Head Tube Angle Question?

[JayRay]

Hi All,
I am still ever so slowly progressing on my LodeRunner
build and have come to a puzzling point.

I am using a larger tire, 24" than the 20" plans call for.
The plans call for a 64 degree head tube angle but
when I set it at 64 there seems to be a problem.

These photos show the head tube angles set at 64 degrees.
(Click all photos to see them full size)
Plan Photo
http://i459.photobucket.com/albums/qq317/JustJay2009/Head%20Tube%20Angle/th_FrontWheelAZ64degrees.jpg (http://s459.photobucket.com/albums/qq317/JustJay2009/Head%20Tube%20Angle/?action=view&current=FrontWheelAZ64degrees.jpg)
My Wheel
http://i459.photobucket.com/albums/qq317/JustJay2009/Head%20Tube%20Angle/th_FrontWheel64degrees.jpg (http://s459.photobucket.com/albums/qq317/JustJay2009/Head%20Tube%20Angle/?action=view&current=FrontWheel64degrees.jpg)

I am getting what i believe to be excessive wheel flop.
When I turn the tire left or right the wheel lays over
and the front of the frame dips. The wheel then has to
lift the frame to return to the straight position.
Left Turn
http://i459.photobucket.com/albums/qq317/JustJay2009/Head%20Tube%20Angle/th_WheelFlop-01.jpg (http://s459.photobucket.com/albums/qq317/JustJay2009/Head%20Tube%20Angle/?action=view&current=WheelFlop-01.jpg)
Right Turn
http://i459.photobucket.com/albums/qq317/JustJay2009/Head%20Tube%20Angle/th_WheelFlop-02.jpg (http://s459.photobucket.com/albums/qq317/JustJay2009/Head%20Tube%20Angle/?action=view&current=WheelFlop-02.jpg)
You can see the wheel off the ground in above photo

I played around with some photos and it seems I will
need to increase the head tube angle to get the same
effective trail as the 20" wheel and decrease the
wheel flop.
Both with 64 degrees
http://i459.photobucket.com/albums/qq317/JustJay2009/Head%20Tube%20Angle/th_FrontWheelOverlay.jpg (http://s459.photobucket.com/albums/qq317/JustJay2009/Head%20Tube%20Angle/?action=view&current=FrontWheelOverlay.jpg)
With my head tube angle "adjusted"
http://i459.photobucket.com/albums/qq317/JustJay2009/Head%20Tube%20Angle/th_FrontWheelOverlay2.jpg (http://s459.photobucket.com/albums/qq317/JustJay2009/Head%20Tube%20Angle/?action=view&current=FrontWheelOverlay2.jpg)

Does the larger wheel affect the head tube angle enough
to cause this or is there something else I am not getting?

I have read many posts here about head tube angles, rake,
trail, etc. but I can not help but think I missing something here.

I know the angle grinder is my friend but
I would rather do this once the right way.

Thanks for your help, opinions, ideas, etc.
Jay

http://www.seamuspuppy.com

[trikeman]

Others here are certainly more expert than me on the intricacies of steering dynamics, and will hopefully chime in, but I always shoot for around 1"-2" of trail and that seems to work for me. When you put a larger wheel on the bike you are making the trail longer at the same head tube angle. I believe you need to increase your head tube angle so that the trail is less in the 1-2 inch range. Even with 2" of trail, I still got some wheel flop on my DW.

[GregLWB]

I am really going to pay attention to this thread because later this summer I will be dealing with similar issues.:1eye:

I don't know that this helps but on the HR I built going with the 20" wheel in front instead of the 26" changed the head tube angle quite a bit. On the 26" version the plans show the head tube going through the main boom at 90 degrees. On the 20" version it's closer to 72 or 73 degrees? I don't remember exactly and I don't have the plans handy.

I think that your change in angle is due to the larger tire in the front. If all your wheels were the larger size your ratios would remain the same. My new guy two cents.:jester:

Greg

[Greenhorn]

I am really going to pay attention to this thread because later this summer I will be dealing with similar issues.:1eye:

I don't know that this helps but on the HR I built going with the 20" wheel in front instead of the 26" changed the head tube angle quite a bit. On the 26" version the plans show the head tube going through the main boom at 90 degrees. On the 20" version it's closer to 72 or 73 degrees? I don't remember exactly and I don't have the plans handy.

