Retrobike Forum Index

It is currently Fri Jul 20, 2018 4:28 am

* Login   * Register * Search  * FAQ



Post new topic Reply to topic  [ 14 posts ]  Go to page 1, 2  Next
Author Message
PostPosted: Sun Feb 25, 2018 12:55 am 
Retro Guru

Joined: Sun Jun 16, 2013 2:57 pm
Posts: 1627
Location: Porto / Plymouth
I've got an itch to build an E-stay bike again, this time with a longer fork like the Surly Big Dummy with a 3" tyre. The Big Dummy has an a to c length 425 mm, which doesn't seem all that much longer than a standard early 90s rigid fork (385mm?), but is it safe to fit a fork this long to a frame that's designed for a 385 fork? It won't rip the head tube off mid-ride, or something like that?


Top
 Profile  
 
PostPosted: Sun Feb 25, 2018 2:38 am 
GOLD | PoTM | Rider | rBOTM
GOLD | PoTM | Rider | rBOTM
User avatar

Joined: Tue Feb 26, 2013 6:26 pm
Posts: 22164
Location: 54 Festive Road Winchcombe GLOUCS Yarp...
Well.............

Image


Top
 Profile  
 
PostPosted: Sun Feb 25, 2018 7:41 am 
Retro Guru
User avatar

Joined: Thu Apr 30, 2009 9:47 pm
Posts: 848
Location: Newbury, Berkshire
Good Grief HM, that bike must have "interesting" handling characteristics - please tell me it's not yours :shock:


ultrazenith wrote:
I've got an itch to build an E-stay bike again, this time with a longer fork like the Surly Big Dummy with a 3" tyre. The Big Dummy has an a to c length 425 mm, which doesn't seem all that much longer than a standard early 90s rigid fork (385mm?), but is it safe to fit a fork this long to a frame that's designed for a 385 fork? It won't rip the head tube off mid-ride, or something like that?


Most of my knowledge regarding the Headtube angle & Bottom Bracket height affecting a bike's handling came from Anthony (currently M.I.A.) :(

With that in mind, you seem to be considering adding 40mm to the intended A-C measurement. Information from Anthony says," The Headtube angle reduces by by 0.6 degree for every cm longer A-C".

If my calculations are correct, fitting a Big Dummy fork would slacken your Headtube by 2.4 degrees, which surely would be most undesirable :? .

Anthony's somewhat complicated but quite interesting explanation on the subject can be found here - scroll down to his post Tue Feb 09, 2010 1:14 pm:

viewtopic.php?f=1&t=88543&hilit

Pip


Top
 Profile  
 
PostPosted: Sun Feb 25, 2018 7:59 am 
The Number Of The Beast
User avatar

Joined: Mon Feb 23, 2015 12:02 am
Posts: 666
Even with a frame buildt for 39.5cm.fork which was common as well, you will slackn your head angle around 1.8° or the other way from aprrox.70° down to modernish 68 - but at the same time you will also slacken the seat angle by the same ammount which together seems to be the worse part. And you not have in consideration yet the 3" wheel you might put up front...

So basically: it will not compromise the intergrety of your frame but I assume.you will never ride the bike in this setup :)


Top
 Profile  
 
PostPosted: Sun Feb 25, 2018 8:30 am 
Retro Guru

Joined: Sun Jun 16, 2013 2:57 pm
Posts: 1627
Location: Porto / Plymouth
Did the police ever catch up with you?

The History Man wrote:
Well.............

Image


Thanks all for your answers. About the 3" tyre, I'd be running it al lower pressure so I've assued tyre sag would balance out the 1" extra nominal height of the wheel. With a slacker seat tube, I'd need an inline seat post I guess. I guess it's a case of just building it up and trying it out.

Funnily enough, they differ a lot from the stuff I've read on 'modern' MTB forums, which usually say adding as little as an extra 20 mm to the fork length will rip the head tube off, which sounds like nonsense. I'm only a physicist so what do I know, but adding 40 mm to a fork length means about 10% extra leverage on the head tube welds, and I would be very surprised if any frame was engineered to such a fine tolerance that an extra 10% stress would snap or crack it.


Top
 Profile  
 
PostPosted: Sun Feb 25, 2018 9:28 am 
GOLD | PoTM | Rider | rBOTM
GOLD | PoTM | Rider | rBOTM
User avatar

Joined: Tue Feb 26, 2013 6:26 pm
Posts: 22164
Location: 54 Festive Road Winchcombe GLOUCS Yarp...
Of course it’s mine, who else would build such an innovative steed? It did did have interesting entertaining handling, dropping into turns, skittering a little at the front etc. If you want predictable normal handling I’d say don’t mess with the geometry. But who wants normal?

As for getting caught, it was a master of disguise and was raced before the frame was snapped up by a discerning buyer at mayhem last year.

Image

Yes I know. Sorry, I don’t do commissions.


Top
 Profile  
 
PostPosted: Sun Feb 25, 2018 10:06 am 
Gold Trader / PoTM Winner / RB Rider
Gold Trader / PoTM Winner / RB Rider
User avatar

Joined: Fri Jul 23, 2010 4:13 pm
Posts: 5081
Location: Royal Forest of Dean (Still looking for the Bear !)
The History Man wrote:
Of course it’s mine, who else would build such an innovative steed? It did did have interesting entertaining handling, dropping into turns, skittering a little at the front etc. If you want predictable normal handling I’d say don’t mess with the geometry. But who wants normal?

As for getting caught, it was a master of disguise and was raced before the frame was snapped up by a discerning buyer at mayhem last year.

Image

Yes I know. Sorry, I don’t do commissions.



Ahh the legendary Harlequin build that had random riders falling over themselves to kiss the front wheel in awe as it sat trackside.

Gotta say it certainly got a lot of looks form those who passed by


Top
 Profile  
 
PostPosted: Sun Feb 25, 2018 12:17 pm 
Old School Hero

Joined: Wed Nov 12, 2008 10:52 pm
Posts: 208
Location: manchester uk
Is it safe to do. Yes. Will it handle terribly. Yes but give it a whirl anyway for a laugh.


Top
 Profile  
 
PostPosted: Sun Feb 25, 2018 2:18 pm 
Retro Guru
User avatar

Joined: Tue Sep 21, 2010 8:08 pm
Posts: 2201
Location: East Lothian
Why not go 24" at the front - that should ease the frame's pain.


Top
 Profile  
 
 Post subject: Re:
PostPosted: Sun Feb 25, 2018 4:05 pm 
Old School Hero

Joined: Thu Aug 15, 2013 6:31 pm
Posts: 198
not sure its worth mentioning, but that rockhopper is a 98/99, sus corrected for 68-80mm. the direct drive fork they come with is either 410 or 420mm, cant remember which. not 390 though.

as others suggest, the handling would be a bit interesting, but youre not gonna snap anything unless you like doing 6 foot dropoffs. id say the main prob with using those surly forks is they will have a 1 1/8th steerer, and not too many 91 frames did.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 14 posts ]  Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 76 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  

About Us

Follow Retrobike

Other cool stuff

All content © 2005-2015 Retrobike unless otherwise stated.
Cookies Policy.
bikedeals - the best bike deals in one place
FatCOGS - Fat Chance Owner's Group

Powered by phpBB® Forum Software © phpBB Group