Anyone know about rake & trail?

[User482]

I'm thinking of fitting a carbon fork to my Thorn Audax Mk1. The only ones on the market for a 1-inch headset have less rake than my current steel fork, (45mm vs 55mm). I understand that this will increase trail which in theory increases stability (not a good thing! It's very stable already). But will the shorter wheelbase not compensate?

Any answers appreciated.

 

[Gerry Attrick]

This may be of help: http://en.wikipedia.org/wiki/Bicycle_fork

 

[skwerl]

I'm thinking of fitting a carbon fork to my Thorn Audax Mk1. The only ones on the market for a 1-inch headset have less rake than my current steel fork, (45mm vs 55mm). I understand that this will increase trail which in theory increases stability (not a good thing! It's very stable already). But will the shorter wheelbase not compensate?

Any answers appreciated.

to some degree but I'd think that each mm change in rake has more effect on handling than each mm change in wheelbase

 

[Chris James]

I wondered about this too, my Dawes Audax fork has a 50mm rake and carbon forks seem to all be 45mm. I doubt 5mm would make much of a difference in my case, maybe 10mm more so.

Is the Mark 1 a similar geometry to the Mark 3 as I see they sell them with carbon forks as an option? What rake does the steel fork option on the mk3 have?

Perhaps try dropping Thorn a line, surely they would help you since you have bought one of their bikes (even if you don't necessarility buy the forks from there?)

 

[User482]

Chris, the Mk1 has different geometry to the Mk3, which was designed to take the carbon fork that they sell. They make it clear that their carbon fork isn't suitable for the Mk1 frame. Having said that, Thorn have some "interesting" views on 700c wheeels, titanium frames and derailleur gears, so this post was asking for any real-world experience that people might have of changing fork rake, and its effect on bike handling & stability.

 

[Chris James]

What's the head tube angle on the Mk1 frame.

A standard road bike was always 73 degrees, give or take, so if yours is within a degree of that I would have thought 45mm offset forks would be okay.

But I agree that you really want someone who has actually done it to post.

 

[jimboalee]

Reducing the fork rake will move the centre of gravity of the front wheel closer to the axis of the steerer tube.

What you have ( and me ) is a gyroscope held by two arms. When the C/G of the rotating mass is further away from the steerer axis, it is more difficult to knock out of stability.

The trail is the distance from the line of the steerer axis when it hits the graound, back to the line which drops vertically from the wheel axle. On a unicycle, it is zero.

A short trail gives a 'twitchy' and manouverable bike – as in a 'Criterium' bike – 74 deg parallel with 40mm rake.

The typical Audax and fast tourer will have a 72.5 or 73 angle and a 50+ mm rake, making a stable bike which can be ridden safely with just a light touch of one finger on the bars.
At the extreme is the Schwinn Stingray and other Cruisers They are difficult to steer.

The other end of the spectrum is a Polo bike. Almost upright steerer with maybe 15 mm of rake. Mind you, those bike spend most of the time on the back wheel.

But, 45mm will be OK.

 

[User482]

Thanks Jimbo. I'm ok with the theory, I just wondered whether adding 1cm to the trail is going to make a noticeable difference. You seem to think not?