Replacing fork

[david1701]

Hi guys, I'm looking at swapping my utility type mtb front fork for a rigid one to save weight and unnecesary suspension, where would you look for rigid disc forks? (26" wheel)

also for replacing them I guess I'd need to replace the head set at the same time ?

I reckon with different bars and slicks (and a front carrier) it could be a really nice as a commuter and about towner that doesn't look quite so nickable.

I've been on ebay but there seem to only be new carbon front forks which work out at 120+ quid and I'd rather spend 30ish max

 

[Globalti]

I bought a really nice carbon fork from here for about £80: http://www.carboncycles.cc/?p=197&

It replaces the Reba fork that I took off for the same reason as you and because it was leaking and needed to go back for repair under warranty. The carbon fork is so comfortable that I haven't bothered refitting the Reba; on normal off road riding you hardly notice and it's only on big hits like kerbs or steps that you realise you haven't got a suspension fork.

The headset was easy, the bottom bearing race just swapped over to the new fork. To my surprise the brake calliper also swapped straight over, I was even able to use the same combination of shims to get the calliper centred on the disc.

If that's beyond your budget, just walk into any bike shop and look up; you'll see loads of unwanted forks hanging from the ceiling that they have taken off customers' bikes and will flog you for a few beer tokens.

 

[Kestevan]

I think you're going to struggle to get a set of new rigid MTB forks for less than 30 quid.
Quick shufty on Merlin/CRC etc shows most of them seem to start at 50-60 quid.

You could try the wanted section here on CycleChat and see if anything comes up.

 

[RecordAceFromNew]

They do exist, here and here e.g.

Suggest you check steerer type/size and axle to crown distance do match to maintain geometry and ring up to confirm disc brake mount type etc.

 

[Zoiders]

If you want steel forks with disc tabs try On-One as I know they do them.

The thing that no one has mentioned is suspension correction.

Modern suspension forks that run around 100-125mm (or more) of travel mean that the frames are starting to be designed around them so they make the head tube angle steeper - this means that if you plug in a fairly generic cheap rigid fork you may find that the steering gets to be a bit quick as the head angle ramps up well past the usual 72-ish degrees or so.

To avoid this make sure it's quite a leggy rigid fork and all will remain well in the steering department.

 

[david1701]

cheers guys, so decide if I want carbon comfort or Ali cheapness. Measure the front rake on the forks to get something that steers the same. So I need to take away some length to account for having weight on it (like leaning on it while someone else measures) to account for the suspension travel?

 

[RecordAceFromNew]

Since you asked, for geometry maintenance purpose your suspension fork's axle to crown distance should be measured with sag (i.e. with your full weight on the bike).

The need to maintain geometry assumes you are perfectly happy with the current steering sensitivity.

Actually to maintain the same steering sensitivity the trail of the fork has to be maintained, and trail is affected by fork rake (otherwise known as offset) and head tube angle, while head tube angle is in turn affected by fork length.

If you want to be precise and are interested in the maths you can read this, this and this!

If not you can just trust me that a 10mm increase in fork axle to crown length will reduce head tube angle by approximately half a degree which in turn will increase trail by approximately 3mm. An increase in trail will make steering more stable/sluggish, a reduction in trail will make the bike more nimble/jittery.

That is assuming rake is the same for both forks; if not an increase in rake by X mm will reduce trail by approximately X mm.

In other words, steering sensitivity can be maintained by a combination of axle to crown length and rake, e.g. a 10mm increase in length combined with a 3mm increase in rake will end up having minimal impact on steering sensitivity.

Then the next obvious question is how significant any amount of change in trail is? This is an impossible question to answer because it depends on how sensitive you are to change. In terms of figures as Dave Moulton said bikes used to have zero trail in the old days, nowadays trail tends to be around 60mm for rigid forks, and around 80mm for mtb suspension forks (larger to ensure safe handling is maintained when compressed, which in turn gave rise to exotic/cult suspension fork designs like the Girvin Crosslinks which maintain trail when compressed).

