LoriRose

Hey guys and gals,

I've an older XC 1999 Rocky Mountain Hammer Race - BikePedia with slicks that I was using as a commuter, and now I would like put some knobbies on it and swap out some parts to use it for its intended purpose and to make it a little more DH friendly.

Now keep in mind, I am a beginner and will not be doing anything too crazy on the bike. I know I need wider bars and a shorter stem, but this is all I'm sure of. The bar that came with the bike when I bought it used is 21" - I'm pretty sure someone cut it down. I've swapped in a 24" riser bar that I had on hand. It's only temporary though as it's the wrong colour. The stem is 90mm with a 25 degree angle.

How much wider do I go with the bars? I'm thinking pretty wide and then cut down if necessary.
What stem length would be best? Aim for the middle and go for 50mm?

What else should I be considering/replacing?

Tire recommendations?

Btw, I'm 5'8".

Thanks!

FrozenK

If by DH friendly you mean lift assisted terrain I'd recommend replacing the entire bike.

LoriRose

Nope, just a few local DH trails. Like I said, nothing crazy. I'm just looking to make a few modifications to the bike, not buy a new one.

FrozenK

You have a 17 year old bike with a 70mm suspension fork and v-brakes. You can throw money at it, but it won't change much. Buy a set of knobby tires (ask locally what people use) and have fun with it until it breaks. Then buy something else.

LoriRose

Thanks for all your great advice

Daspydyr 

Get the feel for the bike as it sits, talk to the people where you ride. See what ideas they have. DANG-I rode a 20 year old BRICK until 5 years ago. I learned a ton about riding and technology. When I did make the move to newer bikes I had a good feel for what I really wanted. Get a couple of months behind you. Make a couple of small changes like the bar or stem.

BEWARE-once you start buying bike stuff, it takes a family intervention to break you loose.

hig4s

It may be a dumb idea, but had a thought. I have read you can use a tapered fork in a straight frame. I am assuming your frame has a straight head. Anyway, thinking get a nice 27.5 120mm fork, convert the front brake to hyd disc. and try it. This should raise the front a little and make the head angle about 2 degrees more slack. Whether it works or not, you then start looking for a great deal on a newer 27.5 bike with a sacked or broken fork to move the whole front end to.

FrozenK

You can use a tapered fork on SOME straight taper frames. It depends on the head tube diameter. And you still need a longer-than usual lower cup for the headset which wil slacken the head angle even more.

It is also not a good idea to install a 120mm fork on a frame designed around a 70mm fork. The change in head angle is significant and will put undue stress on the head tube which can cause failure. Putting a 120mm fork on a 17 year old frame designed for 70mm forks is not a good idea in my opinion. High4s suggestion would be a good one if the OP had a more modern bike designed around 100mm forks.

hig4s

I hear a lot of mention of fork angle stress causing failure, but never met anyone it happened to. Besides that he mentioned he is a beginner and not going to be doing anything wild. But perhaps you are correct.

Another thing that was asked with bar width.. Lot of differing opinions on this. Some of the top bike testers will give somewhat poor reviews to bikes just based on coming with 680mm bars as the feel they are too narrow. You have to remember that wider bars may give you a better feel on high speed stuff but make it tougher on narrow trails. And also as you widen your arms it changes your lean angle so you made need a shorter stem. I go by the push up method of determining bar width.. put a yard stick down and do pushups, when you find your the most comfortable push up position measure from the outside of your hands. add 10mm to 20mm for room to move. My bike came with 730mm and I switched to 680mm and like them better.

FrozenK

Mounting a 120mm fork on a bike designed for a 70mm fork is not a good idea. Mounting a 120mm fork on an old bike designed for 70mm forks so you can do DH trails is an even worse idea. I don't think it will snap catastrophically the first day it is ridden, but it will increase the risk of that happening.

Also, you need to have a semi-integrated headset to be able to run a tapered fork. The OP doesn't have a semi-integrated headset. So the only option is using a straight steerer fork, which could be found used. But unless they find an older one with V-brake mounts, they will need to buy a new wheel and brake. By the time you buy a new fork, wheel, brakes... get a complete new bike and be done with it.

