high trail vs low trail

regrettably, it seems the design i decided on and had plotted won't be quite final. at issue now is how much trail i want.

In the blue shorts: Jan Heine and the iBobbers, advocating trail around 45 mm for general-purpose riding, especially if a handlebar bag is to be used (and I plan to use one sometimes), and especially with 650b wheels (which I'm not planning to use at this point).

In the red shorts: almost everyone else, including Rivendell.

the other factor complicating things is that more rake/less trail would push the front wheel forward, allowing a shorter top tube with even better toe clearance. for those of you just tuning in, i've got a short torso, so a TT c-c of 57 cm is a bit of a compromise (necessitating a shorter-than-desirable stem). so i'm kinda biased towards the iBob side.

another factor is that i could still switch to 650b. the drawbacks would be limited rim/tire/tube availability (and probably higher prices for those parts) and slightly decreased efficiency (wider tries, smaller wheels). upshots are better toe clearance and possibly handling.

see also this on the P/R, and this on crazy Porteur races in Paris

now to hear arguements from the iBobs:

Archive-URL: http://search.bikelist.org/getmsg.asp?Filename=internet-bob.10503.
3069.eml
From: "Tom T" 
Date: Wed, 30 Mar 2005 12:16:23 -0800
Subject: [BOB] VBQ frame geometry- pneumatic trail & geometric trail  an

To follow up on the thread of the subject title a little while back,
here's something you might find interesting. I must say that I was more
than intrigued when Rivendell ( Grant) had that adjustable rake fork
made to test the handling and wrote the  article about it in the Reader
# 31?.

I've ridden that fork with with different settings around the parking
lot but it wasn't long enuf to to make an impression. Then I got the
recent issue VBQ and the frontend geometry article really piqued my
interest so I thought why not?

It just so happens that my Rivendell fixie has identical geometry as the
1954 Alex Singer tested ( bike # 7)minus the the 61 mm raked fork. (Mine
is a 56cm with a 57cm top tube, 73 head angle, 44 rake). And, I also
have a spare fork with 48 mm rake made recently by Roland Della Santa
with the Heron forkcrown and 24mm round blades. (Thanks Todd K.)

I gave Roland a range of 58 min to 61 max. since he didn't know if it
could go that far without rippling the blades ( sometimes it happens).
He has a  jig that can rake to max 70, never been  used up until now.
The one he normally uses can only go up to 55 max. as that is the most
he ever needed rake a fork for most frames built.

The fork came back reraked to 60 mm with the paint intact, was installed
and ridden around the neighborhood hillsides. The initial impressions
are very good to me and a number of shopguys. They were curious about
this latest project of mine, and they did let me use their tools. 2 of
them  commented that it corners so smooth and surefooted , so much
better than the
$3k+ latest Trek carbon Mardone bikes they're selling. Hmm, It was a
good thing it was just outside of earshot range of their buying
customer.;)

Initial impressions seem to bear out Jan's claims of better handling in
the VBQ article.

FYI, currently the wheels are 700c fixed, single speed, Pasela 32c
(actual width, same as the stock Quickbeam tires), inflated to about
75psi. dia. of wheel: 693mm measured. From the Trail-0-matic calculator
http://greenspoons.com/cgi-bin/trailulator I get 43 mm of trail. Front
center is now measured 603.
The fork is now shortened by about 2mm so it isn't a problem with
clearance. That's about what you'd lose for the typical 72.5- 73 degree
head angle for every 10 mm of additional rake. Can someone figure  this
with some simple or not simple math ?
BTW, this reraking wouldn't work if your brake pads are already at the
top of the slot.
The TCO I had with the 32c tires is solved with room for fenders should
I choose to install them later.

Bike looks great.
See: http://members4.clubphoto.com/tom313673/2824469/guest.phtml

Addtional test riding will be done in the next few weeks with Rolly
Polly tires (700x27c, 684 mm dia. gives 42mm trail), with loaded
handlebar bag, and on Mt diablo with twisty fast descent and plenty of
decreasing radius curves.  If I like the handling  with the RP tires I
would have a few more  forks  reraked  the same- with the Saluki being
the only exception, all my bikes have 73 or 72.5 head angle and 45mm to
50 mm rake.

Tom T

***

Archive-URL: http://search.bikelist.org/getmsg.asp?Filename=internet-bob.10503.
2270.eml
Date: Mon, 21 Mar 2005 14:37:58 -0800
From: Jan Heine 
Subject: [BOB] Re: VBQ article freaked me out

>
>That was probably Jones with his URB (UnRideable Bike) research.  He
>tried to establish that stability was dependent on "mechanical
>trail," what Jan called "geometric trail" in his article.

