120mm fork on a hardtail = catastrophic failure?

I just purchased a 120 mm suspension fork (thanks ericfoltz) for my single speed. I am usually a rigid SS guy but I wanted some squish for a change. The fork I had on the bike was 450mm axle to crown and the suspension fork is 499 axle to crown. I have calculated that this has changed my head angle from 71 degrees to 69 degrees. I rode it in the parking lot last night and it felt great. Steering slowed down a bit but I can handle that.

So I am wondering if this will be setting myself up for catastrophic failure? I keep thinking this might over stress the head tube and it will break. The frame is a GT aggressor and it has a huge gussett under the down tube where it connects to the head tube. Time for all of you bike techs to chime in and give me your thoughts.

 

Could be catastrophic......that bike was not designed to run a fork that big....it was probably designed to run a 80mm fork......if the rigid fork in the pic above is the stock fork, then it is probably corrected for 80mm of travel. Even though the HT/DT has a gussett on it, it may still fail.

Might be pushing it. Wouldn't want to bet my teeth or face on it.

 

drop the travel inside the fork.....

but I run a 160mm fork on a hardtail, so i may not be the best advice LOL

 

This is what my future plan was going to be. I am going to run it as is for a while tho before opening it up to do the service/travel reduction. Thanks for this tip... You going to be at rwanda?

 

I am a home mechanic bike hack...at best. I would probably run it.

As far as it being "raked out", I don't think the difference will be that great. The difference is just under two inches, but as soon as you are on the bike the sag will take a big chunk of that away.

I would also think that the suspension fork would also translate to less stress on the frame because it would be absorbing some of the shock and vibration that your rigid fork did not.

I say try it... I would. Remember though, this is coming from someone with much less expertise and experience than alot of folks that frequent this site.

 

Your logic and reasoning is identical to mine. I am a home mechanic bike hack at best as well. So far everything I have managed to "hack" has worked out wonderfully. I will be sure to run a slightly lower air pressure to increase the sag just a tad. Thanks for the response.

 

Yeah, so bump absorption isn't gonna help. The extra stresses created by long travel forks are leverage forces created by the longer lever of the forks. Think twisting, prying, etc.

I would be very cautious about putting a 120mm fork on an entry level xc hardtail frame designed around an 75mm fork. It could turn out to be fine, but I can guarantee that the headtube on that bike has not actually been tested to withstand the forces the longer fork will put on it.

If you are skeptical about whether 2" can make a big difference in leverage forces I suggest you do an experiment with a long handled wrench and see how much more power you get holding it 2" further away.

All of that said, frames are typically engineered with a large margin of safety. So, if you are basically planning on riding the same trails you did with 75mm of travel you may be fine. On the other hand, if you are planning on taking advantage of that extra travel by riding harder over rougher terrain, doing small drops, or dirt jumping, you are asking for frame failure.

 

I will be riding the same trails. Plus I am used to riding rigid so I pick good lines and ride pretty smooth. I dont pound my front end into anything. I guess we will see how it works out!

 

I'm the opposite... if it moves I TRY TO POUND MY FRONT END INTO IT.:lol:

 

Was the first fork 450mm axle to crown rigid? If you you need to calculate the sag of the suspension fork into the equation so determine how much of a difference it will really be. You should only be looking at a 20mm difference in actually riding position if first fork was rigid fork.

 

I just want to say for the record that I wouldn't do this on one of my bikes and I do think it is dangerous, even for the use you describe. "May" be fine is not equal to will be fine or even should be fine.

Could be a good excuse to invest in a full face helmet? :bang:

Oh, and I would make sure to inspect the welds on your headtube for any signs of cracking after each ride. If you get lucky it won't fail all at once if it goes.

 

Another opinion (and real-life experience) from a hack... I ran a 145mm Pike on a cheapo Performance Access hardtail frame for a while (it was a ss conversion too). I dialed it down to 90 mm for the climbs though as the floppy feeling up front drove me nuts with all the out of saddle bar-sawing - even with it locked out. For me, even at 90mm the change in geometry was annoying.

I liked it on the downhills though, but as silverbike mentioned - think in terms of levers. Though it never failed (even with some aggressive riding) I could never get it out of my mind that I was pushing the bike beyond what it was designed for... so i ditched it.

happy trails !

 

I wouldn't feel comfortable with that much change. Not sure your financial situation, but you can get some pretty nice, fairly cheap hardtail frames that are made to run longer forks.....such as an On One Inbred (steel) or Soul Cycles Hooligan (aluminum) frames for around $300 bucks. You could keep the fork and have a nicer frame and piece of mind......

 

I dont think your head angle changes, but your seat angle does

 

Update

Just in case anybody was wondering. I did run the fork at 5 inches and the bike rode completely fine. I still wanted to correct the situation though so I called Marzocchi's tech department. I talked to a guy named Gideon who told me exactly how to reduce the fork to 100mm. Those techs are so informative and helpful its rediculous. Everything went together very easily and now I can ride in peace.

 

Nice. I've found Marzocchi tech support to be awesome as well.

 

Yeah, I was gonna say I've been riding my hardtail with a Fox Vanilla 125 for a couple of years with no structural issues. The problem that I had is that 125mm was too much suspension for the intended geometry of the bike. It never quite handled right so I reduced it down to 100mm. Now the bike handles and feels alot better. It feels more balanced.

 

yea, now you just need to get your wheels trued:beer:

 

:-k Huh?? I don't get it. :-s Why do I need to get my wheels trued?? :?:

 

Hes talking about your avatar I believe.