Over-forking

The 94-95 Mongoose Team SX frames were excellent rides. Carbon tubes glued to aluminum lugs in what turned out to be a perfect design for me. The 1995 black one lasted me as my only MTB from the end of my racing career until mid 2012. The truth is, I panicked.

Crack!

I saw this crack and figured the frame was gone. That the bonding of the seat tube to the bottom bracket had failed, or was about to fail. Even though my riding was intermittent at that point….”OMG I simply cannot not own a mountain bike!” took over my mind.

I ordered up a new bike, stripped the Mongoose down and tossed it into the attic.

During COVID, I put enough parts on the Mongoose to get it moving again. I did this so that I could make the college kid, forced home by Covid, ride with me. It survived a shake down ride under me. A ride that turned out to be a RIDE just because it felt so great to ride it again. Sure the old Mag21 forks barely moved, and the v-brakes sucked compared with discs, but I had a good time. Made the kid ride a few times, so job well done.

Later, the Mongoose was lent to a friend for a few rides and it led to him buying his own bike and becoming a regular riding friend. Job also well done.

Much later, this frame hosted an ebikeling conversion for another child. For reasons. I thought for sure it would eventually die under such abuse. The Mag21 failed, but the frame kept on holding together. I put a cheap coil spring fork on it and it went back into ebike service. After about a year the child stripped off the ebike components and the Mongoose returned to idle.

Another crack

Somewhere along the line it picked up another crack at the top tube junction with the head tube. And it looks like maybe another at the top of the seat tube.

Third is the charm?

The bike did not, however, fall apart.

So, because why have a useless bike when you could have a working bike, I fixed it up. It runs. Gears shift, brakes brake. Flat pedals, my test riser bar and street tires continued from the ebike era. At some point I had put on 9sp grip shifters because they were the cheapest option, and some Performance house brand brakes I had lying around.

It runs pretty well, just tooling around the block. But….

It lives!

It’s “over forked”.

The “sx” part of the frame was bragging, at the time, that the frame was designed for suspension forks. Of the day. Which had about 50 mm of travel. The shortest coil fork I could find is a 100mm travel unit. It is a lot longer than the Mag21.

Over forking

Surprise surprise, the front wheel now flops when you turn it away from straight ahead and overall it doesn’t steer as quickly. This is exactly what is predicted from putting on a longer fork and thereby slackening the head angle. I’m not sure about the difference in rake / offset and trail between the two but that could also be contributing.

This brings me to a brand enthusiast Fb group/page and the repeated queries about upgrading the bikes. As mentioned, Polygon has pretty good deals on what I would call the enthusiast entry-level bike. It is not unexpected that there would be a perceived need by the owners to improve their ride. The “D” models come with 100 (27.5” wheels) or 120 (29” wheels) mm travel forks. The “T” models come with 140mm forks. The Fb page has a lot of folks asking about the wisdom of “upgrading” to a longer fork.

Would adding a 140 or 150 mm fork merely change the 67 deg head angle of the D model frame to the 65/65.5 (depending on size) angle of the T model? I’m not sure, but there may be other adjustment to the frame design. What would putting a 170 mm travel fork do to the T model? My dual suspension bike comes in a 100 mm travel version and a 120 mm travel version. The latter has a one degree slackening of the head and seat angles (68/74 deg) relative to the shorter travel one. So if we guesstimate a degree for each 20 mm of extra fork travel, the Polygon fans are in the 2-2.5 degree range for their proposed overforking mods.

It’s going to mess up the handling compared to the original specification fork length. That’s for sure. The front wheel will flop around. It will fall off line when moving straight ahead. Slower speed travel will be impaired.

It will also put the front and rear suspension travel out of the designed synchrony as well. This seems less severe, after all the front and rear travel on a hardtail is not balanced!

Lord knows I do not object to the concept of upgrading a bicycle. Especially when you start with lesser components and end up with lighter, more durable or more functional components. I’ve spent many a dollar on replacing shifters, brakes, brake levers, derailleurs, tires, hubs, rims, handlebars, seat bolts and cranks with “better” ones. And I’ve changed things out to adjust the fit- stems, saddles and seat posts, for the most part. Mostly for mountain bikes.

Changing the geometry of the bike with a different fork is not something to be done lightly. IMO. It has a tremendous effect on how the bike steers, holds a line, balances on steep climbs, etc.

If you are going to over fork a bike, you had better do your homework on what it will do to the geometry.

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