ApolloCVermouth

Can anyone explain how these three factors affect the handling of a bike? I have an old Nishiki with large amount of rake in the fork- something like 5.7 cm or 2 1/4 inches. I notice other bikes like the Kelly Knobby X have forks that are straight. Would a straight fork tend to make the bike more stable and less maneuverable by increasing the trail, or do I have that backwards? If I put a straight fork of the same length on my bike would it handle much differently? Thanks for the help.

powers2b

The less rake you have the 'twitchier' the handling will be.

Enjoy

DannoXYZ 

Actually the straight-blade fork does have a rake (offset) amount built-in. This is built into the fork by having the blades angled forward compared to the steerer-tube. Same effect as a curved fork since the rake/offset of the hub ends up being the same amount. Resulting in the same amount of trail on the ground. Another way to generate the rake/offset with a perfectly straight fork, like suspension forks, is to use an offset triple-clamp on the steerer-tube.

here's some links with pictures and explanations of the geometries and their handling effects:

https://www.phred.org/~josh/bike/trail.html
https://yarchive.net/bike/rake.html
https://www.velonews.com/tech/report/...es/7322.0.html
https://www.dclxvi.org/chunk/tech/trail
https://www.kreuzotter.de/english/elenk.htm
https://www.coe.uncc.edu/~gkwatkin/Di...n/chapter2.pdf

In the end, it's the trail amount that makes the biggest difference. It's the sum of all the other factors combined. The larger the trail amount, the more stable the bike, the shorter the trail, the more nimble and twitchy the bike becomes.

So if you put a fork with less rake on your bike, it increases the trail and makes the bike more stable (harder to steer). More rake/offset on the fork result in less trail will make the bike more nimble.

https://

FarHorizon

I understand this theory. In practice, the steep steering tube angle on bikes with little or no fork rake steer far more nimbly (or twitchily - depending on your opinion) than bikes with more relaxed steering tubes AND more rake.

Al1943

Sorry but tha's not correct. Increasing rake decreases trail and makes a bike more twitchy.

https://traylorfwd.home.mindspring.com/geometry_def.html

https://www.kreuzotter.de/english/elenk.htm

Al

HillRider

That is exactly the oposite of the real situation. Less rake = more trail. Trail is the parameter that controls responsiveness and handling properties. More trail gives slower, more stable, less responsive handling. Less trail gives quicker, more "twitchey" handling.

Trail is the result of both rake and headtube angle so a fork with a lot of rake can give slow, stable handling if the frame's headtube angle is shallow enough.

Also, forks with straight blades have rake, The fork blades are installed at an angle to the crown and the fork has rake just like a fork with curved blades. Rake is where the axle is relative to the steerer tube and you can get it more than one way.

ApolloCVermouth

Thanks to all for the advice and the links. So if you have two forks with the same amount of trail, one with a slack top tube angle compensated with rake and another with a straight fork a steeper top tube and angled blades will they handle differently? I suppose the second one would allow for a shorter wheel base. Also, does this offset triple-clamp allow you to change the head tube angle?

FarHorizon

Hi Al! - Thanks for the links.

I understand the theory - but in practice it doesn't seem to work that way. Why? If I "reverse" the fork where the rake is pointing backward toward the rider, I've MIGHTILY increased the trail. Based on the "more trail makes for a more stable ride" theory, I shouldn't be able to turn the bike at all! But in actual fact, the bike with a reversed fork is so twitchy as to be almost impossible to keep upright (if I remember my prepubescent experiments with my Schwinn correctly).

Either the theory has additional factors not taken into consideration, I've missed something significant, or the entire theory is bogus. Based on your certainty, I suspect that the second of the above three hypotheses is the correct one. I'll preuse all the links in this thread and see what I can learn. Thanks again!

ADDENDUM TO POST: Eureka! I understand it now - The head angle actually seems to have MUCH more effect on trail than does fork rake. Even a single degree of head angle makes a significant difference in the trail! With older frames (with shallow head angles) there could be SIGNIFICANTLY more fork rake while STILL maintaining lots of trail! As the head angle increases, less fork rake is needed to maintain the same amount of trail. If a heavily raked fork is used with a steep head angle, trail decreases to near zero, causing MUCH twitchier handling. Actually, in my Schwinn example, reversing the fork probably DECREASED trail since the head angle was so lax.

All other things being equal, less fork rake should increase trail and enhance stability. The significant phrase is the first one, though! Head angle makes almost ALL the difference!

Al1943

HTA, rake, and fork length all work together to control trail.
A 1 degree change in HTA would be huge.

Al

halfspeed

Huh?

Forks don't have trail. Top tube angle isn't relevant. Whether or not a fork is straight doesn't affect rake or trail. If two bikes have the same amount of trail they will respond to steering input the same way regardless of the angle or bend of the fork. If you make the head tube angle steeper and reduce the rake you can shorten the wheelbase while maintaining the same trail.

Al1943

Friend of mine made his head tube angle steeper when he T-boned a truck.

Al

FarHorizon

Agreed. Multiple factors, though, affect the trail - head tube angle and fork rake being two. Of the two, head tube angle has a MUCH larger effect on the total trail of the system.

One can obtain the SAME trail with multiple combos of head tube angle and fork rake. However, if the head tube angle remains constant and the fork rake is reduced, the trail increases (increasing steering stability).

andyman

What is the ideal Trail for optimal handling in slow uphill climbs or fast downhill handling if any?

MichaelW

According to one book I read, the head-tube angle affects the sensitivity of the bike to changes in trail.

Al1943

From what I've read there is too much personal preference involved to say that there is an ideal trail. I've learned that I prefer more trail than what an average of all of the well known high performance road bikes offer. My favorite road bike has a trail of about 61 mm. I've read reports that says neutral steering is considered to be about 56 or 57 mm:

https://www.spectrum-cycles.com/612.htm

Al

andyman

I should have been more clear. The above link mentions ideal trail for 700c is between 56mm and 57mm. What about 650c wheels?

Al1943

Good question. I would guess that the ideal trail would be similar, of course the dimensions to arrive at the equivalent trail are different. Just my guess.