Mountain bike time trial

Chipotle

I'm wondering if there are any benefits to getting more aero on a mountain bike. There is a time trial coming up and I am considering using my cyclocross drop bars and some lighter wheels built with DT Revolution (thin) spokes.
I've often heard there is no benefit to drafting at mountain bike speeds. However, this race (not actually a TT, but everyone does get timed individually - drafting allowed) is 24 miles, held primarily on firm two track roads with little single track, some sand and an occasional short hill. My average speed will be 14 mph. In theory I should achieve more speed for my power output but will it be significant?

Woofer

You can plug in the numbers at analytic cycling.com but 14mph IIRC from reading John Cobb's stuff that he used to publish is about the threshold where one would see gains from using aerodynamic equipment. So it's a borderline case - if it's close enough do multiple runs with different equipment. Why not use your cyclocross bike to reducing the rolling resistance and minimize mechanical changes? If there is little technical single track you might even consider using fat road tires.

Smartt/RST

Also...if 14mph is the average, and there are some short climbs (which would reduce the average speed), then there *should* be a significant amount of time between 15 and 20mph where improved aerodynamics could be beneficial, depending on the course profile. But as Woofer alludes to, you must factor in all the variables that will affect your performance: course profile, rolling resistance, technical nature of the course (descending, single track, mud/sand), does your cross bike have the adequate gearing....

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Smartty

MikeHains

Chipotle,

Can I say that from my own experience ... typical mountain bike tyres have high rolling resistance. This is because of the lugs or pattern on the tyres ... and because of low inflation pressures.

I changed to a slick higher pressure tyre on my mountain bike. I noticed a very significant difference in speed for the same power.

Also - a lot of energy is wasted in suspension. Wind any shocks you have up as tight as they will go.

Finally, your speed will have increased because of the two things you've done (above). This will mean you will then feel dramatic improvements by being more aerodynamic on the bike. Really dramatic improvements.

If you aren't sure ... do a "roll down" test on a local hill.

Regards (and enjoy !). MJH

pod

I can't see how suspension wastes energy or am I missing something? To me the suspension is absorbing forces virtually at right angles to the direction of travel. If you had no suspension you'd be bumping up and down more but I can't see how that would make you go faster. Your body would be absorbing the same energy that the shocks would have been absorbing.

Smartt/RST

True, suspension absords shock from the ground, attempting to smooth your ride; how this effects forward motion must be changing from moment to moment, depending on the terrain. But as you say, suspension is absorbing forces; this happens in both directions, from the ground and from the power you put into the pedals. When you get on a smooth road climb, notice how your buddies bob up and down as they pedal; w/o suspension, that energy would be resisted by the frame and more efficiently sent into the wheels and into moving them forward.

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Smartty

MikeHains

Pod, great question.

My experience, is that on a flat road, at high power, an MTB bobs up and down. Not because of bumps - but because of pedal pressure and pulling on the handlebars. This means energy is going into the springs (to compress) and back out (to bounce back). This energy is being "wasted".

This experience is magnified every time I have ever tried an effort out of the saddle on my MTB. My road bike is altogether different.

Here is a study showing clear differences in power between MTB with front suspsension only, and an MTB with dual suspension:
http://www.acsm-msse.com/pt/re/msse...017&nav=forward

The study found some really large differences in power:

"Average power output (W) was significantly lower for ABS FS [Full suspension] versus DS [Dual suspension] (266.1 +/- 61.6 W vs 341.9 +/- 61.1 W, P < 0.001) [266.1w vs 341.9w] ... during the offroad trials ... Power output on the paved course was also significantly different for ABS FS versus DS "

The study concluded, however, that even with the significant differences in power ... there was no significant difference in oxygen cost, or time to complete. I would suggest that this part of the conclusion is inconsistent with the first part - and is likely the result of the distances travelled being too short (at around 10 minutes +- 45 seconds).

Pod, yours is a good question because it is difficult to find a definitive answer, and it is never really satisfactory just to say "well it feels like that to me !".

Regards, MJH

pod

Interesting, I can see that there would be some loss of pedaling power. So if a mountain bike was designed with a rigid frame connecting wheels and pedals but with suspension cushioning the ride at the seat and handlebars, then there wouldn't be this loss of energy?

MikeHains

Some cyclists believe that anything that absorbs power is bad. I note, for example, that shoe manufacturers tout their best shoes as having no flex (ie transmitting every bit of power into the pedals).

Regards, MJH

pod

These were observations re the results of the test rather than conclusions. There was loud silence when it comes to an explanation of these apparantly contradicting results. Some obvious facts not mentioned are:

• 
  An 80 watt (30%) increase in power has to result in a significant increase in oxygen cost
  An 80 watt increase in power has to result in a significant reduction in time for the hill climb unless all of the additional energy was lost through the additional rear shocks. This is a ridiculously high energy loss that is extremely unlikely.
  Trained cyclists cannot suddenly produce 80 watts more. If they could produce 340 watts with the DS then why not with the FS (260 odd watts)

There are two obvious conclusions that can be drawn from this experiment.
1. There is no performance difference between DS and FS as the time to complete and oxygen usage were virtually identically. ie No loss of energy with Dual Suspension over Front Suspension only.
2. Having DS makes the SRM power reading system read much higher power levels relative to a FS system. The power has to be exactly the same if the time, weight and hill climb are the same, assuming similar environmental conditions. SRM should have a close look at this apparent failure of their system.

This is an interesting topic as I'm weighing up options for a serious MTB and the main question I struggling with is a hard tail or full suspension.

MikeHains

Easy ... ride both up the same hill. Make it a nice long hill. One way or another, you'll have your own (real) information, and will make a better decision as a result.


Regards, MJH

MichaelB

Is it not to do with the weight transfer compressing the rear suspension and the shock absorber absorbing some of the power?