a front-rack that moves with and pulls your handlebars/wheel in the flop is a bigger factor than how the bike trails.

So, what I'm wondering is, can this steep head tube idea maybe offer a bike that can carry a front load well (low flop), yet also be a little more stable at higher speeds compared to the lower-trail alternative?
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Old 08-06-14, 07:47 PM   #2
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my understanding is that one aspect of low trail bikes that helps accommodate front loads is simply having the cg of the load behind the axle
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Old 08-06-14, 09:50 PM   #3
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Perhaps I speak blasphemy But I never put a lot of credence in wheel flop/frame drop. I have thought about this since the late 1970s and always felt that the amount of real life steering angle amounts to very little flop, even given a high flop geometry. Given that most steering angles are only a few degrees from straight ahead yet to see much frame drop requires significantly more steering angle I just don't see the importance of flop as a design goal.

Now if we're talking about CG and leverage we have another discussion. Andy.
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Old 08-07-14, 09:01 AM   #4
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Point well taken, Andy. Could you elaborate on this though?

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Now if we're talking about CG and leverage we have another discussion. Andy.
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Old 08-07-14, 11:22 AM   #5
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Subscribe to and read Vintage Bike Quarterly... Jan wrote a lot on that . Ie .. French Porteur bikes
they ferried bundles of newspaper to the Parisian street vendor's kiosks, from the publisher.

the inverse happens I have a Brompton, 35mm of trail , the bike feels better with a load of stuff in the front Bag,
which is mounted on the front of the headtube.

Trail is usually increasing the Rake /Offset .. epitome of flop is low head angles and long chopper forks ...

but , stable as heck in a straight line.. just a long turning radius


Quote:
I'm wondering why frames are not being designed to use a steeper head tube and a standard 43-50mm offset fork.


Build one and see.. 1st Hand.

Last edited by fietsbob; 08-16-14 at 10:41 AM.
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Old 08-07-14, 11:43 AM   #6
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people are generally unhappy with the steering of a 75 degree head tube, except maybe on the track. I fail to see how adding weight would help
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Old 08-07-14, 05:11 PM   #7
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I'll admit to a vast amount of study but a minimal ability to explain my thoughts about steering geometry and load carrying. I have always liked the handling of a long (6-6.5cm) trail bike. I can't really say why but they just feel nicer to my manors of riding. I read VBQ and take issue with what Jan writes (his latest tandem article hits so many wrong buttons IME...) but I do respect his attempts to make sense and explain things.

So what do I feel about CG and leverage?

I like my front load to be low and behind the steering axis. My touring bikes all handle nicer w/out a handle bar bag and with low rider panniers. (but I do use a handle bar bag on tours. Just another example of balancing needs against abilities). I like a rear ward weight bias with a camping tour bike. It's hard to carry all the gear with a front bias when you're hauling 70+ lbs of stuff. So a low and rear biased CG is my choice.

As to leverage- I've thought that a greater trail means that the side/steering forces have to travel further to make the same amount of steering angle change. My rear weight bias has the "tail wagging the dog" nature and minimizing the steering effects of such seems to work for me. (And this is born out on the tandems I have owned and test ridden. For an experienced team a long trail bike handles nicer then a short trail tandem. The experienced stoked doesn't provoke the stoker steer that a newbie does and the more single bike handling nature of the long trail tandem often is preferred by the experienced team).

When I first started building frames I read everything I could on design including some of a series of articles (Mullett's Mechanics) from a British magazine. Is proposed the wheel flop (or the frame raise/drop) theory as to certain handling aspects. Later reading of Jim Papadopoulos et all's research the lever arm/trail aspects seemed to make more sense (although I won't begin to say I followed their math).

Granted this is a bit of a tangent from the OP's question of front end loaded situations. But it's what I have played with for a long time. Andy.
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Old 08-07-14, 06:17 PM   #8
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Andy, did you use trail when you meant rake? thanks
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Old 08-07-14, 08:00 PM   #9
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Eric- I don't think so. My understanding of the steering forces is that they first act on the tire contact point with the road. I could be wrong (have been before, will be again) but I feel that the rake only exists to achieve the trail. Andy.
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Old 08-07-14, 08:31 PM   #10
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it's just that everyone else is singing the praises of low trail, wanted to make sure
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Old 08-16-14, 07:32 AM   #11
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Quote:
Originally Posted by Andrew R Stewart View Post
Perhaps I speak blasphemy But I never put a lot of credence in wheel flop/frame drop. I have thought about this since the late 1970s and always felt that the amount of real life steering angle amounts to very little flop, even given a high flop geometry. Given that most steering angles are only a few degrees from straight ahead yet to see much frame drop requires significantly more steering angle I just don't see the importance of flop as a design goal.

Now if we're talking about CG and leverage we have another discussion. Andy.
I think the trail/flop/load story is ALL about CG and leverage. Ultimately if a bike rides with more or less stability the story is about forces or torque around the steering axis. I don't know if any one has done a fresh-eyes analysis to see if CG (distance of the load from the tire contact?) seems to be a more critical variable than trail. It would all be in the way one looks at it.

I expect head angle and offset really only enter into it to the extent they affect trail - no more magic to it than that. And to the OP's point, you can get the same trail with a large head angle (steep) or a small head angle (shallow). But the rakes would be different between any two cases.

From a frame design point of view, extremes of head angle or rake affect other aspects of the bike: how the calipers reach to the wheel, ability to have fenders, toe interference, rider/load weight distribution, and I think there are others.

So Andrew, I'd welcome seeing a more informed (than I can provide) discussion of CG, leverage, load, and fork/head geometry. I've done some experimenting with trail and load, and I think there Jan has something: between a bike with high trail and a similar (in my case) one with lower trail (~60 mm v. ~40 mm in my case) the lower trail bike is easier to ride with a loaded front rack bag that does not sway much.
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Old 08-16-14, 09:26 AM   #12
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I think of it this way -

Trail is what makes the bike want to straighten out when you aren't giving the bike a steering input. The amount of weight on the front wheel makes a given amount of trail more or less effective at self centering the steering..........more weight on the front wheel makes the bike want to self center more and when you place a bag and weight up front nearly all that added weight acts on the front wheel and gives the steering more self centering.

So the idea is if you start with a lot of trail and then add cargo weight to the front it will have a high self centering force. This makes the bike more 'stable' and it will take more force to get the bike to turn.

So.............the idea is if one wants to carry weight on the front of the bike, and still wants nimble handling, the amount of trail should be reduced so that the added cargo weight, acting on the given amount of trail, doesn't make the bike more stable than is ideal.

My coffee hasn't fully kicked in yet so I hope I didn't just muddy the water with the above.

Have a great weekend.

dave
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Old 08-16-14, 10:43 AM   #13
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+ the size of the contact patch of the front tire, on the street..
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