Tuesday, August 25, 2009

Mansfield Machine

The Mt Mansfield hillclimb, aka Race to the Top of Vermont, is this coming Sunday. Except for the first 200m, this climb is a loose dirt road. The race rules require minimum 2" wide tires, as the post race descent is quite sketchy. I did this race last year, the first time they held it. I raced my hardtail, as-is, with no weight trimming efforts. It is not a light weight machine, weighing in at over 25 lbs. I placed third overall on this bike. I calculated, had I a five pound lighter bike, I could have potentially won the event. There's no way I'll take five pounds off my hardtail. I'm starting with a 3.6 lb Ti frame. It already has an XTR drivetrain. I'm not about to make an investment in a carbon hardtail yet. So tonight I finished cobbling together one of the most bastardized machines yet.

Mt Mansfield is a fairly persistent 12% grade. Sure, there are a couple momentary places the grade levels off, and several section where it is much steeper than 12%, but I'll spend about about 90% of the climb in a single gear ratio. This got me thinking about trimming down my singlespeed for the hillclimb. Stock, it actually isn't much lighter than my hardtail at just under 25 lbs. I have heavy wheels, tires and fork on it.

So I shopped for a good deal on a White Brothers rigid carbon fork. That alone would take 2 lbs off. Then I have these old non-tubeless wheels sitting around I custom ordered eons ago. The disk wheelset weighed in just over 1500g, very light for 26" disk set. Even though I have to add tubes to the tires, this took about another 2 lbs off.

So how do I come up with another pound reduction to reach my goal? I had an old flat bar laying around, silly light aluminum that weighed half the weight of the alloy riser bar I have on the bike now. So I swap the bar over only to realize that the stem is OS (oversize, or 31mm clamp). Argh. Now I have to scrounge for a different stem that doesn't weigh more to fix that problem. Still needed to shed a little more weight.

I pulled the carbon seat post/SLR saddle from my road hillclimb bike. Silly post, it was inches too short. Maybe if I just swap the saddle. The saddle I had on the singlespeed was already a minimalist design, but weighed 250g. The SLR weighed 140g. Perfect, another quarter pound saved. Grab the fish scale, I got 19.95 lbs. Good enough. There are folks out there with fully equipped hardtails out there lighter than this. But this is all I have to work with for now without spending a fortune. I figure the carbon fork will see general reuse from time to time.

I just hope the cobbled up contraption holds up. I'm using a 22x19 gear ratio. That's a 30.1 inch-gear for those that think that way. As you can see, there is no way to test this thing except on a really steep grade. It spins out at 10mph. The test will come shortly after the start of the race. My biggest concern is the chain and chainline. I had to take links out, did not have any spare 8spd chain pins, so I had to reuse the ones in the chain. Bad, bad, bad. The 22t ring had to be mounted on inner BCD too, so I had to move the cog in back closer to the wheel center.

If the Mansfield race were important, bringing an untested rig like this would be hugely risky. Did I get the ratio right? Will I be over/under geared too often? Doesn't matter. The race is not part of the BUMPS challenge. It is just a fun event for me. It would be a riot if a guy on a singlespeed wins it, but I suspect many more people are going to show up this year, making the overall podium a long shot this time. I've always wanted to race my singlespeed. This isn't exactly what I had in mind. A few of us hope to hit trails up that way afterwards to make a complete day out of it. Yes, I will bring a different bike for that.

9 comments:

CB2 said...

Sounds like too small a gear to me, unless you're going to try and sit the whole climb.
That's 2#'s less than the rigid ss I'm racing.
I doubt this is of any help, but the race I did on Sunday was claimed to be 4000' over 24 miles and I used a 34x21 (47" GI). It was good.

CB2 said...

Oh, yeah, good luck!

Hill Junkie said...

Many people underestimate a persistent 12% grade. Every year over confident riders show up with slightly or unmodified gearing at Mt Washington and fail to finish. It's not about whether you can push a bigger gear, but what gear is optimal for power generation for that duration. I do tend to be a spinner and stay seated more than 90% of the time during hillclimb races.

The Mansfield climb goes 4.3mi, and I did it in 39.1min last year. So with a single gear, here's my rational for gear selection:

(4.3mi/39.1min)*(5280ft/mi)*(12"/ft)*(1rev/81.7")*(19t/22t)=73.7rpm

This is actuallly a bit low for me. If I assume I can do it in 37 minutes (to beat last year's winning time), my cadence will go up to 77.8rpm. This is still a tad low but probably close to what I averaged on Mt Washington this year with gears.

Remember, it is all up. Places where I might spin out are nil to 0.1% of the course.

If I transferred my XTR crank over with road Speedplay pedals, I could take another pound off easily. I don't want to mess with a perfectly dialed bike though, the one going to SM100 in 10 days. My hardtail is still in transit from Durango.

GenghisKhan said...

I see 12% as the grade and want to back out of the MMh--and I'm not even entered! Heck, I don't even live anywhere close to the race!

Sounds tough and good luck. Weigh your race kit--you may be able to shed a few more ounces by going shirtless, gloveless, etc. ;o)

rick is! said...

I was going to agree with cb2 until you busted out the math, now my brain hurts too much to argue.

Glen said...

Considering the steepness, and a little slippage possibly on the dirt, I think that gearing will provide all the resistance you'll want. Plus...with all that math..you'll be in the optimal gear.

Hill Junkie said...

I just read my description of the course from last year. I said the grade "undulates something fierce." Yikes, I didn't remember that! I've never done a major climb on a singlespeed before, so this ought to prove interesting. If it goes well, maybe I'll have to do a sub 12 pound road machine for Mt Washington next year.

ha said...

rode it training w/ 33x17 on a 26# 26" ss. had to zig zag the steepest switchbacks. best of luck!

Hill Junkie said...

At Mt Washington, there's always those that show up with monster gears. There's this macho thing in the cycling world that say's only pansy's ride low gears. True, most strong riders can push pretty big gears up these 12% grades, or even Lincoln's Gap 20+% grade. But so far, I'm not aware of any hillclimb results being posted for first, second and third as 33x15, 33x16, 33x17 etc. That would be whacky, wouldn't you say? Think how the knee replacement industry would surge. Results are always posted on fastest times. Biggest gears does not equal fastest times for the vast majority of riders out there (Thom P is an exception). While mashing a stock SS up Mt Mansfield comes with some bragging rights, my MO is to go after optimal finishing times. Maybe it's the engineering geek in me. That's what I do for a living. Always seeking the optimal solution to a problem. FWIW, I'd probably end up walking with a 33x17 gear. I'm a wuss.