As for bar width, it depends on what you like and how wide are your shoulders. Current trend is for wider bars (700mm or more) and shorter stems. For me something between 710mm and 740mm feels good. I'd recommend buying wide, you can always cut down a bar. You can't add to it.

hig4s

From 70mm to 120mm adds about 10% to the length and changes the angle about 2.5 degrees. total increase in frame stress is about 15%.. I doubt seriously if a frame is not built with that much leeway. But that is non suspended stress. Max stress would be on a hard hit with the forks full compressed. Both these forks would be about the same length when compressed. So the actual stress angle would be about the same. And a newer 120mm fork will absorb the hit much better than a older 70mm so there may actually be less stress. Is there anyone with actual engineering knowledge that can weigh in.

FrozenK

I have an engineering degree. Took two years of structural design back in the day.

And where do you get the 10% difference in length? Even if we go with an axle to crown of 450mm for the fork she has now (and it is likely to be shorter, 450mm is the a-c for a modern 80mm fork. She has an old 70mm fork) a 27.5 fork with 120mm of travel is about 530mm of a-c. That's about 18% longer. Account for sag? Yes, we can do that but with 10% sag it still comes to about 17%. And that's just the fork, add another inch for the wheel. Installing a 120mm, 27.5-inch wheel fork on that bike is effectively the same as putting a 150mm fork. That changes not only the magnitude but importantly the direction of the forces to be applied. I would advice against it.

Also, 2.5 degrees is a huge, huge change. A bike with a 70 degree head angle and a bike with a 67.5 degree head angle are two completely different animals. 67.5 degrees starts veering into the downhill rig territory which is going to compromise handling on the flats and make climbing a chore.

hig4s

Ok I assumed the original fork was 480 to 500mm total.. so I am off a little there. and 70 degree to 67.5 is a huge difference in handling, (and would be much better for downhill) but we are talking stress forces. So fully compressed the original for would be 380mm and the new fork would be 410mm. Assuming these lengths and figure for ridge forks, what would the actual change in force on the frame head be?

FrozenK

I'd be more concerned about horizontal impacts that don't fully compress the fork and result in a moment with an almost 20% larger arm... like when a newbie cases a jump or a drop. (Psst, you still forgot to account for the extra 12mm of tire when she changes to a 27.5 wheel size)

hig4s

Ah,, yeah,, bigger tire. figuring for a full 90 degree horizontal force at 42mm (adding tire size) difference in length compressed using F=(WxX)/L I get 8% increase in force, at fully extended I get 18% increase in force. Anything under 10% I would not be concerned with. Unless the tire takes a hit a fully 90 degrees from the head angle some, if not most, of the initial force with be dampened. Without a full integral calculation of the total stresses it is hard to determine the full impact, but I find it hard to believe it would cause a catastrophic failure.

I personally would be willing to try it, but then I built my first wide tired off road bicycle out of scavenged parts back in 1970, long before there were production mountain bikes.

FrozenK

ĪHow does a longer fork increase force applied? It changes the moment but the force applied is the same. I have no idea what that equation you posted is supposed to mean. I'm assuming F is force and W is weight, but why are you dividing it by length? Anyway, changimg the fork not only increases the momeny applied but it also changes the vectors, so the force is applied in a different direction, which is what may cause problems. The total force is the same but the horizontal and verrival components change significantly. Is it going to lead to a catastrophic failure the first time you ride it? No. Is it going to increase stress and cause fatigue faster? Very likely yes. On a 17 year old bike never meant for hard riding I think that is a concern.

hig4s

The increase in force is at the head because the longer fork is a longer lever.

Like I said I would try it, and as a theoretical conversation it is interesting. But as I believe someone mentioned before, not really practical as it would probably cost more to go this route than just buy a whole new bike.

