Fitting a 140mm fork to a frame designed for 100mm!?!

slimboyjim

Hi - I am the owner of a GT Avalanche 1.0 with a Suntour x100 fork. I'm getting into mountain biking in a big way and I am planning on a fork upgrade around xmas after I've saved some money... I like the idea of a 140mm fork (Fox Float 32 for example) as, about a year after that, I would be looking to get a new frame and most of the 'dream' bikes I look at take 140 forks. i.e. When the time comes I want to be able to buy just the frame and not a new set of forks as well... Whilst there are travel adjust forks out there I would like to avoid the extra £100 outlay if possible, expecially as the non-travel adjust models appear to be slightly better performing.

I am aware that fitting am oversized fork would upset the geometry of my Avalanche (I think it has a maximum fork size of 120-130mm) and invalidate any warrantee. However, could the sag of the fork be set low to effectively reduce the length of fork travel and return the bike geometry to acceptable limits? If feasible, would this have any detrimental effect upon the forks operation or lifespan?

In case you are wondering, a brand new full build would be nice but I would struggle to get it past the missus. This is why I'm on a determined (stealth) upgrade route.

Thanks for the advice!

James

nicklouse

you will still have problems every time the fork is fully extended.

If you want a new fork get an adjustable travel one and run it at 110mm to 120mm

stumpyjon

Seconded, pay the extra, get a Talas fork, great bit of kit. Makes hill climbing so much easier.

P-Jay

As above, get a Talas and run that badboy at a max of 120mm until you get the frame.

The other option would be to find a second-hand 100 or 120 fork now, if you get a nice one chances are you could sell it on again for what you paid for it when the time comes to upgrade.

bomberesque

+1 for Talas. if you can't stretch to a talas, try an RS revelation U-turn. If you don't mind heavy (half kilo heavier than Talas iirc) then the Pike is a great fork.

but really, the Talas has about the best rep out there in this category and will allow you to run at a length to suit your frame until you up the frame to one able to handle a longer fork.

I have an Evil Sovereign, which is designed to run forks from 100 - 145 and I have a Pike coil U-turn (95-140) on it. The handling at 140mm (pike is a bit of a long fork for it's travel, iirc) the handling is dire for normal trail use. I use it that way on on DH courses, but run at 100 or 120 for normal stuff

The Northern Monkey

your looking waaaaaaaaaaayyyyy too far into the future...
you want to buy a fork for a frame your going to buy in 2 years?

Buy a decent fork thats suitable for your bike and save for your custom build at a later date.... you may never end up building it 2 years down the line.

stumpyjon

Actually Benj makes a pretty good point. By the time you buy your new frame your new fork may not be compatible, steerer too short or possibly headset standards have changed (they seemed to be heading towards tapered steerers at the moment :roll: ).

slimboyjim

Thanks for the advice guys (and gals?). I knew there must be a reason or people would be doing it!

I know it looks like I'm planning pretty far ahead but it was just a theory question - I could not work out in my head why it wasn't possible (and if a REALLY good offer comes along on a fork I know whether to take it up or not - it's quite easy to waste a bit of time dreaming about bikes).

With regards to the timescales I say 2 years only as, after spending money on a fork, I will have to save a bit again/wait for some good offers/sales! It's not a rigid timescale and, as forks aren't cheap I would rather buy one that works on both bikes...

I will be upgrading my forks within the year and I'll probably look for a Talas when I do. I'll do it around the end of the year so I can get a better impression of whether standards are changing, etc. (thanks for the info re: tapered steerers!)

As I'm a bit of a geek and love knowing how stuff works, why would the excessive extension cause issues - I assume it would only achieve this extension when the bike is in the air so is it the additional forces upon landing?

nicklouse

landing and when climbing the bike would be a pig.

If you are going for a new fork and want to future proof a travel adjust Or travel modifiable for is the only serious way or you will be well hacked of it a piss poor handling bike.

