Mountain Bike setup

[james_95]

I have been running my ibike Aero Gen III on my road bike for about 3 weeks now and I’m loving it. I have an upcoming XC mtb race and I’d like to try the ibike on my mtb. I have read through the forums and it sounds like the data is fairly accurate on climbs and flats (which is what I care about) and maybe a little inconsistent on technical decent or tight switchbacks which I’m okay with.

My question is, what is the best way to setup the unit for the mtb? I have the stem mount and I can set the profile with the weight info but what about the Cal Ride and Coast Downs? Should I find a stretch of dirt trail to do the Cal and Coast or will the data be too inaccurate from the changes in terrain? Most of my XC races will be on hard pack or loose over hard pack trails…sand and mud are rare. Although my bike is full suspension it’s very stiff and I run very low rolling resistance tires. Are there some base numbers I should plug in for the Aero, Wind Scaling, Cda, Fric, and Crr? If not, what is the way to set it up to get the most accurate data?

Thanks,
James

[Velocomp]

You've got a couple of choices here:

1) Use the Fast Start sequence and Cal Ride to establish your Wind Scaling and Riding Tilt factors. I would do this on a regular road, not a dirt road. Then, in the Racr sequence, set Crr to 80 (to account for the higher rolling resistance of the dirt road).

2) You can do your Cal Ride on a regular road, then try coast downs first on a regular road, with suspension locked and Crr set to 80. This will give you a measurement of CdA. Then, move to your dirt roads and repeat the coast downs. What you are really trying to measure here is your Crr, which you add to your measured CdA from your prior set. You may get a value that is somewhat different from 0.008 and, therefore, a minor improvement.

Overall, you may get a minor improvement by option 2), but I'm not sure it is worth the extra effort. Speeds on flats aren't as high as road bikes, so CdA is not quite as important, and on hills, the primary source of energy consumption is climbing, which isn't terribly dependent on CdA or Crr.

[james_95]

I did a cal ride but I forgot to change the Crr to .008 so I ended up doing the ride with a Crr of .0036. I also did 8 coast downs, 4 on the pavement and 4 on the gravel next to the pavement. After analyzing the files and making some adjustment I came up with this: Aero .317, Wind Scaling .666, CdA .224, Fric 8.944, Crr .0080. Do these numbers seem realistic or do I need to set the Crr to .0080 and re-do my Cal Ride? My race is tomorrow morning so I am a little short on time. I have attached my Cal ride file and a couple screen shots for you to look at. Please let me know what you think I should do to try to get accurate data during my race.

Thanks,
James

[james_95]

2 more things

[Velocomp]

It is really good that you posted your ibcd4m file; that made analysis easier.

You had one bad coast down in your sequence (it was grayed out) but that's OK.

All 8 of your coast downs measured CdA, but because you changed from pavement to gravel half way through, your Crr data is a bit strange.

Here is what I did:

1) As you did, I set Crr to Crr adjusted. This let me see the "raw" values of aero and fric

2) I tossed out one coast down, where measured aero was only 0.09 (213PM)

3) From all remaining 7 coast downs, I determined that aero was 0.269 and CdA was .403. It's ok to use all 7 coast downs for CdA determination, even though half your coast downs were on gravel.

4) Because you mixed pavement and gravel coast downs, I used only the gravel coast downs to determine Crr. The value measured was 0.0078, very close to my guess of 0.008

5) So, your coast downs yielded a CdA of 0.403 and Crr of 0.0078

6) I used the Profile/Edit Profile command to edit your CdA and Crr to the measured values. The edited profile is attached; load it in to your iBike and you'll be ready to go!

In summary, by setting the software to "Crr adjusted" we were able to see the actual values measured in your coast downs. Your CdA is independent of your Crr, so we were able to use 7 of your coast downs to determine CdA. You did only 4 coast downs on gravel, and we used those coast downs only to measure Crr.