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650b conversion on new Giant TCX... Stupid?

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650b conversion on new Giant TCX... Stupid?

Old 10-05-14, 11:19 PM
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AlTheKiller
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650b conversion on new Giant TCX... Stupid?

Okay, precursor to this post. I'm working on rehabbing my second broken knee, so I'm going bike crazy in my head thinking about... well, everything to do with bikes really, because I've been out of the saddle too long. I now have the go ahead from doc and physical the****** to do more intensive rides, just short time periods and ice after. Obviously this will be all on road, in more controlled conditions, for quite awhile. In fact, since my crash which broke the knee for the second time in some gravel, i now slow to a crawl near anything dusty and get very paranoid (that's probably an issue for another thread, getting confidence back after a bad crash)

Anyways, I had started working at a bike shop, and obsessed over the TCX for a long time before pulling the trigger. It's a pretty good deal when you get their pricing, IMO, couldn't pass it up. Finally got it, did some light trail stuff, but mostly wanted it for road use and threw some Riv Jack Brown tires on it and loved it(VERY fast for their volume). Fits great, rides great. But I've slowly become more obsessed with higher volume tires on drop bar bikes. Monstercross if you will, but also higher volume "slicks" like the 33.33 JB tire (measures to 35mm on my rims). Not sure if I'd ever ride less than 28mm on a bike purposefully again, preferrably not under 32. Only had it a month or two before breaking the bad knee again...

The only two theoretical issues I have with this bike is the higher standover and higher BB. The BB is not as high as some cross bikes, but not on the low side either. So I've been thinking more and more that I could kill a couple birds with one stone, by putting some 27.5/650b wheels on there. I can snag some Giant branded wheels for stupid cheap probably. I could potentially fit something like a 1.95 on there(would get out the calipers before seriously considering this), or just enjoy a nice 40-42 with a bit lower center of gravity. the 35s it had from the factory are nice, but the hard, not quite smooth trails I hit around here definitely left me wishing for more volume. But maxxing out the clearance on the 700c wheels just keeps pushing me sky high.

I know this affects some of the geometry and handling in less obvious ways, any reason this is a stupid idea? I'd talk to my boss and better mechanics, but they have all basically become "racing bike" zombies who don't really understand crossover traits in bikes, and spend most of their time flipping last year's CF wonder bike for next years CF wonder bike, mostly Enduro/all mountain type bikes, and am pretty sure I'd get laughed out of the shop. I know I've seen lots of Surly type builds with conversions, I just can't see an immediate negative. I'd probably keep the Jack Browns on the stock wheels for long pavement rides.

The bike brand new:
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Old 10-06-14, 09:21 AM
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Seems like a silly idea to me. I mean, if you had a pair of 650B wheels lying around and just wanted to try it out, sure why not.

I'm looking at that saddle height and wondering how standover could possibly be an issue.
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Old 10-06-14, 10:09 AM
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I also don't think you will see any benefit to the wheel and tire change. I have both a Cyclocross bikes and a monstercross bike. I use tires several tires from 700x27 to 700x50. Smaller is always better. The larger tires have been used on mountain bike trails and just don't perform better than a 700x38 tire. On smoother trails, the 700x32 is always faster.

I'm 220 lbs and have never has a problem with smaller tires pinch flatting or any other performance issues.

If you really want to have the biggest safety margin on technical trails, get a 27.5 mountain bike.
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Last edited by Barrettscv; 10-06-14 at 10:16 AM.
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Old 10-06-14, 02:35 PM
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Not Stupid

It's a very good idea on many levels. To achieve the most performance potential using a high volume/low pressure tire, mount them on appropriate width rims. Think 29mm or greater inner width. Appropriate width rims will provide less pneumatic trail, and correspondingly lower slip angle. This performance aspect can be appreciated in corners at any speed above walking pace. My top recommendation is Derby Technical Specs, followed by Nox Composites Carbon MTB Rims and Wheels. If you go hookless carbon with bead locks you can reliably run tubeless with just about any tire, and at comfortable low air pressure. Tech Tuesday ? Wider Rims Are Better and Why Tubeless Tires Burp Air - Pinkbike Take a look at the chart in this article about Specialized/Roval's new rims New Specialized Roval Traverse Fattie All Mountain Wheels Show Fat?s Where It?s At.

