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PostPosted: Fri Mar 02, 2012 12:47 pm 
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Joined: Wed Feb 01, 2012 12:33 pm
Posts: 453
Nice, going full moto there! Rotec used a lot of motox aspects in the original rigs- they were the ones to get moto style- zero reach stems going with long top tubes to accomodate.
I think this one will really stand out amongst current DH bikes !
Very nice work so far :D


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PostPosted: Fri Mar 02, 2012 9:09 pm 
Dirt Disciple

Joined: Fri Dec 09, 2011 12:01 am
Posts: 61
Location: USA
As 13 mentioned, the Rotec was directly influenced by MX bikes and as far as retro DH designs go, could be the closest you will find to a modern DH bike. The head angle is slacker and BB lower (when using the upper shock mount) than DH bikes back then. But the angles are currently relevant. The direct mount bar clamps were available in 25 and 35mm offsets iirc. 75-90mm was typical then, but 38-50 is were things are at now. Also, if you distill the latest suspension designs you will see two popular options: VPP designs and single pivot with rising rate linkage. The Rotec uses the latter. The only thing outdated is the long chainstay length. That is what makes this bike for tall riders only. Short guys just wont be able to float the front wheel and get enough weight transfer fore and aft.

Anyway, those direct mount bar clamps have been unabtanium after months of searching. So I've come up with my own version. I picked up a set of Renthal moto bar clamps. The two hurdles involved with using them are:
Moto oversize bars are 1-1/8". Bicycle are 1-1/4"
Gigantic mounting bolts for the triple clamp

I solved the first by machining the clamps to 1-1/4". There was just enough material.
The other issue took some thinking. I ended up cutting the heads of the gigantic stock bolts and turning them into nuts by boring and tapping them. Now I can run 8mm bolts up from underneath, just like White Brothers intended.

Here is a pic of the modified parts (note giant bolt with head cut off):

Image


And a pic of the assembly:

Image

The 8mm bar clamp bolts are overkill on a bicycle, so I have ordered some titanium replacements. Most of the hardware on this bike will be titanium for that 'Works' look.


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PostPosted: Fri Mar 02, 2012 10:48 pm 
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Joined: Thu Sep 04, 2008 12:23 pm
Posts: 1224
Location: muddy, grimy yorkshire :D
this is gonna be ace! loving the attention to detail


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PostPosted: Sat Mar 03, 2012 1:21 pm 
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Joined: Tue Nov 04, 2008 12:00 pm
Posts: 802
Location: YORKSHIRE!!
mmmmmmmmmmmm!!!!

loving the Yammy paintjob!


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PostPosted: Sat Mar 10, 2012 2:05 am 
Dirt Disciple

Joined: Fri Dec 09, 2011 12:01 am
Posts: 61
Location: USA
The frame is back at my shop and making some progress.

My original frame suffered an ovalized head tube. In response, I have sourced a Chris King Steelset. These are made from stainless steel with extended length skirts to reinforce the head tube:
Image


The needle bearings have been pressed into the main frame in lieu of the original brass bushings:
Image


The rising rate linkage ran on problematic brass bushings. They were not well sealed and would require constant disassembly and greasing. The friction was so intense that I had to find a high temp synthetic grease to survive the heat! Most Rotec owners neglected this regular service and the drag link pins would seize in the bushings. Then the drag links would rotate around the pins, wearing out their mounting holes and eventually breaking. For reasons I cannot fathom, Rotec beefed up the pins and drag links in the MKII bikes. Which of course did nothing to solve the primary issue of the ineffective bushing system.

So like my original race bike, I have converted all the bushings to needle bearings which makes for a dramatic improvement in traction because the wheel can track the ground without all the friction hindering its movement. The bearings have an internal lip seal, and I have added an external o-ring seal for additional protection.

The old pins have been replaced with hardened chrome steel custom machined races and titanium bolts.

Note the use of shoulder bolts for maximum shear strength. Each drag link pin will run on four sets of needle bearings. The frame has three sets, and the lower shock eye will have a set of needle bearings also. PLUUUSH!
The old hardware on the left, new stuff on the right:
Image


Here is a pic of the rocker with the new bearing race/shaft and Ti hardware. The little bolts at the bottom are for plugging the extra holes in the main frame when the seat mast is located in the rearward setting:
Image


The first round of anodizing was supposed to be done last week, then this week, and now they say next week... The swingarm was in that batch, so hopefully next week I can assemble the frame.

I've got a little surprise in the works for the rear shock. It's the last piece to be fabricated, and should be done in a week or two. After that it's all the easy stuff - assembly and sourcing the last few components.

Oh, I just remembered the missing Mr. Dirt chain guide.... Probably going to have to fabricate something there...


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PostPosted: Sat Mar 10, 2012 11:00 am 
retrobike rider / Gold Trader
retrobike rider / Gold Trader
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Joined: Mon Jun 22, 2009 11:09 am
Posts: 9399
Location: Devon
This is the best renovation/build I have seen for so long. Absolutely loving it.


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PostPosted: Sat Mar 10, 2012 1:34 pm 
Retro Guru
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Joined: Wed Feb 01, 2012 12:33 pm
Posts: 453
Velomech wrote:
I've got a little surprise in the works for the rear shock. It's the last piece to be fabricated, and should be done in a week or two.


MMMMM..............I'm guessing moto style......maybe Ohlins ski-do??
http://www.pinkbike.com/photo/33361/


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PostPosted: Thu Mar 15, 2012 10:09 pm 
Dirt Disciple

Joined: Fri Dec 09, 2011 12:01 am
Posts: 61
Location: USA
13 wrote:
MMMMM..............I'm guessing moto style......maybe Ohlins ski-do??
http://www.pinkbike.com/photo/33361/


Close, but not that crazy.


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PostPosted: Thu Mar 15, 2012 10:13 pm 
Dirt Disciple

Joined: Fri Dec 09, 2011 12:01 am
Posts: 61
Location: USA
Update:

The frame is assembled and generally finished.

Shot of the Linkage needle bearings:
Image

Completed frame:
Image


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PostPosted: Thu Mar 15, 2012 10:18 pm 
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Joined: Mon Apr 16, 2007 12:37 pm
Posts: 805
Location: Kenilworth
Fantastic, very impressed!


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