Geometry query - Rake and trail effects on handling
 

I have a custom made frame with which I normally use a columbus fork (Carbon blades, alu steerer).
I specified the whole design myself so I knew up front exactly how it would fit and could make a fairly well educated guess on how it would handle. Suffice to say it's 'perfection' in all regards.
I recently removed the fork to have it custom painted.
In the meantime, I fitted an enve fork (full carbon) to be going on with. The Enve fork has 8mm more rake than the columbus.
As such, I expected the handling to a bit twitchier due to the reduced trail.
However, in reality I surprisingly found the opposite to be true! With the Enve fork I found cornering to be quite hard work and almost scary (understeer).
This is all completely counter intuitive to what I expected and commonly accepted geometry facts...
Now I'm curious as to WHY this might be? Can anyone suggest anything that could explain this?
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Perhaps this is just a classic case of 'There is far more to the way a bike handles than just fork rake & trail'!!

how steep is the head angle?

A lot of communication revolves around everyone using the same definitions for the same words.

More rake=less trail=instability/unpredictability/nervous/more sensative.

It looks like you just discovered first hand knowledge that steering involves more than just lean and moving bars. What you discovered is to keep such instability under wraps involves your core muscles, counter steer, & planning to keep the whole ship upright. You'll acclimate to the new handling.

If you don't have enough trail, meaning rake is too much, your wheel has nothing to "act against" to affect the bike. A bike with a trail of zero would not turn by means other than gyroscopic forces of speed alone. But it would very quickly get there.

Trail, is the distance on the ground between 2 lines meeting at the ground plane ..
1 is your head tube axis line, the other is a plumbline down from the axle...

they cross above the ground, and so wheel diameter can change it..

Trail is a result , Rake is a component part of determining Trail ..
for a given wheel size and head tube angle , (as written repeatedly in Bike Quarterly magazine )

More rake reduces trail.. Porteur bikes use this, a benefit carrying heavy loads up front ..

& a smaller wheel , with HTA and fork bend same , also shortens Trail ..Folding bikes benefit,
as crossing point of 2 axis lines is closer to the ground-plane ..

I like the low trail nature of my Bike Friday as a commuting bike,
as my speed is not fast Except going down hill..
& I have panniers on the front wheel..






.....

Crown to axle distance is critical here as it will effectively change frame angles should it differ from the original spec for the frame.

The head angle is 73º. The columbus fork has a rake of 35mm giving a trail of 67mm. The Enve fork has a rake of 43mm which gives a trail of 59mm.

The axle to crown measurement is identical for both forks.

Interestingly, this guy has mentioned that he preferred a higher trail / shorter wheelbase in contrast to the industry standard of a longer wheelbase with shorter trail.

Interestingly, the 59 mm of trail is actually is already on the upper-end of what you see in most modern era road bikes running 25s (which is usually something more like 55 to 60 with ~57 often held out a good compromise). However, the 69 mm of trail -- although not seen in road bikes nowadays -- would not have been as unusual to see on a ~50 year old touring frame.

The effect you are seeing is exactly what I would expect. A bike with more trail has a tendency to want to steer(turn) when you lean it(look up wheel flop). A bike with low trail requires more steering input from the rider in the form of turning the bars.

That said, 59 and 67mm are not so far out of the ordinary that one couldn't quickly acclimate to it.

"I started racing in the early 1950s and I can say from experience the bikes of that era did not handle and corner near as well as today’s designs. These bikes handled reasonably well because frames were built with much longer wheelbases, wheels and tires were heavier, and tires were fatter.

"Road conditions at that time, especially in countries like Italy and France were often appalling. The long fork rake and the long wheelbase had a dampening affect on the rough road conditions."

~Dave Moulton