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.

1 Like

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.

2 Likes

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.

6 Likes

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.

2 Likes

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…

Here the findings of my field experiment.

Impact on power for a given ‘regular’ (say, 2% to 4%) climb.

  1. open vs locked suspension: 2%
  2. weight: 1% per kg
  3. “faster rolling tires” vs chunkier enduro ones: 4%

I’m yet to crunch these into BBS as per @huges84 's suggestion. I’ll be back

If you want to do this yourself, you could just have the route broken down on a spreadsheet into segments of say 10m, with surface condition, altitude: work done in each interval is equal to the change in kinetic energy and potential energy. Not just your kinetic energy, but the energy of the air you sweep through. The smaller you make the segments, the more accurate this will get. Its basically finite difference modelling.

kinetic_you_power: 0.5 * m * ( v_1 **2 - v_0 **2) * v_ave / s
kinetic_air_power: 0.5 * rho_air * CdA * v_ave **3
potential_energy_power: m * g * v_ave * sin(theta)
rolling_power: Crr * m * g * v_ave

The profile of the descent is important, there are videos online of balls falling on ramps with different shapes, through the same height, with differing final speeds. Your acceleration (change in speed, per unit time) without power output, under gravity is sin(angle descent) * g.

If you assume you are laying down your FTP power throughout, you can solve for velocity and therefore time in each interval. Just sum up the time across each interval. I look at these online bike calculators, and it makes me wonder: how have they calibrated them. There are so many unknowns here, such as your braking, your position on the bike and therefore CdA, wind speed, drafting…

took me a while to realise you had answered. So many thanks for the science.

Mountain biking is complex because power delivery isn’t as easy as hammering on a road bike. You’re constantly fighting traction and gravity + terrain are trying to pull you off the bike.

My SC Nomad is pretty light at 28.5 lbs, but my Specialized Epic at 22 lbs feels like 0 lbs in comparison. It’s just way easier to deliver more power and hold more speed up the climbs.

Last month I did a 4Hr XC race on a 7.7km circuit on my Stumpjumper @ 13.5kg. The next week I bought an Epic Evo @ 12.2kg and went back and rode most of the race circuit.

This segment was one of the climbs and this time is 22 secs faster on the Epic Evo than my best time the previous week on the Stumpy in the race, which was set on lap 2 (~30min laps for me). The time on the Epic Evo was set after riding other trails for ~2Hrs prior. Not only was it 22 seconds faster but my heart rate was 9bpm lower avg for that segment. I wasn’t racing on the Epic Evo and I wasn’t consciously trying to beat my time but I was well aware by then that the Epic Evo was a much faster bike.

I did 7 laps in the race so assuming the difference would be 22 seconds per lap, that’s 2min 34sec faster over 4Hrs on the Epic Evo.

Only a 1.3kg difference in weight between the two bikes. The difference in speed is not just because of the weight, the Epic Evo pedals a lot better and there is noticeable bob on the Stumpy in comparison. I suspect the Epic Evo wheels contribute to the faster speed. Both bikes had proper XC tyres and were both tubeless.

What tires were on both bikes? I know you mentioned they were both XC tires, but there can sometimes be a significant difference in rolling resistance between models (2-3w over that much time can add up quite a bit)

BTW, thanks for reporting, very interesting.

1 Like

The Stumpy has 27.5” Ardent 2.4 front and Ikon 2.2 rear.

Epic Evo has stock 29” Ground Control 2.3 front and Fast Trak 2.3 rear.

I’m not sure how much rolling resistance played a part as the bikes are very different so it’s hard to tell. In terms of how they feel to ride, when you apply power on the Epic Evo it’s converted immediately into speed compared to the Stumpy.

I should add that the race was 62km with 1100m climbing. My FTP was ~280W at the time.

I’ve ridden my gravel bike up a 45 min singletrack climb near me and PR’d the ride by over a minute. I think it’s a combination of lower weight, lower rolling resistance tires, and better pedaling efficiency. Probably the same things you are experiencing with your Epic vs Stumpy.

Yeah for sure. The weight difference isn’t massive but everything adds up.