Need Fork advice for older bike...

There are a few things to keep in mind seeing your bike is currently rigid.

First off, the geometry of the frame was designed with that rigid fork in mind, so pretty much any suspension fork is going to raise the front-end of your bike significantly. Best case this will reduce the head-tube angle and slow the steering down, worst case this may raise your front end enough to induce some "wheel flop" when you turn your front end (think of how a fully exended chopper motor bike steers).

Second off, the structural design of your frame will likely be under-qualified to handle the extra stresses of a suspension fork, especially the weld between the head tube and the down tube.

To get a better ride, you might look at going with tubeless wheel system, there are conversion kits to turn inner tube wheels into tubeless (Stan's). Tubeless wheels let you run lower tire pressures, gets you additional traction and increased bump-absorbtion from your tires. All done without changing the geometry or adding undo stress to the frame.

Cheers,
Juba

Juba said:

There are a few things to keep in mind seeing your bike is currently rigid.

First off, the geometry of the frame was designed with that rigid fork in mind, so pretty much any suspension fork is going to raise the front-end of your bike significantly. Best case this will reduce the head-tube angle and slow the steering down, worst case this may raise your front end enough to induce some "wheel flop" when you turn your front end (think of how a fully exended chopper motor bike steers).

Second off, the structural design of your frame will likely be under-qualified to handle the extra stresses of a suspension fork, especially the weld between the head tube and the down tube.

To get a better ride, you might look at going with tubeless wheel system, there are conversion kits to turn inner tube wheels into tubeless (Stan's). Tubeless wheels let you run lower tire pressures, gets you additional traction and increased bump-absorbtion from your tires. All done without changing the geometry or adding undo stress to the frame.

Cheers,
Juba

Juba is right.
A new bike that's pretty good witha fork can be had for under $400. A decent short travel (<80mm) fork would cost half of that.

Upgrade the bike.. save old trusty in case you break some parts.
fwiw,
MK

mark_kendrick said:

Juba is right.
A new bike that's pretty good witha fork can be had for under $400. A decent short travel (<80mm) fork would cost half of that.

Upgrade the bike.. save old trusty in case you break some parts.
fwiw,
MK

Good Call. I just helped a buddy pick out a bike. He wanted the best he could get for around $350 or less. I looked at GT, Giant, Specialized, Trek, and Fuji. It came down between the Giant Rincon and the Specialized HardRock Sport. I told him to go the Specialized Hardrock Sport. It was $329 at the LBS, and had double wall rims, rapidfire, decent fork with preload adjustment, and a nice alum. frame. A good suspension fork will cost you $250 plus, let alone you'd need the rest of the components to go with it (threadless stem, aheadset, possible a front brake upgrade, labor to put it on , etc). I say clean the trusty bike up, have a quick little ceremony to retire it, and go with the Giant Rincon or the Specialized HardRock Sport...

just my $.02

Juba said:

First off, the geometry of the frame was designed with that rigid fork in mind, so pretty much any suspension fork is going to raise the front-end of your bike significantly.

I've stuck a Marzocchi Comp ETA on an old noname frame that either was designed for a rigid or an old short-travel fork. It looks kinda funky, but the handling isn't an issue. There's no bigger difference between my bike and other bikes than it is between other bikes in general.

Juba said:

Second off, the structural design of your frame will likely be under-qualified to handle the extra stresses of a suspension fork, especially the weld between the head tube and the down tube.

I'm not certain what will be most influential here. Sure, a longer fork will add some stress - but the suspension will also reduce some other stresses. It might all come out even or better.

Then you also have to consider safety margins. An average biker has ample margins with an average bike. As long as he don't start doing high drop-offs or anything else extraordinary the frame should be fully capable of handling any extra stress related to a longer fork.

dabac said:

I've stuck a Marzocchi Comp ETA on an old noname frame that either was designed for a rigid or an old short-travel fork. It looks kinda funky, but the handling isn't an issue. There's no bigger difference between my bike and other bikes than it is between other bikes in general.