Imho if a rigid fork delivers a trail between 50mm and 70mm, you are unlikely to be too unhappy. After all, other factors also affect a bike's steering.

Hope it helps.

 

[david1701]

thats complicated but brilliant RecordAce now I understand what to do and how to do it. I'll ride carefully for the next little while and see if I want to adjust the steering at all then start on the measureing

 

[Globalti]

Those carbon forks I linked are "compensated", I think by 80mm but you'd need to check. This means they will work fine with a frame that comes with a short travel cross country fork.

Look at the seventh column here: http://www.carboncycles.cc/?s=0&t=10&&q=fork_selector&

 

[Zoiders]

Since you asked, for geometry maintenance purpose your suspension fork's axle to crown distance should be measured with sag (i.e. with your full weight on the bike).

The need to maintain geometry assumes you are perfectly happy with the current steering sensitivity.

Actually to maintain the same steering sensitivity the trail of the fork has to be maintained, and trail is affected by fork rake (otherwise known as offset) and head tube angle, while head tube angle is in turn affected by fork length.

If you want to be precise and are interested in the maths you can read this, this and this!

If not you can just trust me that a 10mm increase in fork axle to crown length will reduce head tube angle by approximately half a degree which in turn will increase trail by approximately 3mm. An increase in trail will make steering more stable/sluggish, a reduction in trail will make the bike more nimble/jittery.

That is assuming rake is the same for both forks; if not an increase in rake by X mm will reduce trail by approximately X mm.

In other words, steering sensitivity can be maintained by a combination of axle to crown length and rake, e.g. a 10mm increase in length combined with a 3mm increase in rake will end up having minimal impact on steering sensitivity.

Then the next obvious question is how significant any amount of change in trail is? This is an impossible question to answer because it depends on how sensitive you are to change. In terms of figures as Dave Moulton said bikes used to have zero trail in the old days, nowadays trail tends to be around 60mm for rigid forks, and around 80mm for mtb suspension forks (larger to ensure safe handling is maintained when compressed, which in turn gave rise to exotic/cult suspension fork designs like the Girvin Crosslinks which maintain trail when compressed).

Imho if a rigid fork delivers a trail between 50mm and 70mm, you are unlikely to be too unhappy. After all, other factors also affect a bike's steering.

Hope it helps.

Have you ever ridden a MTB with a long travel fork?

Or converted one to rigid and then tried riding it?

 

[RecordAceFromNew]

Have you ever ridden a MTB with a long travel fork?

Yes.

Or converted one to rigid and then tried riding it?

No, and I have no reason to. The problem of putting conventional rigid mtb forks on a DH/long travel fork bike is not the head angle as your previous post suggested, because these bikes don't have steeper head angles than XC bikes as you said, quite the opposite they have much slacker angles as a matter of geometric necessity. The Commencal Supreme DH, Spesh Demo and Marin Attack Trail e.g. all have head angles around 65 degrees, installing conventional forks would put their head angles back to around the 70 degree mark.

However, because these bikes are not designed for such, amongst other issues you will find yourself sitting on a saddle literally above if not in front of the bottom bracket...

 

[Davidc]

I did my son's bike some years back, when his quite cheap forks wore out. I picked up a replacement fixed pair for next to nothing at the bike shop. The bike was much better to ride with the fixed ones, but as I say it was a cheap bike (well down the range Raleigh) so the original forks were nothing special.

 

[Zoiders]

Yes.



No, and I have no reason to. The problem of putting conventional rigid mtb forks on a DH/long travel fork bike is not the head angle as your previous post suggested, because these bikes don't have steeper head angles than XC bikes as you said, quite the opposite they have much slacker angles as a matter of geometric necessity. The Commencal Supreme DH, Spesh Demo and Marin Attack Trail e.g. all have head angles around 65 degrees, installing conventional forks would put their head angles back to around the 70 degree mark.

