Suspension forks: autopsy and rating

[rickpaulos]

I've been meaning to invent a scale for rating suspension mountain bike forks.  It will mostly be cheap bso forks as that's what have an enaccess to via the local bike coop.

rating system of 1 to 10 based on construction, shock ability, durability, service ability, yada yada yada.

Riders don't have x-ray vision to see what is in the forks so I'll be taking these apart to show whats inside.

Brand: Zoom
model:  Air Ride Comfort Shock
description: welded - steel internal coil springs - no dampening device.
rating: ZERO of 1 to 10.

This first fork gets a ZERO rating due to just being a dangerous final product.  Worthy of a CPSC recall.
It would get a 2 rating if the bushings weren't undersized.

This fork came off a 2006 Huffy cruiser that had nearly no miles.  The molding flashing nubs were still on the tires.  When the front brake was touched, the fork snapped back hard causing the front wheel to lock up. This typically sends a rider flying over the handlebars.  Lucky for me I discovered this messed up fork just walking the bike and using the brakes.  Whew.  The last header I took was on a Mongoose with a really bad headset and brakes.  One more type of flying I don't care for.  I installed another fork from the parts bin on the Huffy and handlebar stem due to the replacement being a .833 fork.

For marketing reasons, most mountain bike suspension forks are called shock forks even though they have no shock absorption technology in them.  Most low cost shock forks get their dampening from friction, later to become stick-tion due to lack of any maintenance or service.  Many finally become rigid forks due to wear and rust.


A small flat blade screw driver is good for removing the plastic plugs to get access into the fork.  Note that many cheap forks have plastic plugs shaped to look like they are adjustments for the internals found in high end forks.  Fakes designed to fool buyers in to thinking they are getting a better fork.

The preferred tool for dismantling many cheap forks is a 3/8" square drive 5mm hex socket on a long 3/8" extension.  The larger socket head self aligns the 5mm bit in to the bolt heads way down in the fork stanchion tubes.  You can buy very long 5mm or 6 mm straight hex shafts then use a 5 or 6 mm wrench to turn those. A plain 5 mm hex shaft is too thin and you have to hunt around in the dark to get that wrench into the socket head.  I've used a 6 mm okay but a 5mm may twist and bent.


pluses: Plenty of grease. Nice chrome on the upper tubes.
minuses: welded crown
performance: not going to risk a ride.
weight: average

the bridge is sheet metal welded to the lower legs.  Rather weak and flexy.  I could easily twist it once off the upper legs. BTY, all suspension forks need a bridge to tie the two legs together to keep the front wheel from flopping side to side.

Small rubber cushions under the bolt heads to cut down on the banging when maxing out the fork.

Rubber cushions at both ends of the springs.

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[JeffC]

Most parents have no clue how poorly made those BSO's are and put their trust in places like Walmart, Target and Dick's which is sorely misplaced trust. In defense of parents in by gone days many had no clue about the workings of bikes they bought for their kids but they had names like Schwinn, Raleigh, Peugeot etc... that had a record of building bikes that were at least safe to ride and there was somewhere to take one of those bikes to get repaired or checked out for problems. Most bike shops today won't go near a big box bike because once they begin to deteriorate they will either cost more to fix than it would to buy a new bike or they can't be repaired.

Those metal pieces that keep the forks from twisting on motorcycles we called fork braces and if you rode in the dirt it was a necessity.

[rickpaulos]

Brand: Zoom
model:  PosiTrack Suspension System
description: welded - internal elastomers - no dampening device.
dated 1996
rating: 2 on a scale of 1 to 10.

Welded steel crown. *
1" threaded steerer.
Two tiny elastomers in each leg.
One side was dry, no lube. The other must have had water in it.  Rust dust poured out. You can see the rust stuck to the elastomers.  The fork barely moved as it was.  I suppose it could have been cleaned and reassembled but not this time.  I was able to extract one of the plastic guides. Most are really impossible to get out. This has a single long guide at the top.  Better forks will have 2 small guides, one at the top and one at the bottom. Less friction/sticktion and less binding up.

A note on elastomers.  All are made of some secret factory formula and they all age differently.  These really have held up, just okay. A bit stiff.  I've seen others that melted, flowed to the bottom of the legs and re-congealed.  I've seen some that looked like they were made of glued cork chips and the glue failed, letting the cork bits fall to the bottom. Another was okay until I left it parked in the sun and it literally melted out. Drip, drip, drip.