I think that your change in angle is due to the larger tire in the front. If all your wheels were the larger size your ratios would remain the same. My new guy two cents.:jester:

Greg


New Guy? You've got 2 builds under your belt! You are approaching Guru status.:)

[Odd Man Out]

My new guy two cents.:jester:

Greg

You a new guy with two builds and 654 posts??????
Uh right.
Don't be afraid to join in. You are fatally addicted. :punk:

[TheKid]

I had the same problem with the DR. I adjusted the angle so it's much steeper. After adjustment, there was about 1" of trail with little wheel flop, which greatly improved the handling for me. I have since changed the wheels from 26" to 20", and there is very little trail. The handling is about the same, but there is almost no wheel flop at all, and still no tire scrub. If you don't plan on going real fast, say over 15mph, a steeper angle should be no problem. But remember that you may have to mod the front tube to keep the main boom level.
It seems that the amount of trail is a matter of personal preference. Some builders advocate 2 or more inches, while others say 1/2" for a bike, 0" for a trike.

[Zwolf]

A quick pair of questions that may not be entirely on topic, but...
How do you guys find the angles your working at/on?
and
What is this talk about trail and scrub? What is it?

Thanx. :)

[GregLWB]

You a new guy with two builds and 654 posts??????
Uh right.
Don't be afraid to join in. You are fatally addicted. :punk:

LOL!:jester: Ok OMO and Greenhorn, I guess I better clarify. On the trike/quad stuff and the chopper stuff I have no/little experience so on those subjects, yes New Guy.

On the other stuff I am far more comfortable but I still have yet to build something totally 'OFF' plan. But I know that if I run into issues I have help.:1eye:

Greg

[Patrike]

A quick pair of questions that may not be entirely on topic, but...
How do you guys find the angles your working at/on?
and
What is this talk about trail and scrub? What is it?

Thanx. :)

Sheldon Brown:
Trail
Trail is the distance from the contact point of the front wheel with the riding surface to the intersection of the steering axis (head tube) with the surface. The trail is a function of the head angle, the fork rake, and the tire diameter. Trail has a major effect on the handling of a bicycle. More trail increases the bicycle's tendency to steer straight ahead. A bicycle with a largish trail dimension will be very stable, and easy to ride "no hands". A bicycle with a smaller trail dimension will be more manuverable and responsive.

My definition: Scrub -- usually used in describing trikes and trailers - when one tire is fighting the other for the direction it is trying to go - it or they will scrub(wear/rub) on the riding surface - pending how much is alighnment is out the more or less scrub.

Hope this helps.

[TheKid]

A quick pair of questions that may not be entirely on topic, but...
How do you guys find the angles your working at/on?
and
What is this talk about trail and scrub? What is it?



The head tube angle refers to the degree of tilt in relation to level ground. If the head tube is mounted straight up and down, it's at 90°. Depending on how much trail you want, which is the difference between where the center of the tire meets the ground and the point to where the center of your head tube would touch the ground if it were long enough. That point is determined by drawing an imaginary line along the length of the head tube to the ground. Wheel scrub occurs when the wheel cannot self center, or does not center completely. The tire will skid, or "scrub" rather than roll when turning. It's usually caused by too little trail. Think of a front shopping cart wheel and how it's mounted. That will give you an idea of what trail is.
This should help:

http://davesbikeblog.blogspot.com/2007/05/trail-fork-rake-and-little-bit-of.html

Search the forums for "trail" and "wheel scrub". You'll find a wealth of info.

[Zwolf]

Many thank yous.

[trikeman]

Unless you are experienced with bike design on a CAD (or in my case graph paper), I think it is a mistake to worry too much about what angle you need the head tube at. Instead, I just adjust for the trail I want, and whatever the angle is, is whatever it is.

[Patrike]

Unless you are experienced with bike design on a CAD (or in my case graph paper), I think it is a mistake to worry too much about what angle you need the head tube at. Instead, I just adjust for the trail I want, and whatever the angle is, is whatever it is.

Hey TM -- your not to far from the gospel on that one -- I have seen the angles all over the place on diff comtraptions. I guess some just like to find the sweet spot for optomization of gravitional deflection -- I made that up, but sounds cool! :jester:

[JayRay]

Thanks All,
For your quick and numerous responses.
Reading all of your answers leads me to believe I am on the right
track. I plan on clamping the front end in place and rolling the
frame around and adjusting it until it "feels/handles right" and
cut and weld it there whatever the angle may be.

Patrike
That means I will be searching for the all ellusive "Sweet Spot"
so I can optimize the "Gravitional Deflection" of my front wheel.
:jester:

TheKid
I have already extended the front frame tube to allow me
to set a steeper head tube angel and reduce the trail.

GregLWB
I should have been more specific.
All three tires are 24" instead of 20".
I can certainly see how just changing the front tire
size would change the whole setup more dramatically.

