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PostPosted: Sat Mar 17, 2012 2:04 am 
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All makes sense, i do wonder then as you do, why they havent gone mainstream for mtb. They did use them in the pivot of one of my old i drives. Maybe its because theyre quite specific sizes for mtb and hence a bit harder/more expensive to get hold of than ever replaceable bushes?
Maybe theyre not as effective in smaller sizes?


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PostPosted: Sat Mar 17, 2012 3:23 am 
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Yea I had one of the first gen DHi's too. GT used needle because roller bearings would have had to have been massive for the size and no doubt weighed a ton.
They ran super smooth but took a lot of maintianing. I think smaller bearings are sealed better?


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PostPosted: Sat Mar 17, 2012 5:45 pm 
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Joined: Fri Dec 09, 2011 12:01 am
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Location: USA
I can't say definitively why manufacturers build and spec the way they do, but I can say this:

There are a multitude of factors at play when designing any product. Most of which are far removed from the specific performance of the product.

Cost
Historical precedence
Brand image
Market demands
Etcetera...

Needle bearings in the shock eye have a limited range of applications, cost more (only a couple dollars more, but multiply that by 5000+ shocks...) require tighter tolerances and give best results if greased on occasion.

A single pivot swingarm bike won't benefit much with needle bearings in the shock eye's because there is only a few degrees of rotation at the shock mount. Where it makes a difference is in linkage bikes with short, rising rate links rotating through many degrees of rotation. And you only need the needle bearing on the end of the shock attached to the linkage. The frame mount end might as well run a DU bushing (technically called a DU bearing, but layman's terminology is "bushing" so I'll go with that)

So you have a better engineered solution that costs more, requires maintenance (while DU bushes are considered disposable) has to be used in the correct application to see any benefits, has not been used previously in the industry on a large scale, and has little market demand.

I would go on to say that because suspension bikes, especially downhill bikes have moved through the wildly experimental early phase of development and have moved into a phase of refinement, we will start seeing better engineered bearing assemblies. What needs to happen is the manufacturers should be able to specify what type of bearing the shock is assembled with on each end of the shock when spec.ing OEM fitments.

Good chat.


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PostPosted: Sun Mar 18, 2012 3:29 am 
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Wow, I love this attention to detail and you clearly have some skill :D This is shaping up to be a beautiful build and I can't wait to see it done.

I'm trying to do a similar thing with the bearings on my Lobo, the plastic stuff really annoys me.
Can I ask what tolerances you used with the rollers?
I'm unsure how to ensure a good strong press fit without causing compression of the bearing. From what I've read online I don't think the shaft has to be be super tight, just snug but the outer needs to stay put.


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PostPosted: Sun Mar 18, 2012 3:59 am 
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Loctite 641 or similar product. should keep it from moving.


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PostPosted: Tue Mar 20, 2012 9:57 pm 
Dirt Disciple

Joined: Fri Dec 09, 2011 12:01 am
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Location: USA
Magsy wrote:
Wow, I love this attention to detail and you clearly have some skill :D This is shaping up to be a beautiful build and I can't wait to see it done.

I'm trying to do a similar thing with the bearings on my Lobo, the plastic stuff really annoys me.
Can I ask what tolerances you used with the rollers?
I'm unsure how to ensure a good strong press fit without causing compression of the bearing. From what I've read online I don't think the shaft has to be be super tight, just snug but the outer needs to stay put.


Shoot for a light press fit (a few thousandths) for two reasons:
The races are thin and too tight of a fit will throw off the bearing tolerences.
Needle bearings carry no lateral loads, so there are no forces to push them out of the bore.

Keep in mind you will need a second thrust bearing for each needle bearing you use on your Lobo. The rotec linkage is independent of the swingarm loads, so it does not require thrust bearings.


Last edited by Velomech on Wed Mar 21, 2012 12:35 am, edited 1 time in total.

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PostPosted: Tue Mar 20, 2012 10:15 pm 
Dirt Disciple

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

After a few hours of looking through catalogs and images, I ordered a MRP chain guide. The BB mount MRP Mini G2 SL looked like it had metal in the right places so I could drill some holes to index the guide with the existing Rotec guide mounting locations.

Here is a shot of the guide installed. I was able to use two of the three swingarm guide mounting locations along with the BB mount to create a sturdy, slip free install:
Image

Keep in mind the Rotec was ahead of its time with a direct mount chain guide design that predates the ISCG standard by several years.

Here is a shot of the guide, cranks and chainring installed:
Image

Another interesting detail is the BB shell width of 80mm. The 83mm BB width standard did not exist back then, but is THE standard for DH bikes now. I was able to run modern cranks with nothing more than the addition of a 2.5mm spacer on the non-drive side.

Considering all the modern details found on these bikes, the Rotec guys may have had a time machine hidden in their facility. I remember back in the nineties and all the negative comments I heard about how Rotec bikes had funky geometry and were too radical. The only real problem with the bikes was that it was released fifteen years too soon.


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PostPosted: Wed Mar 21, 2012 12:25 am 
Gold Trader / MacRetro rider
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Cant wait to see a rolling chassis pic 8)

Those Renthal chainrings look fantastic, shame it gets hidden under a chain :(


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PostPosted: Wed Mar 21, 2012 9:11 am 
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An amazing looking ride, can't wait to see the end result.

Are you taking it as far as ever getting the old Fox RC tuned by stripped cleaned tuned and rebuilt or is it new anyways ?


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PostPosted: Wed Mar 21, 2012 10:37 am 
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Velomech wrote:

Considering all the modern details found on these bikes, the Rotec guys may have had a time machine hidden in their facility. I remember back in the nineties and all the negative comments I heard about how Rotec bikes had funky geometry and were too radical. The only real problem with the bikes was that it was released fifteen years too soon.


I remember the most negitive aspect you would hear about was the way they rode during sprinting or fast pedaling.
The geometry was radical for the time but did get noticed by the big teams, I know cannondale started making their DH bikes longer and running zero reach stems. Myles and Missy ran that style on the falcrums maybe they took a spin on Eric Carters Rotec bitd :wink:
I think a good modern shock will make this bike.
There are a small amount of rear suspension designs like this that were really only let down by shock technology of the time.
If only Rotec lent their time machine to Fox :lol:


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