can l do this?

[Rideron]

Hi! I'm a neewbie so forgive me if I do something wrong. I have a Genesis Fortitude with rigid forks and would like to fit some shock forks to add a little comfort when out on the trail. Can I do this or would it upset the geometry and spoil the ride?

 

[Cyclist33]

There's a lot of TALK about upsetting the geometry, although I've never seen geometry-rage myself ;-D

I think it's just something to be aware of rather than avoided. Yes, unless your rigid fork is the same length as the suspension fork you're looking at, the geometry will change, but it's not something to be necessarily scared of.

If I understand correctly - which is admittedly highly unlikely - putting a longer fork on a bike would raise the cockpit and effectively make the head tube angle shallower, both of which would not be undesirable, as well as raising the bottom bracket height a little, which would also be a bonus?

 

[lulubel]

If I understand correctly - which is admittedly highly unlikely - putting a longer fork on a bike would raise the cockpit and effectively make the head tube angle shallower, both of which would not be undesirable, as well as raising the bottom bracket height a little, which would also be a bonus?

I think it depends a lot on where you're starting from. As you say, a longer fork will raise the bars, and also tilt them back towards you very slightly, but you can usually put it back how it was by changing the stem/bars to ones with less rise (if you want to). Whether the change to the head angle is good or bad depends on what you want from the bike. A shallower head angle will improve handling on rough sections, but won't be a benefit if you don't need to improve the handling on rough sections.

Raising the bottom bracket is generally a bad thing, I think, based on the advice I've read that says to have the lowest bottom bracket possible for the style of riding you're doing. (A higher bottom bracket raises your centre of gravity, and handling is improved by keeping your centre of gravity as low as possible.)

Longer forks would also make the seat tube angle shallower, which is a bad thing if you want to ride up hills.

It depends on how big the difference is, though. If you like the way your bike is set up, it makes sense to keep the fork length as close as possible to the original.

 

[Cyclist33]

I think it depends a lot on where you're starting from. As you say, a longer fork will raise the bars, and also tilt them back towards you very slightly, but you can usually put it back how it was by changing the stem/bars to ones with less rise (if you want to). Whether the change to the head angle is good or bad depends on what you want from the bike. A shallower head angle will improve handling on rough sections, but won't be a benefit if you don't need to improve the handling on rough sections.

Raising the bottom bracket is generally a bad thing, I think, based on the advice I've read that says to have the lowest bottom bracket possible for the style of riding you're doing. (A higher bottom bracket raises your centre of gravity, and handling is improved by keeping your centre of gravity as low as possible.)

Longer forks would also make the seat tube angle shallower, which is a bad thing if you want to ride up hills.

It depends on how big the difference is, though. If you like the way your bike is set up, it makes sense to keep the fork length as close as possible to the original.

Yes, I'll take that last observation as eminently sensible! Although if considering switching to a front-sprung setup, perhaps it goes without saying that one ought to be prepared to adapt/embrace some changes?!

I understand the centre of gravity argument, but on the other hand the raising of the bottom bracket provides more clearance and a slightly lower risk of pedal strike? And centre of gravity is dictated more by rider weight as the heaviest part of the human/machine package is the centre bit of the human?

Regards seat tube angle, could that be compensated by pushing the saddle forward and tilting it forward, so as to restore its effective location relative to the pedals?

 

[lulubel]

I understand the centre of gravity argument, but on the other hand the raising of the bottom bracket provides more clearance and a slightly lower risk of pedal strike? And centre of gravity is dictated more by rider weight as the heaviest part of the human/machine package is the centre bit of the human?

Raising the bottom bracket raises the pedals, which also raises the saddle and the rider, so however much the bottom bracket is raised by, the rider will be raised by the same amount. Yes, a higher bottom bracket gives more clearance, and that was my point about style of riding. If you're going to encounter lots of big rocks, you need a higher bottom bracket than you do for smooth trails. (Road bikes have low bottom brackets.)

Regards seat tube angle, could that be compensated by pushing the saddle forward and tilting it forward, so as to restore its effective location relative to the pedals?

You could probably fiddle around with it a bit, and if you like your saddle adjusted that finely, you'd want to put the nose down slightly as soon as you put longer forks on, to compensate for the lift the forks would cause. I've been looking at a lot of frame geometries in my search for a new MTB lately, and it seems the optimum for an all-round XC bike is seat and head tube angles that are steep and slack relative to each other, to give a decent "over the pedals" position for climbing, but more relaxed handling.

 

[Rideron]

You guys sure know your stuff. Thanks for the reply. I currently have an extension bracket (in height) attached to the head tube to raise the bars up to bring them level with the saddle because I felt uncomfortable leaning too far forward. So the changes you both speak about in regard to bar height sound fine to me.