Weight difference for XC race performance

This is (like) a high school physics exam for @Jonathan (and mostly the rest of you cycling geeks).

I recently raced a 30k XCM circuit on my 32lb heavy enduro bike. Total climb 1100m. TOpography was mostly climb for 18.5k, rest was half steep, quasi enduro descent, and half fire roads that allowed for some serious speed. As fun as it can get.

Race conditions were dry, no wind, warm, but not too much once in motion.
(Irrelevant, but interesting event location on google maps: https://goo.gl/maps/BKmNHjTFXJrvWKpx8)

Total race-ready weight (me+bike+supplies) was 182lb. (Pro tip: Water bottle was only half full at start because aid-station spacing allowed for precise refill. This saved me carrying uphill about 300g XD.

My ftp is 193.
My race NP was 143.
Total work recorded 1175kJ.
My total race time was 2h 48min.

What could have been my total time, had I used my wife’s DC bike that weights only 25 pounds?

What could have been my total time (had I managed to survive the gnarly DH) with an 18 pound scott scale?

My next XCM is a 50km circuit with a 1500m climb.
Who of you would be next to me when I explain my new equipment needs to my wife?

You’d definitely notice the difference between the Enduro bike and the cross/down country 25 lb bike. Not only is the weight significant but also the pedaling efficiency and geometry will play much better going up hill. What you gain on the accent will likely far exceed what you’d loose going down.

The hardtail I wouldn’t recommend as it won’t be that much better than the XC bike and depending on decent it could be significantly slower and less safe.

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Best Bike Split was designed to simulate all sorts of stuff like this to answer these kind of questions. It can even tell you the optimal way to pace.

What it (and anybody that hasn’t ridden the course) can’t tell you is the difference in suspension and handling for technical sections. That you have to judge for yourself, based on the course and your skills. If you will crash or break something, you will probably give up more time than any weight savings.

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2:46 or so

2:44 or so

I’d tell her to tell you to that generally more watts are better than a bike that weighs a little less :wink:

My general rule of thumb is that 1 lb of weight saved is 1-2s per mile of climb at a 7-8% grade. Similarly an extra watt in average power will also be around 1-2s faster.

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I see what you mean. Many thanks for commenting.

I think I’ll crunch the data into Best Bike Split and will report back.

Weight by itself isn’t going to make a huge difference. THe major difference is going to be how the bike pedals and how it is set up. My enduro bike has a slack seat tube (compared to XC) which is less optimized for pedaling, the suspension is FAR more active, wheels and tires (rotating mass) is far higher with more aggressive tread compounds. If I were to just add 5 pounds to my XC bike, that wouldn’t be massively different in my race. That’s like the difference between racing with two water bottles vs none. XC bike is going to handle better at low speeds vs my E29 which handles high speed better, etc.

I XC race a hardtail. There is very little an XC HT can’t handle as far as downhill goes with some training.

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awesome comment; thank you for your time and insight.

During my morning run today I drafted a mini test climb near my house to test my hypotesis. On tarmac for now, bc getting to dirt in my city is complicated.

It’s a 2’ uphill run (info collected with stryd shoe sensor):

  • 280m long
  • 11m alt. difference (though Garmin says 13m)
  • 3.8% grade avg.
    Incline varies along the segment. Will measure+chart on the test day.

My objective is to measure the impact of weight on performance at a given cadence and gear.
If experiment controlled properly, it should answer my original question, on tarmac.

Constant gear: 26T chainring , 28T cog
Constant cadence: 90rpm
Enduro tires with my usual (dirt riding) pressure F/R: 13/14 psi (I’m lightweight and use tire inserts, in case you wondered why so low)
System weight (SW): as of test day, will record

Test runs (on my 29’ enduro 150/170 mtb):

  • Run 1: SW, open sus, open fork, enduro tires
  • Run 2: SW, locked sus, ‘pedal’ fork (I’ve got the lock fork option, but I’d say irrelevant/unrealistic for the field), enduro tires
  • Run 3: SW+7lb, locked sus, ‘pedal’ fork, enduro tires
  • Run 4: SW+14lb, locked sus, ‘pedal’ fork, enduro tires
  • Run 5: same as Run 1, but with my better rolling, slightly lighter XC wheels/tires. Dirt pressure F/R 18/20psi. Will compensate for SW with a water bottle.
  • Run 6: Run 1 again to control for fatigue.

Following conclusions may result:

  1. impact of lower rolling resistance wheels
  2. impact of added weight
  3. added weight impact linearity
  4. impact of bounciness on performance (energy lost through suspension movement)
  5. impact rolling resistance downhill (since I have to go down again I might as well measure things) between each setting

Please pitch in if you think I should consider anything else.

This test will not give you any results you can use.
With a constant gearing and cadence your speed will be the same for all runs, the only difference will be the differing wheel-circumference when changing tires.
Your personal power output will vary for each run though…