I need help with rake and trail

[darwin-t]

I'm building a bike based on the Voyageur - except I'm using a rigid frame - no suspension and am having trouble with the rake and trail.

I have the rake at 70 degrees, but my rake is 3 1/4. It looks like I'd have to go to 75 degrees rake to get a 2" trail. But as I do that, my front wheel is moving back and I may get into a spot where it could interfere with the pedals. I don't know how much trouble that could be. Does one need to pedal while going around corners?

I don't trust my welding enough to add onto the front of the frame....

On the plus side my seat is 6" lower than before (I cut my frame up and started over) - it is 20" from the ground now.

I don't want to proceed until I get some advice from knowledgeable people.

Thanks.

[darwin-t]

Here's a photo of it

http://i2.photobucket.com/albums/y2/darwin-t/V10.jpg

I did the static test, where I turned the front wheel and measured the frame to see if it drops. It drops 3/16"

[locolarry]

Is BB mounted "under" the main beam? If so, why not mount it topside and pick up 1 1/2 inches of clearance?
Loco

[SirJoey]

Is BB mounted "under" the main beam? If so, why not mount it topside and pick up 1 1/2 inches of clearance?
LocoActually, he'd gain about 3", LL.
The thickness of the frame, PLUS the thickness of the BB shell. :)


http://img3.imageshack.us/img3/49/signaturehalloween.jpg

[locolarry]

Sir Joey,
That's the reason You are the "Super Guru"... You are correct! :jester:
Loco

[locolarry]

Darwin,

I have a factory-built SWB..a Lightning Thunderbolt, (See Avitar)that I had to learn to ride by leveling pedals or having "turn-side" pedal on top and coasting through turns.. It doesn't take too long to get used to riding that way. I do, however, prefer to be able to pedal whenever I want to or need to even when turning. There can be "Bad" moments with lots of healstrike!
This bike has a 700 cc rear and a 16" front wheel...can be a bit squirrly on wet or rough pavement, too.

Don't make any moves just yet, wait for the rest of the comments that are coming...The "Oz"'s haven't commented, YET!!! We really DO have some mechanical Genius' on this Krew!
Loco

[darwin-t]

Is BB mounted "under" the main beam? If so, why not mount it topside and pick up 1 1/2 inches of clearance?
Loco

I don't know what "BB" is, but if it is where the crank for the pedals go, yes it was, but I flixed it.

I found a bike with a smaller fork and wheel.


I flipped the main rail to eliminate the interference between the pedals and front wheel. This also changed my rake.

Before rail flip - rake 70 degrees, trail 3 1/4"

After rail flip

With the original wheel - rake was 77 degrees, trail was 2"

With the smaller fork and wheel - rake was 80 degrees and trail was 1"

Here are views of the different configurations - from left to right

http://i2.photobucket.com/albums/y2/darwin-t/3views.jpg

70 rake, 3 1/4 trail
77 rake 2" trail
80 rake 1" trail

I noticed no frame drop when turning the wheel in the last two configurations.

The steering head is just tack welded. I thought I check the angles before changing the rake again.

[bambuko]

I don't know what "BB" is...
yes you have guessed correctly:
BB stand for Bottom Bracket (http://sheldonbrown.com/gloss_bo-z.html#bottom)
Your project is very interesting - I am also thinking about building "simplified" Voyager.

[PeterT]

Looking like you sorted it out without too much trouble!

Well Done!

The beauty of customising things is that there are no directions or positions that are THE ONLY way those parts can go.

If your idea incorporated using a BB lined up vertically, and through patience, trial & error, you got it to work the way you were expecting it to work, when you first had the idea, then everyone would be asking you "How did you get that to work, teach us Master!"

PeterT

[darwin-t]

Looking like you sorted it out without too much trouble!

If your idea incorporated using a BB lined up vertically, and through patience, trial & error, you got it to work the way you were expecting it to work, when you first had the idea, then everyone would be asking you "How did you get that to work, teach us Master!"

PeterT

You give me way too much credit. I started out with a bike on instructables.com, but failed to notice one important point - the bottom rail was supposed to lay horizontally, not vertically. Failing to notice this caused me to chop up the donor bike frame more than necessary and made it harder to attach the rear triangle and the BB, and also the USS. I also set the seat up much higher than it should have been. I also had very narrow tires from an old Ross 10 speed bike which looked really stupid on the heavy frame. I also had 26" tires on the front and rear.

Then I bought the plans for the Voyageur and decided to cut up the frame and use those plans as a guide. I meant to put the BB on top, but did it wrong and didn't even notice for a couple of days. I think I have all of the flaws worked out now. Persistence works as well as intelligence sometimes, it just takes longer.

[savarin]

Just as an insight.
Head angle, rake and trail all go together in the hope of getting steering that works for you.
My experience has been that you can get used to anything, phenomenal flop, twitchy turning etc.
In my humble experience (and I may be totally wrong here) the most important measurement is trail, about 2"-2.5" appears to be my preference.
This should be measured by plotting the steering axis to the ground then dropping a vertical from the wheel axle.This distance between the two marks is your trail.
heaps of trail makes it go straight and may be difficult to turn.
little trail makes the steering very sensitive.
Head angle fits in with this. Steep angles reduce trail, shallow angles extend the trail.
Eventually you get into chopper angles and fork flop starts becoming a problem to engineer out by playing with offset to reduce the huge trail these angles introduce.
All frames will drop a little when turning unless the head angle is 90' The shallower this angle becomes the more the frame will drop.
Accepted wisdom seems to prefer the measurements most manufacturers use but these is a huge amount of flexibility in them and eventually comes down to customer preferences.
Have a look here http://www.tonyfoale.com/
as he is one of the experts.

