69ers

greymarauder71

Yep - I thought it high time we rolled this one out again.

But seriously, I am building a FS 69er at the moment and cannot find any real knowledge out there among the pages of 'opinions'. I was not too bothered about the handling etc (it will all be an experience...) but then I read somewhere that the wrong forks/angle at the front end could result in a broken frame?! Really?
Does anybody have any info on this or be able to point me to a site that does? At present I have replaced my rear shock with a 40mm longer ROMIC to 'balance out' the increased height at the front.

Cheers

nicklouse

well first question is what is he frame.
and your first purchase might have been a bad move.

supersonic

There are so many combinations out there, and the best way to find out if it will work or is warrantied is to contact the manufacturer. However most will say no as the handling could become dangerous, or extra stresses involved as you mention. They are not designed for it.

Your bottom bracket is going to be ludicrously high, and you have altered the rear suspension characteristics too.

Hardly anybody does it because it just doesn't work.

greymarauder71

It is cheap old unbranded frame (thought I would play with cheap parts before breaking any expensive ones) but it is close to a Y style unit. It had a 150mm shock and it now has a 190mm one. I am looking at putting some 100mm travel 29er forks on it...

cooldad

You are being serious?

The Rookie

69ers balance travel by reducing the fork travel by circa 30mm not by ruining the rear as well......junk the shock and fit the right length, having said that if you are using junk BSO parts you won't know if a decent 69er would be any good anyway.

greymarauder71

Yes, I am just messing around with parts and ideas at the moment I admit, but I just wondered if there is anywhere that gives a good understanding of bike geometry from a basic perspective...

supersonic

Yes, the bottom bracket will be far too high.

The Rookie

What is the rear leverage ratio? A 40mm longer shock could raise the rear by upto about 90mm which will steepen the headangle 'a bit' making it rather twitchy, the BB will be so far off the ground you will really struggle with stopping and starting, as the rider will be much higher it will be less stable as well.

Can I take out some life insurance on you, you won't need it but I may as well get more than just a laugh out of the result!

greymarauder71

Okay, for the record for all my searching a chap called Ryan Harry has the best site I have found to date for explaining a lot of this.

I understand the BB issue, I will get to that (although having just been riding a Charge for a while, a higher BB worked wonders for me).

If I am going to die because my head angle is too slack (I understand the increased stresses of slackening the head tube angle), but the bike will be unusable because raising the rear will make it too tight, are we actually saying it is a physical impossibility to create a FS 69er?

And has anybody out there on this forum ever owned one/attempted to build one? Yes, of course you can insure my life, happy to help out there. Good recommended body armour anybody? Actually again, does anybody actually know of a case/person who snapped their forks due to bad geometry?

Cheers,

James

cooldad

greymarauder71 wrote:

...is a physical impossibility to create a FS 69er?

No.
But it is possible to create a total abortion as well.

supersonic

I have seen dozens of snapped frames from improper forks fitted.

greymarauder71

Somthing solid at last...

http://manarea.blogspot.co.uk/2012_09_01_archive.html

It can be done then. I am going to try it and if I survive I will post the results. Now I have spent far too much time looking I have found a fair few that have been run from Stinkys to Y frames and so on. Hey... what's the worst that can happen...

supersonic

No one doubts it can be done. It's what the results will be. The shock there is 6mm longer. Yours is nearly 7 times that.

nicklouse

greymarauder71 wrote:

but I just wondered if there is anywhere that gives a good understanding of bike geometry from a basic perspective...

Yes here but we do actually need to know some things about the BSO you have.

Original shock length and stroke and the new one and then there is the rest of the geometry. Then the suspension design and I could go on and on and on.

The Rookie

greymarauder71 wrote:

If I am going to die because my head angle is too slack

No, because as stated clearly it will be too steep and that may kill you......

greymarauder71

STOP PRESS...

Does fortune favour the bold or am I still in for a shock? Incredibly I put the major components together tonight just to measure the new geometry up and decide whether it was worth proceeding. I have in effect swapped a 150mm shock with a 190mm thus increasing the rear height substantially. However, due to the 100mm fork and the 29" front wheel the front has also been raised considerably also, and after a long time spent double checking (as I couldn't believe it myself), the head angle is exactly the same as the 29er the fork came off. It is 70 (maybe 71) degrees.
As all of the front components are the same and the head tube angle is the same, then the trail and the rake/offset must be the same. Yes, it has of course raised the bottom bracket up to 15", but it appears that there are plenty of people that do actually run such high BB's.
This to me, is looking distinctly do-able, but feel free to keep putting me straight as you guys have all helped a lot with pointing me in the right direction...
My original point is, with the fork geometry just the same, should I at least be safe from snapping the forks?
Thanks again.

cooldad

So what?
One has nothing to do with the other.

Chunkers1980

All else the same you've now got a longer leaver

The Rookie

So you have a 150mm travel bike with a head angle of an XC bike, it's an abortion and will tell you nothing, ride it by all means as a curio, but please try not to kill yourself.

nicklouse

greymarauder71 wrote:

STOP PRESS...

My original point is, with the fork geometry just the same, should I at least be safe from snapping the forks?
Thanks again.

no as what happens at sag or full compression.

brave ... not really. i could say other words.


and you are not the first and you will not be the last.

greymarauder71

Thank you, it has been a great way to get to grips with building up a bike. I would never have really looked into the geometry of bikes if I was not building a 69er, so it has been a good learning curve. I will indeed try not to die.