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I'm rebuilding an AntiGravity Fork and converting it to threadless for my 'Blast From the Past' bike (GT Bravado frame, U-Brake, Bullseye hubs and cranks, 21 speed). I really like these forks and I'd like to bring them back to life.

The elastomers are shot. Some are falling apart, others are crushed and don't expand fully, etc. Given that the company is out of business and the chance of locating brand new AntiGravity elastomers is.... not good, I'm looking at what my options are.

AntiGravity Elastomer Fork: Picture

Option A: Wings-Suspension makes killer kits to convert Manitou/RockShox elastomer forks to springs. I emailed them with pictures and details to see if his springs would be compatible, but that's VERY doubtful. This would be my Option A if it worked.

Wings Replacement Springs: Link | Picture

Option B: Round Tubing from McMasters-Carr. I have been trying to just get something to replace them with, at this point. The main suspension uses 4x 1.25" OD 1/4" ID elastomers, and the rebound is controlled by 6x 3/4" OD 3/16" ID elastomers. McMaster-Carr seems to have close to what I'm looking for, with multiple choices:

Main Elastomers: Neoprene Spring Rubber with 75A Durometer (hard) or Extreme Temperature Silicone Rubber in 60A (medium-hard).

Rebound Elastomers: Only in 1/4" ID (slightly bigger than stock). They have Neoprene Spring Rubber in 75A (hard), Oil Resistant Buna-N Rubber in 75A (hard), and Extreme Temperature Silicone Rubber in 60A (medium-hard).

Option C: ??? (short of buying a new fork)

An answer can either be a link to better replacement material, or which types of the options above are best for a 180-190 lb rider. It would also be really helpful if someone could find a data sheet for this fork showing what the different original durometers were.

In addition, has anyone ever done this before? should I cut them into 4x1.25" and 6x1" pieces, or leave them as one long tube since I'm not swapping them out individually (one of the 'perks' when these were sold)? Does anyone know how to estimate desired durometer / fork force for a given rider weight or desired stiffness? I'd rather be on the stiff side, but I have no idea what 75A or 60A mean in this context - they could be so squishy I bottom them out, or so hard I might as well get a rigid fork.

AntiGravity Fork Internals: Picture

McMasters Carr Round Tubing Link

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I can see a trend to include every possible answers in your questions ;oP –  heltonbiker Apr 7 '12 at 3:58
    
They say you should show research in your questions... I just take that like a fundamentalist does the parts of the bible they like. There are questions in there somewhere. –  Ehryk Apr 7 '12 at 6:14
    
And hey - I left Option C open! –  Ehryk Apr 7 '12 at 6:16

3 Answers 3

I had an elastomer suspension a lot of time ago, and did my own maintenance on the elastomer compartment. I would not take a fork like this and put springs inside. Bike suspension elastomers dampen the motion of the suspension in two ways:

  1. They expand laterally and generate friction against the inner walls of the suspension;
  2. They have a natural visco-elastic mechanical behaviour, that doesn't let the suspension "spring back" too fast.

With springs only (and no dedicated way to control the rebound), the suspension would become too "wild", I'd say.

I think you should take any elastomer kit that fits inside your suspension, with enough lateral clearance, and use it, no fear. Micro-cellular urethane (MCU) was the topmost material used in bike suspension elastomer (before becoming almost obsolete).

It is important, too, that you put them inside the fork with lots of grease, and specifically LITHIUM-FREE GREASE, so that they don't chemically react with one other. Most probably, there are specific suspension greases in bike shops.

Hope this helps

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The correct term is "viscoelastic". (I used to test rubbers for the Air Force.) –  Daniel R Hicks Apr 8 '12 at 2:33
    
Wow, I wrote viscoplastic! My mistake, I actually meant viscoelastic... –  heltonbiker Apr 8 '12 at 3:20
    
Where can I buy Micro-cellular urethane? If I find it in tubing, should I install it in 6" and 5" lengths, or cut it into 6x 1" and 4x 1.25" lengths? –  Ehryk Apr 8 '12 at 6:08
    
I think you could/should take the measure from the debris of your removed elastomers. Other than that, the elastomers should completely fill the empty inner height of the fully extended suspension, so as not to leave any empty vertical space. Another thing to do would be to use smaller sections of tubing stacked over one another, so you could tune the preload by arranging different combination of sections with different hardness. Don't be afraid of experimenting, there would be no harm to the fork. (although a single tube would be better than three or four smaller sections) –  heltonbiker Apr 9 '12 at 2:08
up vote 1 down vote accepted

I went down to a LBS in Minneapolis, The Shockspital, and had a long conversation with a guy that had lots of spare parts, elastomers, springs, and lots of wisdom. He's done many elastomer-spring conversions, and here's how I'm setting it up:

The bottom larger elastomers that handled the load are being replaced with 5" of spring, with an elastomer inside of them providing a small amount of rebound damping by resisting the rebound of the spring, and some compression damping after 40% of the spring is compressed (the yellow elastomers on the rods in the picture). It's the long spring in the picture which will be cut and ground into two 5" segments.

The top smaller elastomers that provide the rebound force and compression damping are being replaced with an all-weather elastomer, all in once 6" piece (the purple elastomers on the rods at the top).

Then I ordered Prothane polyurethane/urethane safe grease to coat the elastomers, rods, and fork internals with to prevent stiction. All in all, this should be a very responsive fork with some interesting characteristics, that will be a bit on the stiffer side of things for my weight.

For what it's worth, he's tried many times with the McMasters-Carr tubing, and it REALLY sucks. It's far too soft to be of any use in forks. It's made of a completely different material (I tried drilling out a piece of it, which didn't work at all and left black slime in my hand). Don't bother with it. I may have been able to use it in this case, because I'm adding a spring and have elastomers resisting compression in two places, but it's not worth the hassle and doesn't have the same type of characteristics of MCU.

Rebuilding AntiGravity Fork

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Go to Manitou website and there are two vendors that have the exact fit elastimers 53 for a kit and 10 for the extended kit. That it.

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I have AntiGravity forks, not Manitou. –  Ehryk Aug 30 '13 at 20:54

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