My ‘ol trusty steed (’97 GT Xizang) has some 70mm travel Marzocchi Superfly forks attached which puts it on the manufacture designed geometry give or take a bit of sag.

[*]seat angle 74 deg[/*]
[*]head angle 71 deg[/*]
[*]Reach 430mm[/*]
[*]ETT 580mm[/*]
[*]BB Drop 35mm[/*]
[*]Arse to Axle 195mm[/*]

BTW my BB->top tube is 460mm and BB->Saddle is 810mm.

My calculations say going for a 120-140mm ish fork and keeping the same stem would give me the following.

[*]seat angle 71 deg[/*]
[*]head angle 68 deg[/*]
[*]Reach 405mm[/*]
[*]ETT 585mm[/*]
[*]BB drop 14mm[/*]
[*]Arse to Axle 160mm[/*]

The revised angles look pretty similar to modern geometry so all is good there, but I’d be running about 20mm higher BB and 35mm less reach than modern geometry so basically I am wondering if this is going to screw everything up or not.

Also wondering if I did go ahead with this would would be a good reasonably priced 1.125″ fork to use with 120mm-140mm travel. Something pre-millenial would be nice as that would keep everything retro, I can only think of the Marzocchi Z1 but I think that weighed about 5lbs, I’m not a total weight weenie but something reasonably lightweight would be good.

Can’t imagine the headtube would be that happy with 140mm if designed around a 70mm fork.

I personally wouldn’t do it. It’ll feel horrible. I can’t imagine an old ti frame taking kindly to a longer fork. That’s a lot more force going through it

I’ve done it with older frames and it works pretty well. Massively improves the ride when going down. It does make them a bit skittish on the climbs. Not a big issue, I just find its quite easy to lift the front wheel so you need to keep your weight forward. Easily fixed with good technique or an adjustable travel fork although you won’t be able to adjust it for every climb. Going retro on the fork would be a compromise too far for me though. The tech has moved on a lot. Some Revs would do the job

Q – Can’t imagine the headtube would be that happy with 140mm if designed around a 70mm fork.

Good thought, the fork being 20% longer when fully extended means that worst case leverage from braking forces on the headtube is also 20% greater – i think this is manageable especially if I don’t allow myself to become particularly heavy. The frame is not the lightest and has a relatively beefy downtube.

Forces from mashing into obstacles would also similarly have more leverage, but this should be offset by the slacker head angle and increased fork plushness.

Don’t do it! It will be a dog and also won’t deliver the performance you imagine IMO.

Just because the head and seat tube angles match modern geometry the rest doesn’t and it will look and handle like a chopper.

My experience of putting 100mm forks on an 80mm fork bike is that it works, once you dial in the extra sag of the fork not much difference. But what you are proposing would put the BB way high even with sag, you’d notice that.

Give it a try – but don’t make your first ride anywhere that’s going to hurt if a) you fall off or b) the bike snaps in two

This is an opportunity to learn all about what happens when you fit a long fork on a bike designed for short forks. Give it a go with some care and enjoy the learning experience 8)

I’ve gone from 60ish to 100. That handled like a dog. It lasted about a week before i spaced back to 80.

You’ll get more benefit from decent forks rather than long forks. (Gone from early elastomer forks (92/93 IIRC) to some mid range air forks from the early 00s.)

I can remember years ago asking Cy @Cotic whether it would be alright (wink, wink) to run sagged 140mm forks on my mk I Soul (which is certified to 130mm).

His response was not a chance. The maximum was stated for a reason as the forces at the head tube could rip it off with the increased leverage (and also that this type of failure was easily diagnosed and it would invalidate my warranty).

I’ll let you take from that what you want…

high bb means it’ll ride like a dog.

Does 20mm higher BB really make that much difference? That is the same height difference as Flats+trainers vs SPD’s and I wouldn’t say that totally kills handling.

What does handling like a dog mean? I’d be chuffed with handling like my JackShihBorderSausage – he does over 30mph even on fairly rough terrain and gets there really quicky, turns on a sixpence too even on loose off camber stuff.

I was originally thinking 100-120mm forks actually, but I did my sums wrong and didn’t take into account enough fork sag, so I posted geometry for 120-140mm and then things got a bit confusing. The Marzocchis Bombers I have are much better than the other elastomer crap some of it’s contemporary’s had but the crown is lightweight and flexy, which when teamed with an already steep head angle gives a nervous ride when on the brakes on steep descents.

I appreciate the concern as separating ones headtube from frame is likely to be ouchful, but does the extra leverage from longer forks really make that much difference? For instance I dropped 23kg of weight recently, over 20% weight loss – doesn’t that counteract 15-20% extra leverage from longer forks? Similarly, I stick to relatively sane stuff nowadays surely the stress from landing large vertical drops etc is an order of magnitude higher than normal, albeit steep rocky and fast, trail riding.
I am not surprised by the response from Cotic, they’d do some worst case calculations with a healthy safety margin and then stick to the conclusion no matter what customer asks. The Xizang of course is from a time before fork length certifications because more than 60mm travel and large wheels etc were inconceivable back then, it does have a significantly larger downtube and meatier headtube welds than a mk1 soul though.

His response was not a chance. The maximum was stated for a reason as the forces at the head tube could rip it off with the increased leverage (and also that this type of failure was easily diagnosed and it would invalidate my warranty).

Maximum leverage from impacts will be AT FULL FORK COMPRESSION so the leverage argument is guff. I’d love to know how thy can diagnose what length fork was fitted.

Properly makes me LOL/shake my head that mfrs (even trusted individuals at SmallBikeCos) come out with crap like this and then punters suck it up.

The fact you are likely to hit stuff harder with a longer fork is a better rationale.

~1 degree per 17mm head angle change.

With longer forks there’s also the fact the fork is likely to be be built stiffer and so absorb less energy itself through bending, plus the increased leverage due to length, plus the ability to ride rough stuff faster, plus the increased leverage when landing to flat due to the slacker head angle. It all adds up. Even if it doesn’t cause premature fatigue failure it’ll probably show up a lack of frame stiffness spoiling the handling.

Regarding BB height, 20mm is a big difference. My full-sus has 12mm of BB height adjustment and even the middle position handles clearly differently to the high and low positions, and that’s just +/- 6mm. My old hardtail’s BB was 40mm higher than my new one and they’re pretty much at the two geometric extremes in that respect.