Mechanical trail, according to Bicycling Science, is the vertical
component of geometric trail. Geometric trail (usually only called
"trail") simply is how far the steerer axis intersects the ground
ahead of the contact patch.

"Pneumatic" trail is a phenomenon that works like geometric trail to
a degree, but is caused by tire deformation. (The tire act as a
rubber band. At the front, it is laid down without deformation, no
matter the side loads. But if there is a sideload, the tire deforms
increasingly toward the back, with the maximum deformation at the
rear. This pulls the steering straight, because there is more
deformation behind the steerer axis than in front.)

I hope Jim Papadopoulos doesn't read this over-simplified
explanation. The VBQ article was the result of numerous discussions
with him and many others...
--
Jan Heine, Seattle
Editor/Publisher
Vintage Bicycle Quarterly
c/o Il Vecchio Bicycles
140 Lakeside Ave, Ste. C
Seattle WA 98122

***

Archive-URL: http://search.bikelist.org/getmsg.asp?Filename=internet-bob.10512.
1090.eml
Subject: Re: [BOB] Rake/Trail vs. Head Angle
From: mlove(AT)questertangent.com
Date: Mon, 12 Dec 2005 10:59:27 -0800

Forbes wrote on 12-12-2005 10:21:36 AM:

> Is there an accepted formula/best design practice
> related to a frame's head angle versus the amount of
> fork rake/trail?  I was taught that the steeper the
> head angle, the less one should rake the fork.  Is
> that commonly accepted practice?  Or is there debate
> on the matter?
>
> Thanks!
>
> - Forbes Bagatelle-Black
> Santa Clarita, CA

Forbes, this has become a bit of a recurring issue across the BOB/RBW/KOG lists,
thanks to Jan Heine's writing in VBQ.  There wasn't much debate on the matter until
Jan began to point out that French cyclotouring bikes of the mid-20th century have
excellent handling characteristics despite the fact that the geometric trail on
these bikes was often much lower than the commonly accepted 50-60mm found on most
modern bikes.  One of the main reasons the old cyclotouring bikes can afford such
low geometric trails is that they used fat, low-pressure 650B tires with a large
contact patch providing a large degree of stability--what Jan calls "pneumatic
trail".

My further readings on the issue seem to have converged on the following heuristics,
and I'm sure that the more knowledgeable among us will correct me if I say anything
blatantly stupid:

- Design for the trail you want, taking into account things like toe-clip overlap,
aesthetics, what have you.  If TCO is a concern, go with a slacker head angle and
larger fork rake, because...
- Within the typical head angle range of 72°-74°, changes in trail will predominate
over changes in head angle.
- If you'll be using 650A/B or 26" tires run at 50psi or so on the road, you can
reduce the geometric trail somewhat--even more so if you'll be loading the front
down with a rack or large handlebar bag.  The old French bikes had geometric trails
as low as 11 mm, but most were up around 30 mm, if I recall correctly.
- Conversely, bikes with skinny 700C high-pressure tires may benefit from higher
trail--the J.P. Weigle randonneur tested in the latest VBQ had 52 mm of trail with
the fork designed for lightly loaded front ends, and 42 mm with the fork for loaded
front ends.  The Rambouillet and Model P have trail figures in the high 50s, the
Atlantis and Surly LHT in the low 60s.

Murray
www.vintagebicyclepress.com
Dublin, Ca

*** the definitive post from J. Heine

Archive-URL: http://search.bikelist.org/getmsg.asp?Filename=internet-bob.10512.
1158.eml
Date: Mon, 12 Dec 2005 17:43:01 -0800
From: Jan Heine 
Subject: Re: [BOB] Rake/Trail vs. Head Angle

>>An interesting thing is that a mountain bike with a skinny slick often
>>feels more twitchy than a road bike with stock size tires.

Wheel size plays a role. That is why 650B is so popular - it feels
different from 26"

>>For low trail lovers, my Bike Friday has 33mm of trail (from the small
>>wheel with normal fork design of 73deg, 40mm offset).  Some riders
>>have found that they prefer to reduce the fork often to get more trail
>>and more stable handling.  15mm of offset would give it 59mm of trail,
>>closer to a road bike:
>>http://kogswell.com/trail.php?h1=486&i1=73&j1=14&e3.x=13&e3.y=16
>>

Another example that shows that simply looking at trail is
meaningless. A 650B bike with fat tires and 33 mm trail will be
lovely. Yet I never could ride my Bike Friday (no idea about that
bike's geometry, but probably was similar) no-handed at less than 30
mph. And I rarely go that fast. On the other hand, as a teenager, I
had a Peugeot folding bike - awful contraption - with the same wheel
size, yet riding no-hands at moderate speeds was not that difficult.