Are you trying to say you took a frame designed for a rigid fork, put on a 105mm travel fork, and you cannot feel the difference? Sorry to sound harsh here, but you need to pay more attention when you ride. That fork has more then likely taken your headtube angle from a XC 71'ish degrees to a laid back downhill 68'ish degrees or even shallower. With headtube angles that slack, the steering responsiveness at slower speeds is sluggish.

dabac said:

Then you also have to consider safety margins. An average biker has ample margins with an average bike. As long as he don't start doing high drop-offs or anything else extraordinary the frame should be fully capable of handling any extra stress related to a longer fork.

Wrong, wrong, wrong. Have you seen the injury that can happen when a critical component like a handlebar/crankarm/frame unexpectadly breaks? Ever had to carry a guy back to the car with a pair of broken ribs because he was riding a handlebar that everyone told him was too light for his weight/riding style? Ever had to relocate someone's shoulder on a trail in the middle of nowhere when the headtube of his bike snapped clean off? Are you an materials engineer or frame builder that is qualified to assess the extra stress loads of a suspension fork on a frame?

It is not only the extreme freeriders that putting big stresses on their frame. Something simple like not paying attention and botching a bunny hop over a curb when you are cruising along at 30 km/hour can put more stress on a mountain bike then a freerider who properly lands a 6' drop off.

Cheers,
Juba

Juba said:

Are you trying to say you took a frame designed for a rigid fork, put on a 105mm travel fork, and you cannot feel the difference?

1) I don't know if it was intended for a rigid fork, or an early short travel fork. Fork travel HAS increased over the years.
2) using the max extended position is rather misleading, since the fork won't carry much load there. Start comparing at sagged length for a truer comparison.
3) A difference between what? I have never ridden the bike with the original fork, so what should I use as a reference? It handles faster and better than my old Asahi, but not as quick as my brother's old Kona. His Stumpjumper is somewhere in between.

Juba said:

Have you seen the injury that can happen when a critical component like a handlebar/crankarm/frame unexpectadly breaks?

I have only seen one disastrous failure due to massive and sudden overload, and that was a snapped crankarm when a guy did a massive air on a regular street bike and overshot his landing. All other failures have been wear and fatigue related. Maybe because I basically only ride XC with skinny guys?

Juba said:

Ever had to carry a guy back to the car with a pair of broken ribs because he was riding a handlebar that everyone told him was too light for his weight/riding style? Ever had to relocate someone's shoulder on a trail in the middle of nowhere when the headtube of his bike snapped clean off?

Yeah right. Let's settle who's got the bragging rights for goriest injury witnessed instead. That's soo much more productive.

Juba said:

Are you an materials engineer or frame builder that is qualified to assess the extra stress loads of a suspension fork on a frame?

No I'm not. But I've built and ridden far stranger things than this without any accidents and I wouldn't hesitate to do it again.
Are you a materials engineer or frame builder that is qualified to say that I'm wrong?

Quoted post said:

Something simple like not paying attention and botching a bunny hop over a curb when you are cruising along at 30 km/hour can put more stress on a mountain bike then a freerider who properly lands a 6' drop off.

Which is sort of my point. Anything sold to the public is usually designed with impressive safety margins as a protection against getting sued for releasing unfit products. As a side result there is usually a lot of leeway in wht you can do with the stuff before it breaks. I have done the maths on the front suspension of a car once, and I have no reason to believe that the bike industry shouldn't use the same sized margins.
Cheers,

Where to begin, where to begin...

dabac said:

1) I don't know if it was intended for a rigid fork, or an early short travel fork.

Your bike was not the question. The original poster's bike was, and he clearly stated his bike was originally a fully rigid bike.

dabac said:

Let's settle who's got the bragging rights for goriest injury witnessed instead.

Bragging? Seeing my friends in pain is not something I brag about. If you feel this is something to be proud of, well, I kind of feel sorry for the people you ride with. I was illustrating that the reality is a person can get hurt, so I for one would not feel comfortable suggesting he modify a bike in a way that was not intended. If you are comfortable making those kinds of suggestions without even seeing what he is trying to modify, maybe you would also be willing to fund his hostipal bills on the odd chance he does as you suggest and ends up hurting himself?

dabac said:

Are you a materials engineer or frame builder that is qualified to say that I'm wrong?

No I am not, nor do I profess to be. But then again, all I am suggesting is that he uses the bike in the method IN WHICH IT WAS DESIGNED, so my argument is automatically supported by the manufacturer of the frame who DOES have qualified engineers. You are suggesting he use the bike in a method in which it is not intended, therefore your argument is not supported by the manufacturer, and the oweness is on you to back up your suggestions.

dabac said:

3) A difference between what? I have never ridden the bike with the original fork, so what should I use as a reference?

