Monthly Archives: May 2014

Ironman Lake Tahoe Bike Calculator

I’m not a triathlete, but offer this bit of advise as somebody who has lived and cycled extensively in the Lake Tahoe area. I think that a lot of people underestimate the difficulty of the climbs in this race within the context of an Ironman event. It’s one thing to do the loop in training or blast up Brockway Summit for fun, but it needs to be treated with a lot more respect if you’re going to run a marathon afterwards. I saw a lot of talk on the Slowtwitch forums before the 2013 race and got to thinking. Here is what I came up with.

To start with, you’ll need to know your FTP. Then, take 10% off the top due to the altitude. Then take 75% of that to get the average power that you’re shooting for in an Ironman event. Some examples.  For a FTP = 350W your IMLT power would be (350*.9)*.75 = 236W.  For FTP = 300W, IMLT power = (300*.9)*.75 = 202W.  For FTP = 250, IMLT power = 169W.

So now we need to consider what kind of gearing you’ll need to ride up Brockway at your IMLT power at a reasonable cadence.  To help with that I found an excellent calculator.  Punch in your IMLT power, your weight, the elevation, % grade, distance, and temperature.  Let’s start with our 350W FTP (sea level) rider.  We’re going to use a weight of 75kg (I’m a metric guy, but you can use english units if you like).  The segment data can be found on Strava here. So, 350W FTP => 236W for the climb.  Elevation at the start is 1890m.  Grade = 7.1%.  Distance = 4km.  For temp, we’ll go with 15 degrees C.  That gives me a speed of 12.74kph and a time of 18:50.  FYI, the Strava KOM is 11:59.

Okay, what kind of gear do I need to go 12.74kph at a reasonable cadence.  For me, I want to be able to spin at least 80rpm.  Find your favorite gear calculator.  Sheldon Brown’s is always a good bet.  I use a compact crank so I’m going to cut to the chase and use 50 and 36 for my chainrings.  I picked the Campy 11 speed 12×27 freewheel and found that at 80rpm in a 36×27 I would go 13.8kph.  For me, that’s slightly too large of a gear!  A 36×29 worked out to be just about right at 12.8kph.  Think about that for a second.  A guy with a sea level FTP of 350 watts who weighs 75 kg, that’s 4.67 W/kg at threshold needs a 36×29 for this course!  Sure, a person with that kind of power output could easily get around that course in a 39×25 or even a 39×23.  IF they wanted to walk most of the marathon… How about our 300W and 250W sea level FTP riders?  11kph and 9.25 kph.  At 80 rpm that works out to needing about a 34×32 and 34×36 low gear respectively.

The important thing to think about here is what you CAN do versus what you SHOULD do. My opinion is that pretty much everybody but the pro’s should be on a compact.  I can hear the wailing and gnashing of teeth now.  “But what about the flats and downhill sections?  I need my 55×11!”  You know what?  No.  You don’t.  Seriously.  a 50×11 at 80rpm gives you 47kph.  Just a smidge under 30mph.  Please tell me where you’ll be going that fast except on the two big descents each lap at 236W.  The answer is nowhere.  “But what about the stretch from Squaw to Truckee?”  Again, NOT at IMLT power.  If you do, it will be on some short section of about 30 seconds or less and if you are smart you will either suck it up or learn to spin a bit in those short fast sections.  If there is time to be lost due to “only” having a 50×11 it will be more than countered by the time you would lose trying to get over those big hills in a 39.  If you really need/want something bigger, then a mid compact with a 36×52 might be the ticket.  But frankly, in my opinion only the strongest riders will be okay even with the 36.  I suspect the charts will tell most people to use a 34.  The 34×52 combo will likely lead to dropped chains.  Trust me, you lose more time stopping to put your chain back on than you will if you are “limited” to a 50×11 top gear.

Now comes the ugly part.  How are you going to get that gearing on your bitchin Shiv or P5 considering that right now you’ve got a 42×55 crank set with an 11×23 cassette.  Answer?  Get out the credit card.  If you’ve got a standard crank you’re going to need to pony up for a whole new crank set unless you’ve got a new Shimano 9000 11 speed crank (110 bcd rings) or one of the SRM models (like my Dura Ace 7800) that has a 110 bcd inner ring and 130 bcd outer ring.  Then, if you decide you need something bigger than a 28 or 29 cog, you’ll need a new long cage rear derailleur.  It used to be that you needed to buy a MTB derailleur and it was a lot trickier picking the right one.  Now it seems most brands have a long cage version that will fit the bill.  I believe that all levels of the current SRAM rear derailleurs com in long cage versions and I know Shimano makes them too.

FYI, here’s 2013 IMLT segment:

1 Comment

Filed under Uncategorized