This fork has double seals that should have kept the water out. 
The aluminum bridge is bolted on. Two bolts on each side, one of the bolts doubles as the canti-brake mount.
I found a new home for the bridge, next to the horse shoe that has been on my house for 38 to 95 years.

* one of the more common CPSC recalls was for welded crown mountain bike forks.  I think that was one reason the chinese made bikes started getting assembly dates on the sticker on the frame. That way they could recall just a weeks worth of production instead of an entire year.  My guess is the badly welded forks were from new workers.


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[rickpaulos]

more photos

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[rickpaulos]

Brand: Manitou
model:  Spyder
description: swaged construction - steel steerer, aluminum crown, steel inner legs, aluminum lower leg - elastomers & steel spring - no dampening device.
rating: 4 on a scale of 1 to 10.

Long elastomers and a spring with a soft plastic preload adjuster.
Plenty of grease. So much so the elastomers were encased in it which probably kept oxygen away from them so they held up.

Reassembled and installed on a bike.

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[rickpaulos]

This next fork is going to be double posted.  it sure qualifies for the Bad Mechanics thread but it also shows the internals for this thread.



Brand: Manitou
model:  2
description:
rating: 5 on a scale of 1 to 10.

Long elastomers with 2 preload adjusters on the bottom end, 2 rebound adjustments on the top. Same hardware in both legs.

stock photo
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I found the exploded diagram for it.
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The elastomers are labeled as compression rubber.

This particular fork had been dismantled before. Surely to replace the rotted elastomers. However the repair job was McGuyvered with junk found in a garden shed somewhere.
Lots of red stuff included as maybe a grease.  Looks more like dehydrated tomato paste.

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[rickpaulos]

Once I removed the red stuff, I found the space of the elastomers (1 1/2" + 1 1/2" + 3/4" ) plastic spacers and a couple sections of wooden chair legs.  My guess the amount of travel was about 1/4" vs about 1 7/8" for stock.

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I ordered a rebuild kit for this fork as it's worth restoring. The crown, and legs are high quality Easton aluminum.  Suspension fork parts on line store carries repair kits for some models of forks.  Mostly those with elastomers.

the model "2" seems to correspond to the year.  1992.  During the heyday of mountain bikes, the fork companies were in a cut throat competition so each came out with new models ever year.  Rock Shox, Manitou, Fox, etc.

[JeffC]

Do the repair kits include new wooden spacers?. The grease looks like it bled out all the oil and left the wax behind.

[rickpaulos]

A video featuring the Manitou elastomer fork.  Featuring a pretty cool bike from 1993 with twin forks !!?

[rickpaulos]

Brand: Kawasaki
model:  no model listed on the fork
description: welded - steel internal coil springs - no dampening device.
rating: 2 of 1 to 10.

weight 4.42 pounds. 


From a Kawasaki KDX126fs bso mountain bike. 
Chinese made licensed product (Cycle Source Group 2006) from the motorcycle branch of a huge Japanese conglomerate.
One of those "dirt floor factory" bikes. The steel used has numerous surface flaws you get when manufacturing in a filthy facility.

I give it a 2 rating because the top of the upper legs are flared out a bit making it very easy to work on. 
basic coil spring with 3 rubber bits to quiet it up.  The 3rd is in the bottom of the bottom legs. They didn't want to come out easy so I left them in place.  No loud slamming noises at the ends of range of motion. Steel welded crown. 

Tools.
Here are the basic tools used for these forks.

Craftsman set of socket-hex-bits.  Bought these 20 years ago or more.  The big advantage of them is the large 3/8 drive socket head self-centers in the bottom of the fork tubes making it easy to get the hex bit into the bolt head.  The down side is the hex bits will get stuck in the bolt heads so when you pull the socket out, the bit stays behind.  You have to counter torque to break it free.  There may be other brands or models of 3/8" drive socket hex bits that won't come apart so easy.

3/8 drive ratchet wrench.

very long 3/8 extension.  I like this one because it has some knurling at the end so it's easier to turn with greasy hands.

I also have an extra long 6 mm hex shaft for those forks that are too narrow at the top for the socket-hex-bits to fit inside. Also useful for pushing stuff around inside the tubes.  Some forks use 6 mm bolts, some use 5 mm, and rarely you get a regular hex head bolt.  I've seen some that were so rusty (full of water), you can't really tell what size was used. Most forks don't hold water.

An extra long 5mm hex shaft is probably going to twist too easy.