Fork rake... and other things

Sniper8052(L96A1) said:

I have been drawing out the RSR plan for a recumbent as found on the
Dutch Speedbike web site in Autocad. Having dimensioned everything and
checked the angles I find I cannot get the rake of the front fork in
front of the centerline of the front wheel, in fact it is 5.19mm behind
the center. As it is clearly illustrated in front of the wheel center
either I am in error or the plan was not true to the bike originally built.

The questions are,

1 How much should the rake of a fork be ahead of the perpendicular
center of the front wheel?
2 Has anyone else built a bike from this plan and found the same error?

Have you taken the size of the tyre into account? when compressed by a
rider?

Silly question as I am sure you have not calculated the trail on the
rim seat diameter..

...d

David Martin said:

Sniper8052(L96A1) said:

I have been drawing out the RSR plan for a recumbent as found on the
Dutch Speedbike web site in Autocad. Having dimensioned everything and
checked the angles I find I cannot get the rake of the front fork in
front of the centerline of the front wheel, in fact it is 5.19mm behind
the center. As it is clearly illustrated in front of the wheel center
either I am in error or the plan was not true to the bike originally built.

The questions are,

1 How much should the rake of a fork be ahead of the perpendicular
center of the front wheel?
2 Has anyone else built a bike from this plan and found the same error?

Have you taken the size of the tyre into account? when compressed by a
rider?

Silly question as I am sure you have not calculated the trail on the
rim seat diameter..

..d

I have only drawn this out on 'paper' ,as it were, to get an
understanding of the design. This was the problem that I encountered
when I tried to get the front end to line up with the listed
measurements and geometry. I have put in a 20inch rim but no tyre
although a tyre would only move the centre of the wheel upward in the
same plane IE: surely the perpendicular would contact the ground in the
same place infront of the illustrated fork angle/trail.

http://uk.pg.photos.yahoo.com/ph/sniper8052/album?.dir=/74f4scd

should illustrate the problem.

Sniper8052

In article <[email hidden]>

Sniper8052(L96A1) said:

I have been drawing out the RSR plan for a recumbent as found on the
Dutch Speedbike web site in Autocad.

A URL might be helpful.

Quoted message said:

Having dimensioned everything and
checked the angles I find I cannot get the rake of the front fork in
front of the centerline of the front wheel, in fact it is 5.19mm behind
the center. As it is clearly illustrated in front of the wheel center
either I am in error or the plan was not true to the bike originally built.

I think maybe you're confusing the terminology.
Rake is the offset of the hub from the steering axis - in other words
how far you've bent the fork.
Trail is the horizontal distance between the steering axis and the tyre
contact point - it is positive when the contact patch is behind the
steering axis.

Quoted message said:

The questions are,

1 How much should the rake of a fork be ahead of the perpendicular
center of the front wheel?

A small (12mm or less) positive trail seems to be accepted as about
right for recumbents.

Quoted message said:

2 Has anyone else built a bike from this plan and found the same error?

Don't worry about it - you can easily adjust the rake to set the trail.
Here's a handy calculator: http://www.kreuzotter.de/english/elenk.htm
There are other factors in recumbent steering design, such as the tiller
effect, but I don't know if the bike you're building is long or short,
front or rear drive, over- or under-seat steering ...

Sniper8052(L96A1) said:

David Martin said:

Sniper8052(L96A1) said:

I have been drawing out the RSR plan for a recumbent as found on the
Dutch Speedbike web site in Autocad. Having dimensioned everything and
checked the angles I find I cannot get the rake of the front fork in
front of the centerline of the front wheel, in fact it is 5.19mm behind
the center. As it is clearly illustrated in front of the wheel center
either I am in error or the plan was not true to the bike originally built.

The questions are,

1 How much should the rake of a fork be ahead of the perpendicular
center of the front wheel?
2 Has anyone else built a bike from this plan and found the same error?

