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PostPosted: Sun Dec 12, 2010 11:07 pm 
aka Leo Swayer
aka Leo Swayer
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I'm in the process of turning my Rocky Mountain Vertex hardtail into a 69er, more out of curiosity than anything else.
The frame is designed around an 80mm fork. I've attached some 445mm a/c P2's which is almost exactly the a/c of the 80mm fork that was on the bike originally.
If I now throw in a 29er front wheel it shouldn't upset the handling too much should it?
It will raise the front a further 30mmish which would be more or less the same as fitting a 100mm fork, or would it?
Anything i've overlooked? :D


Last edited by Grannygrinder on Mon Mar 07, 2011 11:04 am, edited 1 time in total.

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PostPosted: Mon Dec 13, 2010 12:05 am 
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Even if you can maintain a similar head tube to ground dimension as you had with the 26" wheel, the handling will still feel a bit different, as you end up with more trail with the 29" wheel. Ideally, you need forks with more offset to counteract this but I don't know of any that are short enough axle-crown for your purposes (you could get away with as little as 425mm) except the forks that Singular Cycles fit to the Hummingbird 69er, which I think are 445mm. I'll measure in the morning.
The extra offset also has the advantage of providing you with a little more toe clearance.

You'll probably end up slackening the frame angles by about 1.5° - 2°.

Try it anyway - in my experience you soon get used to any handling changes, which are no more extreme than you notice when changing from one bike to another.


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PostPosted: Mon Dec 13, 2010 10:23 am 
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An update on this -

The Hummingbird forks are 445mm a/c dimension.

Head angle is 70° and fork offset is 55mm (compared to a more usual of about 38mm) which with that head angle and the 29" wheel gives a resultant trail of 75mm.

This geometry just works and is an almost magical blend of low speed poise and agility and high speed stability and "plantedness" - and yet you can change line almost just by thinking - the best handling and most fun bike that I have ever ridden, just amazingly good. I wish I'd had something as capable as this 20 years ago, when I could have done it more justice :wink:

Anyway, Sam was selling these forks on their own a while back- you could always drop him a line- sam@singularcycles.com and see if he has any available.


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PostPosted: Mon Dec 13, 2010 11:50 am 
retrobike rider / Gold Trader
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I tried a 29 wheel on my Cotic when using a 440mm Pace RC31 fork. Handling was fine but i kept hitting my toes on the wheel when turning which buggers things up a bit.
Dare I say it was a bit fugly too. :?

Si


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PostPosted: Mon Dec 13, 2010 12:41 pm 
Posh Mark
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There was some advice from Cy (of Cotic) about this over on STW.

He said that the shorter the AtoC (clearance allowing) was the best way forward.

A AtoC of 425mm is closer to the geometry of an 80mm fork.

I have a Salsa Cromoto on my 63mm travel adjusted frame and that works fine with a 29'' wheel - maybe a bit tall but not so much to be a pain - about the same as it was with 100mm forks on.

I would have thought that a 440mm fork and a 29'' wheel would jack up the front end on your bike too much :?

May not though :wink:


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PostPosted: Mon Dec 13, 2010 5:48 pm 
retrobike rider / Gold Trader
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letmetalktomark wrote:
There was some advice from Cy (of Cotic) about this over on STW.

He said that the shorter the AtoC (clearance allowing) was the best way forward.

A AtoC of 425mm is closer to the geometry of an 80mm fork.

I have a Salsa Cromoto on my 63mm travel adjusted frame and that works fine with a 29'' wheel - maybe a bit tall but not so much to be a pain - about the same as it was with 100mm forks on.

I would have thought that a 440mm fork and a 29'' wheel would jack up the front end on your bike too much :?

May not though :wink:


440 is roughly about the same as a 100mm fork and the Soul is designed for a 120mm fork but works really well up to 140mm. The Bfe which shares the same geometry is designed around anything up to a 160mm fork so there is room for manouvre.
Depends on the frame, going slacker is no crime and all the rage with the kids. I'm running a 170mm fork on my Banshee (designed for 140-160mm).

I would worry about toe clearance more than head angle tbh.


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PostPosted: Mon Dec 13, 2010 7:18 pm 
aka Leo Swayer
aka Leo Swayer
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Thanks for the info everybody :wink:
This is how the bike is looking currently, with its 26" wheel and 445mm fork.
Chainstay is still sloping slightly down towards the BB, which I think means I haven't gone too far yet :D
I'll try it with the 29" front, if it turns into a horrible handling nightmare, i'll try and lay my hands on a slightly shorter fork.
I'm wondering if a bigger diametre(rolling circumference) rear tyre and a narrower smaller diametre front might help slightly?


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PostPosted: Mon Dec 13, 2010 7:26 pm 
aka Leo Swayer
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Andy :D how are you measuring the offset? If it was a motorbike I would use a level/straight edge to continue the angle of the headtube in a straightline down towards the front spindle. Then measure, on the horizontal, from the spindle centre to the level/straightedge which would give me the offset?
Does this sound right? :D
If its right the offset of the P2's is approx 45mm, which isn't a million miles away from ideal i think?
The bike rides nicely at the moment. If the 29" doesn't suit I may stick with how she is now and start saving for a Singular :D


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PostPosted: Mon Dec 13, 2010 8:44 pm 
Posh Mark
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Dr S wrote:
letmetalktomark wrote:
There was some advice from Cy (of Cotic) about this over on STW.

He said that the shorter the AtoC (clearance allowing) was the best way forward.

A AtoC of 425mm is closer to the geometry of an 80mm fork.

I have a Salsa Cromoto on my 63mm travel adjusted frame and that works fine with a 29'' wheel - maybe a bit tall but not so much to be a pain - about the same as it was with 100mm forks on.

I would have thought that a 440mm fork and a 29'' wheel would jack up the front end on your bike too much :?

May not though :wink:


440 is roughly about the same as a 100mm fork and the Soul is designed for a 120mm fork but works really well up to 140mm. The Bfe which shares the same geometry is designed around anything up to a 160mm fork so there is room for manouvre.
Depends on the frame, going slacker is no crime and all the rage with the kids. I'm running a 170mm fork on my Banshee (designed for 140-160mm).

I would worry about toe clearance more than head angle tbh.


Should have been clearer - the example did not pertain to one of his models.


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PostPosted: Mon Dec 13, 2010 8:46 pm 
Posh Mark
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Grannygrinder wrote:
Thanks for the info everybody :wink:
This is how the bike is looking currently, with its 26" wheel and 445mm fork.
Chainstay is still sloping slightly down towards the BB, which I think means I haven't gone too far yet :D
I'll try it with the 29" front, if it turns into a horrible handling nightmare, i'll try and lay my hands on a slightly shorter fork.
I'm wondering if a bigger diametre(rolling circumference) rear tyre and a narrower smaller diametre front might help slightly?


That looks steep as it is.

With a 29'' wheel it will be quite chopper like :?

A shorter fork would offset the difference made by the 29'' wheel.


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