Trikeman
The reason I focused on the head tube angle is that
it is one of the only "specific dimensions" in the plans.

All who have purchased AZ plans will understand what I mean by that.
For the others, the plans give you a step by step text and photo walk
through of the bike/trike build. It explains how, when and why everything
goes where it does and what they do. This allows you to understand the
design and build it exactly as the plan shows or modify it to your taste/needs.

Now all I need to do is find the time to get back out there
and work on my LodeRunner. I am so close to a rolling frame!

Thanks again all for your help!
Jay

[25hz]

To get a better understanding of the rake-HTA-trail relationship, you'll need to experiment a little. You also need to remember that the forks everyone uses (unless you build your own) are all built for upright bikes, and very few of them, or none, have HTAs of 64º. The easiest way to do it at first, is just to duplicate the HTAs of other uprights and your handling will usually always be very manageable across a large speed range.

Another way to solve the wheel flop issue (which means there is too much trail) is to take some of the rake out of the fork legs (the way they curve near the dropouts). Lots of people do this with simple wooden jigs with a supported axle or bolt and gently pull down on the steerer tube to slowly remove the curve. Still another option is find a 24" fork with less rake and turn it around backwards.

Less trail helps with higher speed handling, but makes things feel "light" and skittish at low speeds. More trail helps handling at lower speeds but makes higher speed handling "heavy" and difficult to make subtle steering corrections with. Often, 20" forks are built with less rake when they come off BMX bikes because you want light handling, even at low speeds. Less rake probably works fine with a 64º HTA. 24" forks are generally made for small MTBs and slower speed bikes, so they have more rake, to produce more trail (at standard HTAs) which in turn makes them more comfortable to control at lower speeds. This extra rake (which in turn produces more trail) is likely the source of your wheel flop. Now, without being able to compare the rake and trail for each fork, I could be completely full of crap (it's happened many times before :)) but in my experience, that's generally been the pattern of the forks I;ve used and chopped up.

For most designs in the 2 wheel family, I try to stay in the mid 70s for the HTA with about 2" of rake for a 20" wheel. Is this always optimum? No, probably not, but then I've never built a LWB and all the bikes so far have quite satisfactory handling in the ~5 to 90 kph range, so I let sleeping dogs lie, so to speak. :) I have no problem visualizing or figuring out what geometry changes do to the steering of a tadpole, but a 2 wheeler requires a little more finesse, at least in my mind. While not having hit a "homerun" with perfect geometry yet, it is very satisfying to slowly chip away at it and have plans come to fruition that exceed expectations. I am very liberal when it comes to handling expectations after having built and ridden a few Pythons and swing booms.

[25hz]

It was pointed out to me that, in no uncertain terms, my 2nd paragraph of, uhh . . . "advice" was completely out to lunch. The third paragraph isn't much better. I usually re-read my posts at least once to make sure I'm saying what I intended to say and pick up spelling mistakes, but I'm not sure what the hell I was thinking because the info in that paragraph is completly ass-backwards. My apologies. I hope the OP was smart enough to ignore my "advice" and he managed to rectify his problem. My advice of taking some of the rake out to reduce wheel flop and trail, would in fact do the exact opposite. The wheel flop would INCREASE and so would the trail. At least I appended the 3rd paragraph with the proviso that I could be completely full of crap, because as it turned out, I was.

I did a more applicable, and I think usuable post here, so hopefully it is of more use, in pennance.
Post #9

http://www.bentrideronline.com/messageboard/showthread.php?p=555283#post555283

[TheKid]

With the Meridian, I found that a more laid back head tube angle, and 3-4" of trail, produced too much wheel flop. The result was at slow speeds and starting, I had to fight to keep the bike from tipping over. However, once reaching approx. 5mph, the bike was a dream to ride. Steering with so much trail required leaning into the turn, rather than actually turning the the wheel. So at higher speeds, the laid back head tube angle made for easier riding. I compromised, and adjusted the angle so I had less than an inch of trail. It was much easier to control when starting and at very slow speeds, and at higher speeds, turning the wheel slightly more than I did with the laid back angle, in addition to leaning into the turn, gave the same ease of riding at higher speeds.
With the trike, 3" of trail also produced wheel flop, but there was no danger of falling over due to having 3 wheels. However, it felt uncomfortable for me, so I reduced the trail to almost zero by replacing the 26" straight fork with a curved fork, and using 20" wheels. Much easier than cutting and rewelding the head tube.
I'll be switching to a 24" wheel up front, with a straight suspension fork, and 26" wheels in the rear. When I mocked it up, there seemed to be about an inch of trail, which should be okay. If not, I'll have to change the head tube angle.