Northwind

Can those floats have their height adjusted internally? It adds cost but it might make it more viable. Still not a good plan but not as terrible maybe

peachos

i was wondering the same thing recently. have just bought a DMR Trailstar which i'm going to build up as a SS/thrash/do it all bike. I'm, not sure of the year but i think it's a few years old so probably only designed for a 100mm fork. The bike actually came with a set of MX/MZ comps which i'm going to fit at first but will look to upgrade later once i have the bike up and running. The forks appear to be 100mm. I would like go for something like revelation/pikes/bombers that are around 130mm.

So what affect will this actually have on the ride of the bike? Will an extra 3cm impact on head angle that much and what effect will this really have on the way the bike handles?

The Northern Monkey

peachos
read nick's reply 2 posts above yours...

peachos

landing and climbing would be a pig. yeah i read that - it's not really giving me any specifics though is it. why would it be such a pig?

i just cant se why or how a few cm would have such a significant impact on how a bike handles, hence why i posted a my question in this thread.

but thanks for your input bigbenj.

supersonic

It will slacken the head angle, raise the bottom bracket, affect the handling due to this, and when climbing the fork will extend more if you simply run a lot of sag.

But check the crown to axle heoights.

peachos

well looking at the DMR website the head angle for the 100mm frame is 71degrees. am i right in thinking that a few extra cm's would lower this to around 68/69 degrees or have i got that the wrong way around???

i suppose the best thing would be to stick my 150mm bombers on the frame for a bit and see how it handles, if not too bad a set of 130's would probably do the trick. if i fall off the back whilst climbing, well...

The Northern Monkey

peachos
Its not just the head angle etc... frames are warranted for a certain length fork. One of the reasons for this is the strength of the frame.
A longer fork and different geometry could put extra stresses on other areas of the frame making it more susceptable to breaking...

Northwind

Yup. A fork is basically a lever, make it longer and you increase the strain a lot.

DJBOMBSCARE

This thread is really interestin me cos im running 140 foxs on my 575 and as a backup winter thrash hack thing Im buildin a yeti arc hardtail and would like to run the same forks just for feels sake

Now no-ones really got into rake trail etc and just said geometry and warranty issues. and coming from a motorbike background Ive spent loads of time adjusting fork lengths. To make steering sharper or greater straight line stabilty for different tracks. As well as compression and rebound settings

So i hope people dont mind if I ask a couple of questions



Sus forks always say eg xxxmm of travel. Does that actually relate to a 140 fork being 40mm longer then a 100mm, or is it less then that.

Has anyone really had any frames mess up from running longer or even shorter forks as that would do exactly the same. Surely the stress pressure would be in the forks not the frame. The pivot point being around the rear axle. None of the angles would actually change within the frame as such just their relativity to the ground. So as a cage, the frame should remain structurally the same.

Sorry if they are slightly stupid questions. But hopefully knowing where im coming from might help explaon why im asking them

Thanks in advance

RealMan

Yes - genius! 3% [ 1 ]

No - you are an idiot! 96% [ 31 ]

Who was the one guy who voted yes then.. Go on, own up..

CycloRos

it was me cos it'll be p*ss funny seeing a dude on a relatively cheap bike completely over forked

to the OP, seriously just get a travel adjustable fork if you want to future proof it a bit. My Duster currently runs a Pike 90-140mm and anything above 120 just makes it feel all wrong regardless of any sag.

supersonic

DJBOMBSCARE wrote:

This thread is really interestin me cos im running 140 foxs on my 575 and as a backup winter thrash hack thing Im buildin a yeti arc hardtail and would like to run the same forks just for feels sake

Now no-ones really got into rake trail etc and just said geometry and warranty issues. and coming from a motorbike background Ive spent loads of time adjusting fork lengths. To make steering sharper or greater straight line stabilty for different tracks. As well as compression and rebound settings

So i hope people dont mind if I ask a couple of questions



Sus forks always say eg xxxmm of travel. Does that actually relate to a 140 fork being 40mm longer then a 100mm, or is it less then that.

Has anyone really had any frames mess up from running longer or even shorter forks as that would do exactly the same. Surely the stress pressure would be in the forks not the frame. The pivot point being around the rear axle. None of the angles would actually change within the frame as such just their relativity to the ground. So as a cage, the frame should remain structurally the same.