Tony Oliver, in his book "Touring Bikes," has a diagram that
indicates that bike with MORE trail are LESS stable - exactly the
opposite of what most Americans believe. At first, I thought it was a
misprint... but it isn't.

However, it isn't a matter of either Americans or Oliver being right.
The thing few people understand is that two factors play a role: More
trail increases stability, especially at high speeds (to the point
where it is difficult to change line in mid-turn), yet it also
increases wheel flop (which persists into moderate speeds). So you
have two counteracting phenomena, which is why so different bikes can
handle similarly.

When you add a front bag, you increase wheel flop, and you really
need a low-trail geometry. Tire size and pressure also plays a role -
no point rehashing the VBQ geometry article here (Vol. 3, No. 3).

The Berthoud tested in VBQ Vol. 4, No. 1, had about 65 mm of trail,
and it worked OK at moderate speeds, but not well at low speeds (<10
mph), where it suddenly would sverve. This especially with a
handlebar bag. (This lack of stability is due to wheel flop.) At very
high speeds, it was rock-steady (high trail taking over), but once
you picked a line, you couldn't change it. If there was a pothole in
mid-turn, you would hit it! Also, the Bertoud was sensitive to
cross-winds at low and moderate speeds, but not at all at very high
speeds.

The Weigle tested in VBQ Vol. 4, No. 2., had about 41 mm of trail. It
was wonderful at low speeds even with a full bag (riding no-hands
climbing at 7 mph! / very little wheel flop), it was great at
moderate speeds, and it was good at very high speeds. I liked being
able to change line in mid-turn, but in a full aero tuck at 50+ mph,
it lacked the confidence that the Berthoud had (less trail and
"stability"). Putting my hands to the drops made it totally fine. The
bike was not sensitive to cross-winds at low and moderate speeds, but
was more sensitive to them at very high speeds.

So which is "more stable?"
--
Jan Heine, Seattle
Editor/Publisher
Vintage Bicycle Quarterly
c/o Il Vecchio Bicycles
140 Lakeside Ave, Ste. C
Seattle WA 98122
www.vintagebicyclepress.com

***

Archive-URL: http://search.bikelist.org/getmsg.asp?Filename=classicrendezvous.10605.0870.eml
From: 
Date: Fri, 19 May 2006 11:10:42 +0100
Subject: [CR]Peugeot frame design.

It was asked:
Does anyone know why Peugeot would have spec their bikes like this? To be
different in a French way? A trend at the time? A famous racer had a
bike with these angles?

The July 1971 issue of Le Cycle shows "The plan of service des courses
Peugeot of a racing frame" It may be the frame used by Delisle in the 1970
Tour but I am not certain of that.  The details are:-
Head angle:73   Seat angle 72 Top and seat tube 56cm
Front centre 588mm   Rear centre 430mm
BB drop 80mm    BB height 262mm
Fork rake 55mm    Trail 46mm
An interesting dimention is measured horizontally from seat lug line to
rear dropout line and is 248mm
The angle between the seat and chainstay is 90
All this seems entirely conventional, if perhaps a little old fashioned
for 1970. For example Eddy merckx had a fork rake of 40mm and a rear
triangle of 415mm on his 1970 Tour bike.
I can only think that if Peugeot did sell bikes with a 76 degree head
angle and a long fork rake it was so they felt "lively" during a quick
test spin round the block.

Ray Green, Brighton, England

*** and last my post and a response

Archive-URL: http://search.bikelist.org/getmsg.asp?Filename=internet-bob.10608.
0877.eml
Subject: Re: [BOB] What do with a slack HT angle?
From: mlove(AT)questertangent.com
Date: Wed, 9 Aug 2006 10:42:56 -0700

Ethan wrote on 09-08-2006 10:24:51 AM:

> HowdyBobs
>
> This is usually the type of thing that gets hashed out on the
> Framebuilders' list, but I'm searching for Bob insight because the
> frame in question is quite Bobbish.
> Here's the situation:  I've got long legs and a short torso.  I'm set

> on using 700c wheels front and back, mostly because the lugs and tube
s
> I have will only work for that (I think).  So to avoid toe overlap an
d
> let the top tube be as short as possible, I made the HT angle
> relatively slack (71.5°).
> I had originally planned to go with ~59mm trail, meaning 52mm fork
> offset with a 30mm tire.  After reading what I could find on low trai
l
> designs in the archives, I'm a convert and ready to go low.  But how
> to acheive this with a slack HT angle?  To get low trail, I'd need to

> increase the rake/offset, which means yet more wheel flop, which so
> far as I know is undesirable.

Ethan,

Wheel flop is a function of only HT angle and trail.  Increasing fork
offset will decrease both trail and wheel flop, all other things being
equal:

WF = Trail * sin(HT) * cos(HT)

Murray