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Road bar on mountain stem?(12 posts)

Road bar on mountain stem?haimish
Jan 16, 2002 12:45 PM
Is it safe to use a road bar (26.0mm diameter)on a mountain stem (25.4mm diameter)? The bar is a Performance Forte, stem is an open-face Bontrager.

Jan 16, 2002 1:17 PM
While it is possible to squeeze a road handlebar into a MTB stem, it is not recommended, and is not particularly safe.
No,No,No,NO!!!!!!!!!!!!!!1Rusty McNasty
Jan 16, 2002 3:04 PM
Even with a beer-can shim, the d@mn thing is going to pivot on you, probably when you land after a jump. This wouldn't be a big deal with a straight-tube MTB bar, but with drops, it is!
Besides, the drop bars will force you to sit with your hands, what, 50-70mm farther forward-does that work for you?? It don't for me!!!
Jan 16, 2002 10:54 PM
Seeya in the rearview strapped to my fat ass rusty!!!Seems as you allways have some smartass comment.apology to everyone who reads this but rusty.Later,Joe
What are you trying to accomplish?nee Spoke Wrench
Jan 17, 2002 6:57 AM
There are several moderately priced road bars available that come in the 25.4 clamp size. Dimension and Nitto are two brands listed in the QBP catalogue so any LBS should be able to get you one.

I think that the relationship of the handlebar relative to the seat is one of the real keys to bike fit so I would take some time to think through what this change is going to do to your position on the bike.
I've tried this kind of stuff.dzrider
Jan 17, 2002 7:28 AM
The clamp diameter and the bar diameter should really match. I once got a drop bar on and couldn't get it back off without a saw, and I have no confidence in shims holding drop bars in place.

Back in the mid 80's I tried straight bars on road bikes and drop bars on mountain bikes and all sorts of combinations on hybrids and cross bikes. I ultimately concluded that frame geometries were designed to fit either a straight bar or a a drop bar and didn't work very well with the other. A possible alternative is a 'moustache' bar which is like a flat drop bar - the bends go forward.
My wife has a set of these on an old Bridgestone and they are incredibly comfy for beating around.
how to make it work....C-40
Jan 17, 2002 9:53 AM
If the MTB stem is not an open-face type, I wouldn't do it. If it is an open-face type, a slight modification will make the stem work.

The open face stem will have a tendency to pinch the bars at the center. To eliminate the pinch point, the gap between the stem and face plate must be increased to approximately 6mm in total width to eliminate the pinch point. Most open face stems have a gap of only about 2mm. Filing a shallow angled chamfer across the edges of the face plate and stem will effectively increase this gap to eliminate the pinch point.

Ignore the comments about shims. A shim would never be required when the bars are larger than the stem.
...or not.grzy
Jan 17, 2002 5:25 PM
What you're really dealing with is a difference in *diameters* which presents the problem all the way around the mating surfaces. To avoid _any_ pinch point and potential failure caused by the stress riser, you'd need to accurately remove material along the entire circumfrence. A milling machine would be best - a hand file probably one of the worst methods. Anything less and you'll end up distorting the bar in a region of maximum stress. Given the frailties of heat treated aluminum and carbon bars you'd really be asking for a premature failure in either the stem or more likely the bar. Remember that the stem wasn't designed or tested to have this kind of material removed. Given the risk involved I find it difficult to believe that any one would do this much less recommend it.

I find C-40's advice problematic and dangerous.
problematic and dangerous!!!C-40
Jan 17, 2002 7:38 PM
Wow, so much concern.

I have worked as a machinist for ten years and a manufacturing engineer for the past 20. If you carefully analyzed the geometry involved, you wouldn't be so skeptical.

Aluminum bars typically aren't very round to start with (trying measuring with a micrometer). I measured my 26.0mm Easton EC-90 bars before mounting them in a 25.8 diameter ITM Millennium stem. The diameter varied between 25.8 and 25.9. At no point was it 26.0.

Aluminum bars are also not made very accurately. Although a brand may be advertised to be a "nominal" 25.8 or 26.0 diameter, you could find examples of each that are the same size.

Aluminum bars can easily deform the small amount need to conform to the curvature of a 25.4 open-face stem. Some bars might even exhibit better roundness after they are clamped in 25.4mm stem.

I think you worry too much.
At this point,TJeanloz
Jan 18, 2002 12:09 PM
I think it's critical to say something. I'm not a mathemetician, I'm not an engineer, I'm not a machinist. I'm a bike shop guy. It is my professional opinion that putting a road handlebar in a MTB stem is not safe. That's not to say that it can't, or shouldn't be done- I have just such a setup on my TT bike, which has the beefiest cowhorn bar around and gets ridden almost never.

However, I have seen many bars break when subjected to this practice. It's usually not very pretty for the rider when this happens. Repeatedly riding a 26.0 bar in a 25.4 stem will cause failure. It's not a question of if, it's a question of when.

I don't doubt that with the proper tools (and I have no idea what those tools are), a machinist could make a 25.4 stem work with a 26.0 handlebar. But it's not something somebody could do with a hand file- at least with today's bars and stems. It's possible, but it is not safe.
Thanks for the (various) inputhaimish
Jan 18, 2002 7:39 AM
I ended up finding a Nitto 115 bar with the 25.4 clamp diameter, so I'm going with that. However, I checked around with a few shops, and they all said a 26.0 bar wouldn't be a problem with a open-face 25.4 mtb stem.

I was trying to build up a soma cross frame using only parts that I have laying around; got lots of mtb threadless stems and road (26.0) drop bars, annoying that the relevant diameters aren't the same.
watch for the pinching...C-40
Jan 19, 2002 5:14 AM
Before I catch any more hell, remember that I only recommended this practice, if the pinch point was eliminated. Depending on the gap between the face plate and stem, the amount of edge chamfer required to eliminate the pinch point may be quite small. Let us know how it works.

Reboring the stem would be the best idea, but unless you have a friendly machinist, it would cost more than the stem. A one-time setup to do this takes a lot of time.