There are many high volume/low pressure tire options in 27.5. If you want gravel specific tread with knobs, they're available, if you want supple, low rolling resistance, comfortable tires, they're available. If you prioritize road use, Soma Grand Randonneur green label tires are outstanding. Grand Randonneur Tire (Green Label) 650b x 42mm | SOMA Fabrications

Images below: Top - 42mm Grand Bois Hetre on P35 rims, second - Hetre on Derby rim, third - 38mm Pacenti Pari-Moto Derby rim, fourth - 42mm Soma Grand Ranonneur Derby rim.


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Old 10-06-14, 04:03 PM
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I like the idea and I'm going to try it on my bike later this year.

This guy does it and it looks great to me.



EDIT: If my memory is correct, TCX already has huge tire clearance and doesn't have seatstay and chainstay bridge. What's the widest tire size you can put on the bike?

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Old 10-06-14, 04:09 PM
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I like the idea. The biggest problem in a 650b conversion is that you may get heel strike once you drop the BB; that's not an issue here because the bike has a high center of gravity. A little fatter tire at a little lower pressure is good. That said, not worth doing unless you can do it inexpensively since what you have is pretty good and this will be an incremental improvement at best.
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Old 10-06-14, 06:05 PM
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I can concur with the "Not Stupid" camp. I have been running the Grand Bois Hetre on my Trek Crockett disc and can't say enough good things about them. The ride of the bike was transformed. With the right tires this is much more than an incremental improvement. Mine are set up Tubeless on Velocity Blunt SL Rims. There is zero penalty riding them on smooth roads and huge gains in rolling resistance and comfort as the roads turns rough or too dirt. I've never had an easier time riding on freshly graded or loose gravel. The problem with large 700c tires on the road is that they start to feel like wagon wheels. The smaller 650b keeps the overall diameter in check and reduces the gyroscopic affect of all that weight spinning at the edge of the wheel.

The Hetres wouldn't be my first choice for trails but that is what the newly introduced 650b Soma Cazadero, Surly knard, and Bruce Gorden Rock and Road will be for when I get my hands on them. These are all 40+mm light knobby tires. If your bike can fit 2" tires there are options from Vee Rubber, Schwalbe, Maxis, etc. I would go for the biggest tires on the trail to avoid pedal strikes as mentioned above. Obviously a drop bar cross bike is never going to be a complete replacement for suspension MTB on trails - but I think you knew that.
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Old 10-06-14, 06:41 PM
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that doesn't really look like it's going to clear 650bx50, maybe 40mm? If you can get a 40mm tire in there, it isn't a bad idea. It really depends on your road surfaces. Around here, there seems to be a pretty big budget for gravel, and the roadbed base is 3" rocks, so bigger is definitely better. With 30mm tires I was getting regular pinch flats. None this year on 38mm tires. My guess is you can go with 40mm slicks and not suffer much.
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Old 10-21-14, 09:32 AM
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Originally Posted by unterhausen View Post
that doesn't really look like it's going to clear 650bx50, maybe 40mm? If you can get a 40mm tire in there, it isn't a bad idea. It really depends on your road surfaces. Around here, there seems to be a pretty big budget for gravel, and the roadbed base is 3" rocks, so bigger is definitely better. With 30mm tires I was getting regular pinch flats. None this year on 38mm tires. My guess is you can go with 40mm slicks and not suffer much.
The bike will easily clear 700x40, and theoretically up to 42. so 650 should widen that up a bit. How much it widens up can only be told by some calipers, which I have yet to do. There is no height penalty on the rear, however the stays obviously widen out the further out it gets, which is where I'd be getting the extra width. Not sure I'd gain a whole lot of width up front.
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Old 10-21-14, 10:10 AM
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Originally Posted by AlTheKiller View Post
the stays obviously widen out the further out it gets
I'm not sure you'd get ANY extra clearance. The limiter is almost certainly the chainstays, and on newfangled carbon bikes they don't necessarily get wider as you move toward the rear. Remember we are talking about moving the rim less than an inch rearward.

If swapping to 650B got you into that 1.8-2.0 tire realm then I'd say, go for it. But I'm just not seeing it.
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Old 10-21-14, 10:18 AM
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I did that conversion on my old Salsa Vaya. Worked like a charm. You can read about it here.
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