Currently riding a Look HSC4 43 rake -380gms

On a Eddy merckx Ti frame 57cm (pretty relaxed fit and geometry)

need a new fork due to number of "incidences" with it - not the forks fault!

Was considering changing the rake but unsure of the consequences of higher or lower

Would like a sharper turn in and more stable on high speed decents (45mph +)

Love your thoughts

one of your demands sacrifices the other sadly. Just fitted a HSC4 to my bike. Very nice I must say. I think the only forks that can possibly be better are the Stork forks perhaps.

The end result you are looking for is a change in trail (or, more precisely, a change in caster angle). An increase in trail increases stability at speed and requires more force to change direction. To increase trail, you must decrease the rake, assuming no other changes. If I remember correctly, Looks are on the low end for trail, which means their bikes are less stable than most, and require less force to turn than others.

When switching forks, keep in mind that some forks have different lengths than others and that putting a fork with a different length on your bike will change the head tube angle. However, most carbon forks are close enough that it doesn't really matter in practice. Putting the wrong fork on your bike will not make it unrideable. Most likely the change will be slight and you will only notice it on technical descents and when riding with no hands. You will still be able to descend just as fast and you still be able to ride no hands, they will just feel a little different.

Sean

Oops, I misread the original post and thought your bike was a Look, not just the fork. I don't know what the combination of your bike and fork make, yet.

Sean

I did some quick searches, but Merckx doesn't seem to list their head tube angles. In spite of that, let's make some guesses and run the numbers. A good online calculator and picture is here if you want to continue playing with the numbers: http://www.kreuzotter.de/english/elenk.htm

Merckx does list the head tube angle of the carbon GX2 as 72.5 deg on a 57 bike. This gives a trail of 60 mm with a 700x23 tire. Most people think the sweet spot for trail is 55-60 mm so this is on the stable end of the normal range. Increasing the rake to 45 mm decreases the trail to 58 mm. This is a very small change and is still in the normal area, and may be described as "nuetral" by many people. Good luck finding a 47 mm rake fork, but if you did the trail would decrease to 56 mm, which could be described as "quick", but still within the normal range. A 41 mm rake would increase the trail to 62 mm, which is stable, verging on over-stable.

These are relatively small changes and can be thought of as fine-tuning rather than radically altering the handling. Some extreme examples, to give you an idea of possible trail ranges, include motor pace bikes and my hacked Trek Y-Foil.

Bike used in motor paced racing have forks that look like they are turned backwards. In truth, they are designed this way in order to give them a lot of trail (70+ mm) so that they are very stable at high speed. My Y-foil, on the other hand, had something like 35mm of trail becuase the fork I used, an Easton EC90, was a full inch shorter than the original fork. This lowered the front end and increased the head tube angle to more than 76 deg. The bike was very rideable and very eager to turn. I had to pedal very smoothly when riding no-hands, but it soon became second nature. I even completed a number of double centuries on it.

Sean

thanks wanderingwheel.

Great information and links. I will take all this into consideration.