fork travel

jeffers696

hi, wana get a new fork for a trek 4300, but just wondering how mutch travel is the maximum i can go for befor it starts to effect the handeling of the bike ect

Anonymous

Any change in travel will affect the handling to a certain extent. What really matters is whether or not you approve of the change to the handling.
I went from a 100m fork to a 150mm on mine years ago, and really like the change it made. Since having the longer fork, I'm always in the "attack" position on the bike, meaning that even if I hit a drop or something unexpectedly, I'm ready for it.
The only downside is that I have to consciously put more weight over the front on very fast, loose-surfaced corners, but it became second nature after a few rides.

jeffers696

what about the frame, could a lardger fork put more strain on it or something?

nicklouse

jeffers696 wrote:

what about the frame, could a lardger fork put more strain on it or something?

Yes. but 10mm extra would be OK.

Anonymous

See, I've been pondering, and I don;t get this whole "longer fork causing more stress" thing.
Surely, the moment of greatest stress is when the fork is compressed to it's limits, putting the greatest amount of force on the head-tube.
But, when the fork is compressed, it's the same length as any other fork, give or take small design differences in crown design.

So is it really, really true that it puts more stress on the frame?

jeffers696

i thought of it diffrently, as you put a longer fork you increse the head angle (i think) turning the bike in to more like a chopper design. but i dont really know what i am on about tho :P

but you think if i lob a 130mm on there it will be ok?

hoathy

Jeffers is right, basically as the distance between the crown and the axel increases, the distance between the area the force is being applied (where the tyre touches the ground, and the place where the force is affecting the frame (the headtube, expecailly the weld between the head tube and down tube) is increased, this is because the effective turning force (torque) is a function of applied force (the force of the earth reacting to the mass of you and the bike pressing down on it) multiplied by the distance from the head tube to the bottom of the tyre. So a longer fork = a bigger force, it is slightly more complicated than this, as the whole think pivots around a point (the rear axel) - so the distance between the headtube and the rear axel is fixed, lifting the front of the bike higher increases the head angle and as such the 1 dimensional distance between the headtube and the font axel (i think this is called the trail or something), increasing this distance also increases torque at the point of the headtube but in a simultaineous but different moment.

hoathy

oh yeah, and I wouldn't go above a 100mm fork on one of those trek frames, it will increase the stress, it will ruin the geometry, it will ruin the handling, and it will void your frame warranty

jeffers696

thanks Hoathy

Anonymous

Hoathy, you make it sound like you're quoting from an instructional text!

I still don't think it has as much of an effect on the headtube stress as is commonly accepted - the stress when just sitting on the bike with the fork extended is absolutely minimal compared to the force on the headtube during a heavy impact. the leverage action of the longer fork happens at a very low load state of the frame.
I've never seen a bike that wouldn't be strong enough to do front wheel hops either, and this puts much, much more stress on the headtube than a longer fork would.

Maybe it's one for Mythbusters?

hoathy

yes, i agree, but of course putting a longer fork on doesn't just affect the bike when you are just sitting on it, it also means there is more stress when you do those things you mention, so 'hopping' with a longer fork is far more stressful than with a shorter one. The rules apply all the time. A longer fork definately increses stress, alot.

The thing you said before about the stress being greatest when the fork is compressed isn't really the case. The force that will break the frame is the turning force that is trying to rip the head tube of the downtube about the pivot point produced by the headtube-top tube join. you see its all a matter of levers, the moment produced by the force of the wheel on the groud (A) (whether that is when the bike is just standing there, or if you just did a 50ft drop on it) is the result of: distance from tyre/earth juction to the headtube/toptube join X force applied to tyre by earth. and the counteracting force (B) is the result of the equation: distance between headtube/toptube join and headtube/downtube join X force applied by headtube/downtube join. The piviot is the headtube/toptue join.
In order for the frame not to break, the moment A should never exceed the maximum possilbe moment B (for which a major determining factor is the strength of the weld).
Ands then theres when you hit thing head on... thats a whole other can of worms (but a longer for is worse here too!)

supersonic

Trek do not warranty anything above 100mm on this frame.

Another reason is that people are more likely to use the frame beyond its limits of normal usage with a longer fork. It is not meant to be thrashed.

hoathy

wow, that is a less complicated reason, wish i'd thought of that...

supersonic

Lol, but of course you are right too, there is more stress placed on the headtube in normal riding conditions. Most are actually testesd with more travel, but of course it has to exceed the margins by a long way. Many downtubes are very thin on XC bikes, and sometimes eve a minor knock head on into a tree folds them!

jeffers696

cheers guys, very indepth answer that was way over my head but i am sure the debate will continue. here is another spanner tho, the RockShox Revelation 426 Air U Turn Fork has adjustable travel would this ok at 100mm or is still to long?

supersonic

Keep it at 100. But even if you went over, and broke the frame, wind it back in again ;-)

But why the extra weight of a Rev? Why not get a Reba?

jeffers696

ye just fancy a bit more travel than 100mm as my 100mm tora shakes me to the core but that could be the fork. dose they weight make that much diffrence as its only 130g?

supersonic

Thee tora may be internally adjustable to 115 or 130 with the correct spacers. Which Tora is it? is it set up right?

