Viewing 16 posts - 1 through 16 (of 16 total)
  • Time trialists, slower with disc wheel
  • wee-al
    Free Member

    Right, i’ve been very tempted to try and buy some speed for a while now and finally broke and bought a disc wheel. It arrived last week, i ordered the same tub as i have on my 82mm rear wheel i’ve used until now. My previous best was 22:59 (@ c280w) on my local 10m TT course (5m out uphill, dead turn 5m back downhill). https://www.strava.com/segments/1404218 . Lately i’ve been doing 23:15 or so on this and other courses @ c260-270w. I fitted the disc for Tuesday night. Conditions were fairly typical and all the other riders times were where you would expect yet i did 24:22 at 270w!?!

    I didn’t know what had happened. My minute man (normally 20-30 seconds faster had caught me before the turn and i made barely any inroads to the guy i was chasing.
    I thought my power meter cal must have been off. but after looking at my data my HR correlated what i was feeling which is that i was definitely at my limit.

    To make sure, i rode the course again today, in broadly similar conditions. All other factors, tyre pressures, skin suit, helmet, shoes etc were the same as the other TT’s (on 82mm and disc wheel). Power and HR were at the same threshold +/-.
    Results were the same. +1m odd.

    I’ve looked at brake rub, bearing friction, alignment, flex etc and the wheel seems good.

    To my mind a disc wheel (even a basic flat one) should be faster than a spoked wheel in any conditions barring possibly straight on brutal cross winds. The weight penalty isn’t huge, especially on the kind of gradient experienced in the test.

    So, is it a flywheel type effect?

    Is the lack of smooth cadence and flat power curve dumping energy into the wheel that i’m not getting back?

    Would a more expensive, lighter stiffer wheel negate some of these losses? Would i still be slower?

    Do i have to persevere and find out how to get the best out of it (that’s a big deficit to overturn?

    If you can tell me what i’m missing or otherwise make sense of my confusion, please do.

    Cheers, Al

    jambalaya
    Free Member

    Warmer / colder weather. Atmospheric pressure ?

    angeldust
    Free Member

    There are so many variables (many which you cannot control) that could affect your time, you would need a lot more data to confidently pin it down to the wheel. Try it 10 more times at least, before you draw any conclusions.

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    mrblobby
    Free Member

    Go test it. Find say a 5 min out and back. Repeate it a few times swapping between disc and spoked for back to back runs. Compare times.

    chilled76
    Free Member

    Headwind on the downs on the run with the new wheel?

    damascus
    Free Member

    Bearings on the new hub?

    Not used to the side winds?

    aracer
    Free Member

    I don’t know what it is you’re missing, but given you have two wheels to compare I’d find a suitable hill to do a rolldown test on and try them back to back (repeat a few times to eliminate experimental error). That should at least tell you which is more aero.

    RamseyNeil
    Free Member

    If I read this correctly you have swapped from an 82 mm deep section rear wheel , which is a very deep sectioned rear wheel , to a disc which could well be a significant amount heavier and have , at best , minimal aerodynamic advantage . If that’s the case then your slower time is not a surprise .

    wee-al
    Free Member

    Angeldust I have loads of data for the course on the 82mm and 2 runs on the disc at the same power, HR and cadence, in as close as conditions as the real world will allow, with the same tyre at the same pressure in all the same kit, with the same gear ratio. Surely this is enough data to pin a 1/23rd difference to the wheel? I’m not saying the wheel is at fault but it’s definitely changing something

    fifeandy
    Free Member

    @ramsey, a heavier wheel really shouldn’t much difference on a relatively flat TT, maybe 5s top, certainly not a minute.

    As others said, only way to be sure is to spend a day testing it. If race pace is 270w, go ride it 8 times(4 with each wheel) back to back at 220w.

    MaryHinge
    Free Member

    I picked up some Zipps recently, 60 front 82 rear. They just spin forever with the bike in the work stand. I was excited and looking forward to riding like a god.

    It was harder, I was slower. (Hilly TT)

    I persevered and somehow seemed to get better after a few rides, and now I use them as my main road wheels too. So yeah, you might just need to stick with it and work it out for yourself. I don’t know what I changed TBH, but something clicked.

    I felt a difference above 22mph to start with, but round here it’s not possible to hold that speed for long.

    Having said that, I was on PX 30mm alloys before, so a much bigger difference than just swapping an 82 rear for a disc.

    angeldust
    Free Member

    Interesting. Okay, if you have a large number of replicate runs on that course, you should be able to work out the variance you experience with your old set up. It sounds like you have a pretty good idea of your spread of times, which does make it more likely that the new variable (the new wheel) could be the cause. Next thing to think about is if there are any other variables the slower time could be attributed to? Tired legs, wind, temp etc. Reading your post again, it looks like you have a pretty good handle on it, but some of these variables can be very hard to identify. I would still give the new wheel a few more tries on different days before you conclude it is slowing you down.

    sockpuppet
    Full Member

    Stick the old wheel in and see what happens!

    wee-al
    Free Member

    I like the idea of the roll down tests, that should certainly give an idea of aero properties and any difference in friction between the two.

    I certainly need to give it a few more goes before it becomes the worlds most expensive frisbee.

    Maybe i need to work on my pedalling technique and smooth everything out, but that would be a huge change for me, i don’t produce a lot of torque and can be up and down the block a lot, especially on that course.

    TiRed
    Full Member

    Ride a circuit back to back at a constant: but not race power and heart rate. Swap wheels and repeat. I do this at Hillingdon where I ride 11 laps at a time.

    Although the tub is the same, is the mounting the same could it be Crr? A roll down test would help here perhaps.

    Just about to swap from a Jet 9 to a Jet Plus disc, I’m not expecting huge gains. 90 mm is a deep wheel for most yaw angles.

    cyclistm
    Free Member

    What wheel is it and was it new?

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