Sorry if they are slightly stupid questions. But hopefully knowing where im coming from might help explaon why im asking them

Thanks in advance

Fork crown to axle lengths do vary. Longer forks do stress the headtube area more due to the extra leverage - ride into a tree and you'll see what happens! Also consider braking forces and hitting smaller objects.

Basically - check your warranty.

paul.skibum

Take the money, buy premium bonds, wait until you can afford frame buy both together and hope you won a million in the meantime.

I would go for the travel adjust and or correct fork for your frame. A U turn Revelation sounds perfect or a Fox Talas to let you run the right length now with possibility to go longer when frame allows. And you can try it at 140 and see what happens and report back to us!

skullthaw

did you see they marin b17 in this months mbuk almost exactly the same as will happen to you

Northwind

If you ignore the frame-snapping-in-half-and-breaking-your-wrists issue...

Not all bikes will handle poorly with a longer fork. Look at, frinstance, a Cotic Soul- you can run it at 100mm and go racing or stick in a 130mm for bouncing down mountains- but it'll climb at 130mm and descend at 100mm. Then, look at the BFe- it's the exact same frame geometry, but built up for a longer fork (up to 160mm I think). Though it's not the sweetest of climbers with 160mm! It still steers beautifully with a short stem despite its silly fork.

All a bit irrelevant when you're sitting at home with 2 broken wrists mind, but it's not true that it'll automatically handle poorly- it all depends on the design.

Hercule Q

theres a guy riding a saracen hardtail from '03 i think and its got 170mm travis' on it the thing looks and handles like a hog i'm just waiting for the day it snaps

DJBOMBSCARE

Northwind, thats similar to where i am coming from.

Its all rake and trail and at the right angle longer forks can steer better than a shorter one. Thats why im wondering just how much longer an extra 40mm of travel will make a fork. Then i have software that can work it all out. Hence my questions.

And I do understand about the stresses, but would I be right in saying the movement of the suspension up and down, rider weight, tyre size and pressure would also change the geometry slightly. So would that move the stress points?

I'd assume those were all tolerences built in at the design level. However could it mean that a larger person would be able to ride the same frame better with bigger forks than a smaller lighter person with the reccomended ones

sorry for all the questions I am genuinely interested in this stuff, and soaking it all up like a sponge

Northwind

It also depends on how you ride. Like, if you like to do huge drops onto the nose of the bike, you're going to put a huge load on it. If you mince about and never brake hard or otherwise test the front end, it'll not be a problem. All relative.

That aside, I've been considering sticking 140mms into my Soul- Pikes or 09 Revs with a maxle- and I consistently chicken out in case I snap it like a twig

Anonymous

supersonic wrote:

Fork crown to axle lengths do vary. Longer forks do stress the headtube area more due to the extra leverage - ride into a tree and you'll see what happens! Also consider braking forces and hitting smaller objects.

Basically - check your warranty.

ahem sonic, i hope you have checked your warranty my man.

nicklouse

DJBOMBSCARE wrote:

Northwind, thats similar to where i am coming from.

Its all rake and trail and at the right angle longer forks can steer better than a shorter one. Thats why im wondering just how much longer an extra 40mm of travel will make a fork. Then i have software that can work it all out. Hence my questions.

And I do understand about the stresses, but would I be right in saying the movement of the suspension up and down, rider weight, tyre size and pressure would also change the geometry slightly. So would that move the stress points?

I'd assume those were all tolerences built in at the design level. However could it mean that a larger person would be able to ride the same frame better with bigger forks than a smaller lighter person with the reccomended ones

sorry for all the questions I am genuinely interested in this stuff, and soaking it all up like a sponge

25mm (1") in axle to crown length will change the head angle by approx 1 degree.

MacAndCheese

Made this mistake with my old Kona Lava Dome back when I was about 14...bought a ridiculous pair of Bomber z1 Drop-Offs (I thought they where so cool at the time) Must of been about 5 inches of travel on a frame designed for rigid P2 forks...the top tube cracked and luckily I noticed before a full on snap at high speed...

passout

Use this bike till it breaks or gets uneconomic to replace bits (eg forks & hubs go) - then buy a new one with nice new 140mm forks.