It's that time of year when hundreds of BSO show up at our facility, bought by employee donations, and destined for military kids for Christmas, and I'm "in charge" of volunteer assemblers.

I've got a bunch of SS/coaster-brake models, and I can't see any appreciable trail/rake whatever in the fork, and I'm not sure which way to rotate it.

Is it possible that for a non-performance kid's bike, the fork is actually 100% straight (incl the steerer), and the head-tube angle provides all the caster angle that is necessary for riding stability?

Is it dangerous if I (or an even more ignorant volunteer assembler) get the fork backwards?

backwards it has a lot of trail, Bicycling Science MIT Professor David Gordon Wilson sent a handlebar less bike with out a rider on it
down a hill and it went straight , with all that steering trail.. a video must be on the internet somewhere.


trail is a ground plane distance between 2 lines 1) the head tube angle, to the ground, 2) the plumb line from the hub axle..

You can measure that, No?





...

OK, then there is trail, because the head tube is not purely vertical. I guess what I'm saying is there's no rake? The amount that the fork dropouts are forward of the extended head-tube line? Or at least I can flip the fork this way and that, and it looks to be the same angle to me either way.

You would actually get more stability by having a normal fork installed backwards (at least if the front tire would still clear the down tube). Forks are raked in order to reduce the stability and therefore make it easier to steer around turns. But if there's so little rake that you can't tell (or none at all) then it shouldn't hurt to install it either way.

One measurement is worth a thousand assumptions (or in this case two measurements)


With front wheel secured in fork and pointing straight ahead... measure from the BB to the front axle. Swing the front wheel and fork 180* and re measure. Are these dimensions the same? If not there is a rake, even if not obvious. Andy.

There are different ways of attaining offset, is there any offset at the dropouts? "Leading axle" (axle in front) should be correct if there's offset at the dropouts.

Some 12" and 16" wheeled bikes do indeed have straight forks.

None of the 20" ones I saw at Walmart last year had straight forks.

Wrong. Just. Wrong.

start by studying this chart, then go research the subject further ok?

https://bikearama.com/wp-content/uplo...5912863770.png

less RAKE equals less STABILITY.... Rake is also called Caster.... MTB bikes use a 67-68 degree rake... they need stability, especially when plunging down some rocky, slippery trail....... road bikes use a 70 deg. or so rake... they need to turn into corners easier since more of the rider's weight is over the front tire...

TRAIL adjusts how much and how quickly the bike tips into the corner... trail can also be used to increase stability.... get it wrong, and the bike becomes spooky to ride, not inspiring confidence in the rider.... the front wheel might tuck in suddenly, or not want to turn in, sending the rider off a corner.... OUCH!

get the head angle, or RAKE, too shallow, and the front wheel might shake like a wet dog at speed...

the above reply is the condensed version... the nuances involved are endless...

the forks the OP mentioned are lacking OFFSET... see the chart i linked, ok? that INCREASES the TRAIL specification, making the bike... What? look it up somewhere.

RAKE AND TRAIL demystified... and let's include OFFSET into the equation...

info time....

rake and trail

If there are not any components such as caliper brakes that mount on the fork/steerer tube assembly or the way the fork ends point for the axle dropout that show which way it needs to go, then it probably makes no difference. But if these are being brought in by bike shop owners, you might ask them.

Is there an assembly manual available? If not in the box maybe the mfr's website.

Is it possible for them to be 100% straight? Yes.

Put more simply, offset is aka "fork rake", as opposed to "rake angle." So decreasing rake (offset) increases trail and stability but making turns slower or more difficult.

Got my terms straight, with the help of Dave Moulton.

There is trail, because the headtube angle is not 90deg. But there is no rake or offset that I can see (but I still haven't measured)

Thx, "possible for them to be 100%" straight is basically the answer I was looking for.

On these simplest SS/coaster brake bikes the forks are completely bare and the fork ends are stamped tubes with dropout slots in line with the fork arms.

These are not being brought in by anybody, we have a mess of boxed bikes all shipped straight from mfr/distributor, which we will be donating to military families for xmas, and some of them we need to assemble, some we just give in the box (per family preference)

That's kind of what surprised me as well. And in previous years the 20" bikes had (superfluous, nearly non-functional) caliper brakes on the front wheel. Those brakes might have been totally straight as well, but I wouldn't have noticed because the brake would tell me which way the fork should go.

Maybe you should do some studying yourself. What motorcyclists refer to as 'fork offset' is called 'fork rake' by bicyclists and is measured in mm (typ. about 45 mm) rather than angular degrees.

From Wikipedia:
"Fork offset
The fork offset is the perpendicular distance from the steering axis to the center of the front wheel.
In bicycles, fork offset is also called fork rake. Road racing bicycle forks have an offset of 40–50 mm (1.6–2.0 in).[6]
The offset may be implemented by curving the forks, adding a perpendicular tab at their lower ends, offseting the fork blade sockets of the fork crown ahead of the steerer, or by mounting the forks into the crown at an angle to the steer tube."

The forks mentioned by the OP apparently have either a 0 or very small fork rake which results in a relatively large trail and correspondingly high stability which would be good for beginning young riders.

the 2 lines, mentioned earlier, typically cross above the ground, so all else the same,

a smaller wheel will have a shorter trail ... My Brompton, is I was told, has a 35mm trail.

Got any pictures of it? I would like to think that one would be able to tell by looks, but if you're stumped, I'm intrigued.

Here's one from Walmart.

just spin the wheel around and check the clearance to the downtube. Install fork with the lower (or the one that doesn't clear) forward. Most bikes other than kiddy bikes will have an offset. The fork blades may be straight with an offset at the crown and or dropouts attached on the front of the fork.

"In bicycles, fork offset is also called fork rake."
that wiki needs corrected, and that passage was written by an ignorant fool.

or maybe we should all rewrite the english language some more, and call inclines "flat land"
... or begin calling pressure "flow".

a long standing error is still an error.

The terms are used differently for bicycles and motorcycles, and you're muddying the discussion rather than clarifying it by insisting on the motorcycle uses here.

+1, confusing at times, but true.

Dave Moulton's write up is excellent for bicycles.

Aha. Boy, that sure looks like a fork that would work either way.

Plugging the numbers into Jim G's tool, it looks like the small wheels on kid's bikes allow you to use zero-offset forks without sending the trail number through the roof. Pretty fortuitous for designing simple and inexpensive bikes like this.

I think this might be the 20" boys model Looking carefully at that straight-on pic, there might be a tiny bit of rake in that fork, it's hard to tell. This black one looks 100% straight, but that's not the one we have.

DM sez: