How fast actually are ‘fast’ gravelly tyres on tarmac?

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  • How fast actually are ‘fast’ gravelly tyres on tarmac?
  • Premier Icon w00dster
    Subscriber

    Hi Trailwagger,
    Have a look at Hambini’s blog about aero wheels (https://www.hambini.com/blog/post/bicycle-wheel-aerodynamics-which-one-is-fastest/). There’s also a fair bit of info from ENVE and ZIPP on the subject. I will dig out some of the info later tonight.
    Its never as simple as saying 23mm is faster, or 32mm is faster. Far too many other variables and we are also talking about a small number of watts.
    So some newer wheels (ENVE SES for example) are designed for 25mm tyres. These have an internal rim width of 21mm. For these wheels the fastest tyre is the 25mm.
    My Reynolds wheels (17mm internal rim) are designed for 23mm front and 25mm back. The back is hidden away so a slightly bulbous profile isn’t an issue. Reynolds actually say of there newer wheels there is no difference between 23mm or 25mm – but I think that is them sitting on the fence. Just visually I can see the 25mm is slightly more bulbous that the 23mm race tyres I normally have fitted.
    From Zipp (https://www.zipp.com/support/faq/faq.php)
    Firecrest wheels with wider tire beds
    The best aero and lowest rolling resistance is obtained with front 23mm and rear 25mm width tires running at the recommended tire pressures.
    More details on that FAQ with Zipp even recommending 21mm as the fastest for older non Firecrest rims when performing time trials.
    There is an ENVE version of the same info which I will dig out- also on one of the other forums I frequent both a ZIPP and an ENVE engineer also involved in the discussions specifying that on certain rims 23mm is the tyre to use. (But to reiterate – that is older ENVE rims)

    In my opinion there is so little in it between 23 and 28mm that its not really worth discussing. I do find that 32mm is slower, but that just my observation.

    Premier Icon w00dster
    Subscriber

    Trailwagger – another link worth a read…
    https://www.bikeradar.com/road/news/article/cannondale-systemsix-52536/

    “The super-wide rim (well in excess of 30mm externally) and 21mm wide internally is shod with a 23c tyre, which sounds strange in these times of larger and larger rubber.

    David Devine tells us that the wider internal means that a 23c tyre actually measures up at 26mm wide when fitted, and that the bike can take up to a 30c tyre, but aero wise it’s at its best in this combination.”

    I tried the bargain sports contacts from planet x – maybe I went too big as the 42mm felt like anchors anf seemed to self steer on the front…didn’t help slashing a side wall on the first ride either. I really want the whole ‘do it all’ tyre thing to work, but no luck so far…maybe compass are the answer but can’t spare 50notes/tyre to.find out!

    Thanks for the links woodster

    bigrich
    Member

    about 5km/h slower for the same effort

    Premier Icon ampthill
    Subscriber

    One of the confusions is the difference between “faster” and “rolling resistance” and “feels faster”

    I think feels faster is largely put to bed. We use to think that harsh skipping feeling was fast, we now know its not

    Although I agree that tyre rolling resistance measurements aren’t perfect it seem pretty clear that bigger tyres have less rolling resistance. This is exactly what you would expect once you understand where rolling resistance comes from

    https://www.bicyclerollingresistance.com/specials/grand-prix-5000-comparison

    There is a Gp4000 comparison as well I think

    But of course rolling resistance isn’t the whole story. In a fast group tiny amounts of acceleration differences matter so weight is a factor. But most importantly bigger tyres are less aerodynamic. Whci is why cobbles aside 25mm is the norm for the pros. When you’re as slow as me aero matters less hence the balloon tyres

    submarined
    Member

    38c GKs fitted, 50PSI. And the answer to my question is…
    Every bit as fast as the tyres that came off. With a ‘bit’ of a tailwind, the morning commute was within 10% of my fastest time, and the fastest I’ve done in the last month, with a 2nd fastest time on one section. Journey home was a lot more leisurely for various reasons, but just as quick as most of my journeys over the past month. Excellent. £56 well spent. I’ll hack up the towpath when it dries out a tad and I’ve built some tubeless wheels.
    FWIW they weighed in at 328 and 329g. Not too shabby at all.

    I suspect any loss in speed was balanced out by the 47% increase in awesome that the tanwalls bought to the party.

    View post on imgur.com


    Thanks all 🙂

    Only downer is I now need to do the 4d jigsaw of trying to stop the ‘previously ample clearance’ guards rubbing.

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