Have you taken the size of the tyre into account? when compressed by a
rider?

Silly question as I am sure you have not calculated the trail on the
rim seat diameter..

..d

I have only drawn this out on 'paper' ,as it were, to get an
understanding of the design. This was the problem that I encountered
when I tried to get the front end to line up with the listed
measurements and geometry. I have put in a 20inch rim but no tyre
although a tyre would only move the centre of the wheel upward in the
same plane IE: surely the perpendicular would contact the ground in the
same place infront of the illustrated fork angle/trail.

http://uk.pg.photos.yahoo.com/ph/sniper8052/album?.dir=/74f4scd

should illustrate the problem.

I'd try a couple of things:

1. see what the trail is on an upright.
2. Work out what the angles would be with the fork loaded - you have
drawn a suspension fork in which pushes the headset up and makes the
steering angle shallower. This will become steeper/lower with the
contact point moving forward relative to the steering axis when loaded.

...d

Rob Morley said:

In article <[email hidden]>

Sniper8052(L96A1) said:

I have been drawing out the RSR plan for a recumbent as found on the
Dutch Speedbike web site in Autocad.

A URL might be helpful.

Quoted message said:

Having dimensioned everything and
checked the angles I find I cannot get the rake of the front fork in
front of the centerline of the front wheel, in fact it is 5.19mm behind
the center. As it is clearly illustrated in front of the wheel center
either I am in error or the plan was not true to the bike originally built.

I think maybe you're confusing the terminology.
Rake is the offset of the hub from the steering axis - in other words
how far you've bent the fork.
Trail is the horizontal distance between the steering axis and the tyre
contact point - it is positive when the contact patch is behind the
steering axis.

Quoted message said:

The questions are,

1 How much should the rake of a fork be ahead of the perpendicular
center of the front wheel?

A small (12mm or less) positive trail seems to be accepted as about
right for recumbents.

Quoted message said:

2 Has anyone else built a bike from this plan and found the same error?

Don't worry about it - you can easily adjust the rake to set the trail.
Here's a handy calculator: http://www.kreuzotter.de/english/elenk.htm
There are other factors in recumbent steering design, such as the tiller
effect, but I don't know if the bike you're building is long or short,
front or rear drive, over- or under-seat steering ...

Sorry, URL for the site is http://www.dutchbikes.nl/

Also my second post has my drawing showing the problem I encounter with
the published plan.

Sniper8052

Rob Morley said:

A small (12mm or less) positive trail seems to be accepted as about
right for recumbents.

Er, I don't think so. The original Kingcycle had 44 mm, which Grandpa
Kingsbury reckoned to be optimal but which a lot of people reckoned made the
bike's hadnling too twitchy for their liking.

The Kingcycle Mango had 15 mm trail, and look what happened to /that/...

--
Dave Larrington
<http://www.legslarry.beerdrinkers.co.uk>
God was my co-pilot, but we crashed in the mountains and I had
to eat Him.

In article <[email hidden]>

Dave Larrington said:

Rob Morley said:

A small (12mm or less) positive trail seems to be accepted as about
right for recumbents.

Er, I don't think so.

Fair enough, that was just the impression I gained from a quick Google.

Quoted message said:

The original Kingcycle had 44 mm, which Grandpa
Kingsbury reckoned to be optimal but which a lot of people reckoned made the
bike's hadnling too twitchy for their liking.

What was the head angle?

Quoted message said:

The Kingcycle Mango had 15 mm trail, and look what happened to /that/...

I have absolutely no idea. :-) But to change it all you have to do is
bend the forks (assuming the clearances don't mess up).

Rob Morley said:

In article <[email hidden]>

Dave Larrington said:

Rob Morley said:

A small (12mm or less) positive trail seems to be accepted as about
right for recumbents.

Er, I don't think so.

Fair enough, that was just the impression I gained from a quick
Google.