You will notice weight differnce on the front end.

jeffers696

its the one that comes standard, tora sl 100mm i think. i think its set up correctly, do you know a guide to setting them up?

supersonic

The instructions ;-) Downlaod them from SRAM if don't have them. The booklet covers all varieties of the fork. Bascially you set the sag, then the rebound adjuster, and with some models compression damping.

jeffers696

:roll: lol cheers try that then nice one sonic

hoathy

It could well be set up wrong as these forks shouldn't really be like that. How much do you weight, if you are considering an upgrade maybe try to keep the travel as low as possbile. Apparently, though I haven't tried them the Magura Odur are good forks if you want something strong that will take the trail well without having bags of travel or ruining your bank balance.

jeffers696

i am about 13 stone with a 23" frame (6'6), gona try the set up first befor splashing out

Anonymous

Hoathy, I still think you're missing a fundamental part of the reality of the forces acting on the headtube. Given a static example, or a rigid fork, I think your view is 100% accurate, however..
When the fork is extended, any force acting upon the bike from the ground will compress the fork before it acts to break the head/down tube connection. My proposition is that this will always happen to such a degree that when the force is large enough to damage the headtube, the fork will be compressed. this means that the longer fork will not produce any more torque than a shorter one, since at the point of causing damage, they will both be compressed, and will therefore be of similar lengths.

(I'm not saying you are specifically wrong, I'm just challenging the commonly accepted belief with what I think is a reasonable alternative. I might be wrong as well, but I'd like to see some in-depth tests - I know, I'm a geek)

I think supersonic's explanation holds more water - that a longer fork will lead to a more aggressive riding style.

supersonic

Also worth noting that some longer travel forks have stronger, stiffer steerers and crowns. In a front ender, a big one, some forks bend - sometimes its the frame! This is why twin crown forks need reinforced headtubes, as they place a lot more stress on the frame.

Anonymous

Who said they need reinforced headtubes. I've run a dual crown on what is essentially an XC frame for 8 years, and all is good. It's not exactly the lightest frameset ever, but it was not intended for hucking!

viva la revolution!

supersonic

Either a tough frame, or luck! I have seen many frames with headtubes ripped off, and most that are intended for double crown forks have bigger gussets and thicker tubing. The stress they place on the front end is a lot higher, no give at all - hence why they are a lot stiffer fore/aft.

Anonymous

ssssh, there's nearly 10mm of material on the headtube all round - don't tell anyone!
I know what you mean though sonic, but to be honest, I think my frame comes from the days of being overbuilt for safety, and I believe most of the main frame is actually shared with the B-17, not the original mount vision, so not really an xc frame . Also, the Junior Ts, although relatively stiff at the time, definitely feel quite flexible by today's standard.
my next fork will definitely be a single crown though, the twin crowns of today are far too long, fitting one would make it handle like something off "American Chopper"!

supersonic

Ive seena few fitted to Zaskars too! But the B17 is a tough cookie, and erm, I actually had a set of Marzocchi Mr T on an old Mt Vision! These were odd forks, you could unscrew the stanchions, and turn into a single crown.

Anonymous

really? wow! certainly can't do that with my juniors! how strange!
I've seen a load of zaskars with triple clamp forks over the years, they were probably the original hardnut hardtail.

hoathy

OK, back to the pointless mechanics arguement....

I agree that your point makes sence assuming that all the force is applied directly along the axis of the forks, but it isn't (your forks would have to be pointing striaght down, like a prst!). the point of hitting a tree is a highly valid one, and translates also to hitting any obsticale, when you hit a small rock at speed (even one the suspension is going to handle) there is a signicant force hindering your forward motion applied through the forks and directly into the headtube.
I do see what you mean about the travel compresion thing (with respect to landing on the forks), but lowers on long travel forks tend to be longer too (my nixions are longer than my f80s), meaning that at maximum compression the leverage will still be greater with a long travel fork.
with respect to fitting triple clamps, i was thinking of doing this on my tandem! It did once have a some RST mozo comps on it and they were ok, except sagged like 60%! Theres this guy who rides around exeter with a cheap saracen and boxxers or something mental on it (he looks like evil kinevil with a motorbike driving ban), its quite funny, he's never going very fast... i wonder why??