FrozenK

The force doesn't change when you use a longer lever, what changes is the moment. Think torque. The same force with a longer lever=more torque. And in this case, because force is not applied perpendicular to the fork, the horizontal distance from axle to crown will drive that moment. My quick math says that mounting the 120mm fork (26 inch wheel) results in a 30% increase of that horizontal distance. That is a significant change. I still think that the cased jump scenario is more likely to result in a failure, but a longer fork will result in significant stress increases=more fatigue of the frame. Safe practice is to keep the fork length within 20mm or so of the intended. That won't stress the frame too much and it also won't affect handling too much.

As for practical, I was the one that already pointed out that installing a fork would be too expensive. If it were me, I'd plop better tires and ride it as it is. If the bars are too narrow buy new bars. But other than that keep that bike as a commuter.

hig4s

So you are saying the amount of force on the short end of the lever does not change when you lengthen the long end of the lever.
It also seems you are making assumptions without doing the actual math. Conventional wisdom is almost always wrong, so i would like to see the calculations.

FrozenK

You don't seem to understand the difference between moment and force, and I already tried to explain it. I have better things to do than continuing down this rabbit hole.

Frozen out.

CrippledKonaBoy

A longer fork makes it easier for the frame to snap just behind the head tube. Engineering math be damned, there's a wealth of precedent out there for it.

Just playing around a bit on DH trails? No such thing with what amounts to an antique bike. It's just not capable of it any more. It's like showing up for a "Fast and the Furious" street drag with a '57 Chevy. LOOKS good...for an old tank.

Just trying to keep you walking without crutches.

RJM

This is for the op and I'm skipping the fork/lever/internet engineer argument....

riding your current bike down a DH course isn't going to be fun and your brakes are not up to the task. Better to rent a bike than to take your current bike down it. That's my feeling on the matter. Why risk tearing your current bike apart on a course that your bike isn't designed for?

Don't spend money on the bike, especially buying a new fork. Ride it like it is and go slow and safely as you can or just skip the idea of riding your current bike down that course.

LoriRose

Maybe I didn't stress enough that I am merely looking to ride down some smaller hills at a somewhat leisurely pace. While it might be nice, I absolutely do not need a new $3000 downhill mountain bike to do that. I'm a 37 year old woman looking for a little more fun on the bike - could someone just humour me and answer my question?

FrozenK

First, try it as it is. If you must buy a new handlebar try a 700mm or 710mm one. Stem, try a 70mm. Tires, as I already said ask around your town what works best.

If it is going to be wet/muddy you may want to put some Kool Stop salmon brake pads.

And sorry for derailing the thread a little bit.

Diablito

For a quick revamp on your bike for some not so aggressive DH fun you would need a new suspension fork and new front brake. If your gears and back brake work well just throw on the MUST have parts. If you want to go bold get a front shock with a disc brake, a front disk brake wheel and a disc brake. Maybe $600 if you look for parts on ebay. Lower your seat and have fun. The handle bars and stem are good to get after you buy the basics first. Once you get the new fork, it will change the geometry of the bike bringing the front end up so wait to see how the bike will feel with the new geometry before buying the new stem and bars. If you have an older bike you have to find out what width the steerer tube is so then you can figure out what fork you want. Your bike should be fitted with an A-Head set. If you don't have an A-Head set your gonna have to buy a new headset once you figure out the steerer tube width and your existing stem will automatically have to tossed out the equation.

Its a lot easier done then typed out. To be honest, if you love your bike just do the upgrades BUT I would suggest save the $600 or so dollars until you have $1200 or even $1500 and you could get an AWESOME FULL SUSPENSION with disc front and rear brakes that will Recharge your love of Biking. I was in the same boat as you just 3 weeks ago. I read a ridiculous amount of articles and spoke to every shop near me and came to the conclusion that upgrading my 1995 Voodoo Canzo with Fox alps 4" rear shock and Marzzochi Bomber 6" travel mtb was gonna be expensive. I ended up getting a 2016 Trek Fuel EX 8 29er. A tremendously long name for a bike but the bike is amazing. Every part of the bike has been redesigned from what I was use to. Now all I have to worry about is making this bike last me as long as my Voodoo did. Best wishes with your bike. If you have any questions or need advice just shoot me a PM.