Quoted message said:

The original Kingcycle had 44 mm, which Grandpa
Kingsbury reckoned to be optimal but which a lot of people reckoned
made the bike's hadnling too twitchy for their liking.

What was the head angle?

Pass. But trail varies with head angle and fork rake. Something like a
Tour Easy, for example, has a very relaxed (by upright standards) head angle
and a lot of fork rake to get the required trail.

<URL:http://www.easyracers.com/images/side.jpg>

The Kingcycle had straight fork blades with the dropouts on the front
thereof, so Someone Clever could probably work it out from first
principles...

Quoted message said:

Quoted message said:

The Kingcycle Mango had 15 mm trail, and look what happened to
/that/...

I have absolutely no idea. :-)

First it went into a tank-slapper at 70 mph and fell over, slid 200m and got
savaged by a Toyota MPV. Latterly it got into another at about 55 mph, fell
into a hole and broke :-( The very short wheelbase probably didn't help.

Quoted message said:

But to change it all you have to do
is bend the forks (assuming the clearances don't mess up).

I don't think there was any clearance for /anything/ in that thing. See

<URL:http://www.bikerevuk.com/rob/racing/whpsc/whpscsun.html>

and subsequent pages.

--
Dave Larrington
<http://www.legslarry.beerdrinkers.co.uk>
Frozen gorillas can be used to control the temperature of a warm
and stuffy room.

In article said:

Rob Morley said:

What was the head angle?

Pass. But trail varies with head angle and fork rake. Something like a
Tour Easy, for example, has a very relaxed (by upright standards) head angle
and a lot of fork rake to get the required trail.

Incidentally, if you are Googling for pages on how fork geometry affects
handling, it's worth noting that motorcyclists tend to use "rake" to mean
what cyclist call "head angle", and "offset" to mean roughly what we call
"rake".

In article <[email hidden]>

Dave Larrington said:

Rob Morley said:

In article <[email hidden]>

Dave Larrington said:

Rob Morley wrote:

> A small (12mm or less) positive trail seems to be accepted as about
> right for recumbents.

Er, I don't think so.

Fair enough, that was just the impression I gained from a quick
Google.

Quoted message said:

The original Kingcycle had 44 mm, which Grandpa
Kingsbury reckoned to be optimal but which a lot of people reckoned
made the bike's hadnling too twitchy for their liking.

What was the head angle?

Pass. But trail varies with head angle and fork rake.

Indeed, plus there are things like tiller and flop to take into
consideration.

Quoted message said:

Something like a
Tour Easy, for example, has a very relaxed (by upright standards) head angle
and a lot of fork rake to get the required trail.

<URL:http://www.easyracers.com/images/side.jpg>

That looks to be about 57 degrees with 3" rake, 3" trail and 8" tiller.
I suspect that the different weight distribution of a LWB machine makes
quite a difference to the relationship between handling and steering
geometry.

Quoted message said:

The Kingcycle had straight fork blades with the dropouts on the front
thereof, so Someone Clever could probably work it out from first
principles...

Actually the online calculator that I linked to
http://www.kreuzotter.de/english/elenk.htm
can do it - 1 1/4" rake, 1 3/4" trail and 20" wheel gives 73 degrees.

Quoted message said:

Quoted message said:

Quoted message said:

The Kingcycle Mango had 15 mm trail, and look what happened to
/that/...

I have absolutely no idea. :-)

First it went into a tank-slapper at 70 mph and fell over, slid 200m and got
savaged by a Toyota MPV. Latterly it got into another at about 55 mph, fell
into a hole and broke :-( The very short wheelbase probably didn't help.

Quoted message said:

But to change it all you have to do
is bend the forks (assuming the clearances don't mess up).

I don't think there was any clearance for /anything/ in that thing. See

<URL:http://www.bikerevuk.com/rob/racing/whpsc/whpscsun.html>

and subsequent pages.

Maybe they should have left the stabilisers on. :-)