However, because these bikes are not designed for such, amongst other issues you will find yourself sitting on a saddle literally above if not in front of the bottom bracket...

Current XC bikes - not just "downhill bikes" are coming equipped with as forks with as much as 150mm travel these days.

To stop the fork kicking the head angle back too much head tubes got steeper, not slacker, seat layback or seat tube angle can be tweaked as well to avoid any BB problems.

Bikes geared toward gravity sports have always been slacker as fork travel is extreme and steering needs to be a bit slower on the DH course or the bikes becomes far too twitchy - this is not the case with most hardtails aimed at the XC market.

You can't just plug a non corrected rigid fork into most new frames as it screws with the handling - and yes slacker angled bikes do get converted to rigid forks as rigid bikes are preferred by some jump or street riders.

Leading link forks such as the Girvin and later the Whyte bikes BTW were prefered for the ability to climb up and over obstacles under compression as the axle path moved forwards and up, they also resist diving under braking force, trail was not so much the consideration.

 

[RecordAceFromNew]

Current XC bikes - not just "downhill bikes" are coming equipped with as forks with as much as 150mm travel these days.

To stop the fork kicking the head angle back too much head tubes got steeper, not slacker, seat layback or seat tube angle can be tweaked as well to avoid any BB problems.

Bikes geared toward gravity sports have always been slacker as fork travel is extreme and steering needs to be a bit slower on the DH course or the bikes becomes far too twitchy - this is not the case with most hardtails aimed at the XC market.

You can't just plug a non corrected rigid fork into most new frames as it screws with the handling - and yes slacker angled bikes do get converted to rigid forks as rigid bikes are preferred by some jump or street riders.

Leading link forks such as the Girvin and later the Whyte bikes BTW were prefered for the ability to climb up and over obstacles under compression as the axle path moved forwards and up, they also resist diving under braking force, trail was not so much the consideration.

Zoiders earlier you said "Modern suspension forks that run around 100-125mm (or more) of travel mean that the frames are starting to be designed around them so they make the head tube angle steeper". Now you are saying current XC bikes are coming equipped with 150mm forks, and again insisting head tubes got steeper and not slacker.

Given we can probably all agree that typical 80mm/100mm travel XC bikes have head tube angles at around 71 degrees, where are these XC hardtails with a) 150mm forks and b) head tube angles steeper than that? Can you provide links to make/model/geometry to prove the point?

I am asking because afaik 150mm travel hardtails are firstly rare, secondly they are never billed as XC bikes but as freeride or trail bikes, finally they have head tube angles at around 67 degrees, i.e. much slacker than the typical 80mm/100mm XC bikes, but not as slack as the DH bikes which have even longer forks.

As I said there aren't many hardtails designed for 150mm forks, but examples include the On One 456, Commencal Ramones Crmo and the Ragley mmbop. They all have head tube angles at around 67 degrees.

I am raising this question because I can't see how bikes with longer travel forks can come with steeper head tube angles as you said - firstly that will mean the handlebars getting higher and higher (although that can theoretically be addressed by installing riser bars upside down - but I haven't seen much evidence of that on the market), but more importantly if the angle was steep to start off with then when the forks are compressed it will approach 80 degrees, geometric trail will drop like a rock even with minimal rake/offset, and secure handling goes out of the window when it is needed most.

Rather imho it is nearly inevitable that longer forks mean slacker head tube angles - evidence is that 180mm/200mm forks are at around 65 degrees, 150mm forks at around 67 degrees and 80mm/100mm forks at around 71 degrees. Given sag has to be accounted for and each 10mm of fork shortened increases head tube angle by 0.5 degree, putting typical rigid forks for xc mtb on bikes designed for longer travel forks will actually bring their head tube angles back in line. However, one would be wise to check the trail figure is not crazy since it is also dependent on the new fork's rake/offset, and to ensure saddle position, bb height, wheelbase etc. are also acceptable as they will all be affected.