If you have never ridden a bike with the original fork. Yet you are arguing there is no difference. How would you know? Like you said, you have no referance. How do you know your old Asahi was not a wicked fast handler before you put the wrong size fork on it? I mean your argument is so illogical it is hurting my head trying to figure out a way to tell you that your argument is illogical.

dabac said:

Maybe because I basically only ride XC with skinny guys?

And you know for a fact this guy is also skinny? What if you were to find out this guy is actually 250lbs? Would you still make the suggestion to throw caution to the wind?

dabac said:

Anything sold to the public is usually designed with impressive safety margins as a protection against getting sued for releasing unfit products.

Unfit products? You mean like a company that slapped a suspension fork on a bike that was not designed to support a suspension fork?

dabac said:

I have done the maths on the front suspension of a car once, and I have no reason to believe that the bike industry shouldn't use the same sized margins.

And the meat of your argument. What works for a car MUST work for a mountain bike right? Wrong. Do not pass go. Do not collect $200. Cars are overbuilt. So are lawn mowers. Bikes are not. Know why? If they were overbuilt, a bike would have 5 speeds and weigh 50bs. But no one would buy a 50lbs bike with only 5 speeds. They want something that has 27 speeds, 4" of plush suspension front and back, hydraulic disc brakes AND they want it to weigh less then 30lbs.

Stop and think about the incredible technology that they pack into your mountain bike and are still able to keep the weight down under what a single wheel on a motor bike weighs.

You want to do crazy modifications to your mountain bike? Hey, all the power to you bud. But you are not talking about your bike here, you are answering a question on a public message board, and your suggestions are [censored] poor.

Cheers,
Juba

Juba said:

...he clearly stated his bike was originally a fully rigid bike.

WHat I said was that my bike MIGHT have been fully rigid, or it might have had a short travel fork. I have no way of knowing.
What is evident is that the current geometry isn't stock any longer.
The bike is fully rideable and enjoyable. It has well over 1000 k on it and seems to be holding together just fine. It has worked for me.

Juba said:

...Bragging? Seeing my friends in pain is not something I brag about.

Then why did you feel compelled to make such a list of injuries? Bad things happen when equipment fails in a dynamic situation, no need to get graphic about it.

Juba said:

..fund his hostipal bills ...

I think I could cover those, whatever they are.. 😄

Juba said:

all I am suggesting is that he uses the bike in the method IN WHICH IT WAS DESIGNED,

Granted, your assumtion is the safer one. But you don't know enough to disprove my statement either. In my experience slapping a 4-inch longer fork will work just fine, and for all that I know it might even fall well within design specs. If you're so keen on logic, then why are you so eager to claim that the usage A automatically disqualifies the usage B? It's unknown, not disproven.

Juba said:

If you have never ridden a bike with the original fork. Yet you are arguing there is no difference.

Here are my words again. Please read them ALL this time before replying. There's no bigger difference between my bike and other bikes than it is between other bikes in general. My bike fits neatly into the span of handling characteristics that different unmodified bikes would display anyhow. It is not identical to any other bike, but the differences aren't outstanding either.

Juba said:

And you know for a fact this guy is also skinny?

I wrote that as a theory as to why I don't see many disastrous failures, nothing more.

Juba said:

What if you were to find out this guy is actually 250lbs? Would you still make the suggestion to throw caution to the wind?

You're right, I didn't think about this. An overweight rider will have less margins to experiment with and should be more careful. But it's quite common in these forums that people post their weight if they think it might be an issue.

Juba said:

Cars are overbuilt. So are lawn mowers. Bikes are not.

You should put that as "High-end bikes are not", and I'll agree. His bike is not a high-end bike. It's a starter bike. The reviews of it that I found regularly mentions "sturdy frame". Building it stronger than needed (and heavier) makes for easier manufacturing all around is easier and more cost effective than building it fully optimized. ASSUMING he's an average guy doing average things he should have margins for experimentation.

And what would you say about the people who build tandems out of ordinary bikes? Now you're talking serious overload here - what makes their bikes stick together if frames are such fragile things as you believe?