[darwin-t]

Thanks savarin.

I found a little freeware program on the website you provided a link for. It figures steering geometry. It asks for rake (measured from vertical, hot horizontal, so I subtracted the rake from 90) and "yoke offset". Would I be right to assume that is how far the axle is in front of a line extending down through the steering axis?

If you know those two and the diameter of the tire, it pops out the trail. This could be a handy little program.

http://www.tonyfoale.com/progs/castor1.exe

[savarin]

Yoke offset is the amount the fork legs are offset from the steering axis.
Most motorcycles have a triple tree (so called) that the fork legs fit into and the steering tube is also fixed to this yoke.
A lot of home built chopper bicycles also use this idea.
This allows straight fork legs whilst also producing an amount of trail.
Bicycle forks often curve the bottom of the fork leg forward. Modern straight fork legs tilt the steering tube to produce the same effect.

[John Lewis]

The difference between the vertical to the ground from the axle and the point at which the extended axis of the steerer tube meets the ground is trail. The curving forward of the forks is rake.

Rake can be used to some extent to adjust the trail. On my LWB the trail was around 6". Way too much. I chose to use pretty extreme rake to bring the trail down to around an inch.
The handling was dreadful prior to this. If you plan to rake the fork it really needs to be considered when you are planning the build.
There is good advice here.
http://bikesmithdesign.com/Design/12Steps.html

John Lewis

[darwin-t]

The difference between the vertical to the ground from the axle and the point at which the extended axis of the steerer tube meets the ground is trail. The curving forward of the forks is rake.

Rake can be used to some extent to adjust the trail. On my LWB the trail was around 6". Way too much. I chose to use pretty extreme rake to bring the trail down to around an inch.
The handling was dreadful prior to this. If you plan to rake the fork it really needs to be considered when you are planning the build.
There is good advice here.
http://bikesmithdesign.com/Design/12Steps.html

John Lewis

I'm confused. I think that I have heard both the angle of the steering axis and the curvature of the forks referred to as "rake" even though they are different things. I realize that curving the forks more moves the axle forward which is how it reduces the trail. Right?

[savarin]

The major problem is that the word "rake" is used differently by motorcyclists and bicyclists.
I'm too lazy this time of night to look it up but generally its used to describe:- how far the front wheel is from the frame, or, the angle of the forks, or, the steering head angle, or, the distance the wheel axle is in front of the fork axis if they were straight.

Steering head angles between 68 - 74' then play with fork offset (rake) to give a trail of between 1.5" - 2.75" seems to give the most reasonable handling.

[John Lewis]

Perhaps this will help to clear it up.

http://images.google.com.au/imgres?imgurl=http://bp0.blogger.com/_SZx2oaxuJSA/Rjr_RgZu19I/AAAAAAAAAcA/3juu90abktk/s320/Trail30rake.jpg&imgrefurl=http://www.wellsphere.com/biking-article/trail-fork-rake-and-a-little-bit-of-history/236726&usg=__0sVfqwC0pHWShgXoRo3eiQuWv9g=&h=320&w=260&sz=13&hl=en&start=1&tbnid=F4NH6t5paoDlgM:&tbnh=118&tbnw=96&prev=/images%3Fq%3Drake%2Band%2Btrail%2Bbicycle%2Bfork%2 6gbv%3D2%26hl%3Den%26sa%3DG

Reasonable diagrams and explanations here.

John Lewis

[savarin]

nice clear article John.

[darwin-t]

Very informative AND interesting. I bookmarked your archive page for further reading. Thank you.

[John Lewis]

You're welcome darwin-t. Hope it assists with your build. There's lots of information out there. Finding it is the problem.

John Lewis

[darwin-t]

I wrote to a guy that has a mathematics web page and he graciously gave me a formula I asked for. My wife's cousin wrote a simple spreadsheet for the formula. You input the steering axis angle, tire radius and rake and it pops out the trail. If you use zero for the rake, it will show you the trail of a straight fork and you can easily see how much rake you need to get the trail you want. 1" rake reduces the trail by 1", etc.

http://darwinsastroworld.com/Trail.xls

I'll try to write a stand alone program for it, or if someone here can do it, please do so. The formula is in cell B5.

[John Lewis]

Have a look at this online calculator for recumbent bicycle trail etc

http://www.wisil.recumbents.com/wisil/trail.asp

John Lewis

[darwin-t]

I remeasured everything and ran the numbers through my calculator. It came out with 1.77" trail with the larger wheel, 0.76" with the smaller wheel.

Would 1.77" be okay, or do I need to change my steering axis angle?

[darwin-t]

The trail came out to 1.52" or .76". Is 1.52" enough trail?

[fultondp]

Anywhere in that ballpark should be fine. Try not to get too wound up on the perfect amount of trail, it's not as big a deal as I sense you are trying to make it. You are well inside the range you want to be for a nice neutral ride. The chopper I made for my daughter has 9" of trail, and is still very easy to ride. Unless you are unusually sensitive, the recumbent experience ,especially with USS, will probably overwhelm any slight difference you would notice in the trail and head angle. After your 3rd of 4th bike you might start noticing the differences. By then you will have enough experience to know how to change it if you don't like it. :sunny:

Darren