Optimized tires width for fulcrum wheels (racing zero db)

I am trying to get hold on their rim-tire-pressure comptability table but i cannot find it only a ‘scribd’ link seems to be available.

I am wondering which tires would best fit for these pair of wheels. the internal rim (channel) seems to be 19c and the external 23.8. https://www.fulcrumwheels.com/en/wheels/road-bike-wheels/racing-zero-db

just looking around i came across a certain 105% rule
what is this 105% rule? what would be the 105% rule for my wheelset? based on the specs.
i want to get a set of fast tires with the most aero gains on these wheels. i have my eyes set on the gp 5k tl tires. (the s-tr seems a bit too fragile).
I heard that two wide tires will ballon out or cause a lightbulb effect. ut i do not understand if this is good or a bad thing…


Hold up first. These rims are 30mm deep. That’s not aero. I suspect it doesn’t matter what tires you put on these wheels.

Now, the 105 rule is a rule of thumb. The rule holds that to maximize aerodynamics, it’s probably best to aim for the rim’s external width (23.8mm) to be 105% of your tires’ inflated width. That puts you at a tire with 22.7mm measured width. A tire that’s wider than the rim will be bad, it will add some drag. How much? I don’t know, the penalty may be in the low or high single digit watts. Not sure, probably depends on how much the overhang is.

It’s moot here, because literally nobody makes a tire that mounts to 23mm on a rim with 19mm internal width. There aren’t that many tires that are nominally 23mm to begin with (the 23mm GP 5000 non-tubeless clincher mounts at 24.5mm or so to rims that are 19mm wide internally; Conti doesn’t even make 23mm for the GP 5000 S TR). The Fulcrums have pretty old rim specs. Maybe they figured that with only 30mm depth, the rim to tire interface might not matter, so they just reused the old die (aluminum rims are extruded out of dies).

Basically, I would just put whatever width tires you were planning to run anyway. I would not have bought those wheels. My sense is that many Fulcrum wheels lag behind other manufacturers in terms of rim dimensions. This is a bit odd, because Campy’s newer wheels have more contemporary measurements (Fulcrum is owned by Campy but branded differently).

Even in a windtunnel under optimal circumstances the effects of tyre width is only marginal but I think in the real world the small aero loss is offset greatly by the lower rolling resistance/ impedance of the wider tyre

If you’ve got nice roads, I’d go 23mm front / 25mm rear.

Do you already own those wheels? I’d just put some 25mm GP5000s on them and call it a day. Since the rims are not very aero at all, it doesn’t make a lot of sense to try and aero optimize the tire to the rim.

If you don’t own those wheels, then don’t buy them unless you have an older frame with clearance limitations.

what are contemporary measurements? w

what makes them so bad. what is a common width to put 25mm wheels?
i want to use them for climbing. to replace stock wtb i23 stock tires that are a tank.

is the the inner rim width or 19c that make it bad or the fact they are not 60mm depth?

i cannot afford carbon for 1k i’d only get cheap carbon. i prefer high quality alloy than cheap carbon.

if you would upgrade to some high end alloy wheels what would be your choice?


I never said they were bad. They just aren’t an aero wheel and you were talking about optimizing tires for aero. You can just use whatever tire size you like as long as they fit in your frame.

How much are these wheels? When I search for them, I come up with $900 for a set of alloy wheels. They seem like older tech for $900. But maybe you are finding them on a deal? Anyway, you can find many carbon wheels in the same price range.

I rode a set of Campagnolo Eurus wheels for years with 25mm Continental Grand Prixs. Great wheels until I broke a spoke. It took 2 weeks and $25 to get a single replacement (bladed aluminum spokes). But then I got 50mm (25mm wide) Farsports and they were a huge improvement in speed.

my question is why are they considered obsolete or as i quote not contemporany?
which king of rim width is expected on 'today’s tires
isn’t having a wide wheels exclude some of the tires width? can a 25mm tire be safely put in a 23mm rim? without danger or burping out?
for the record i have these wheels for the last year and I wanted to upgrade the tires to some gp5k and i was just wondering what would be the right size.
another thin i wonder… most my rides are rolling hills, even with the shallow depth (30mm) i feel the crosswinds I cannot imagine what would it be with 60mm or 80mm depths.

a last question what should I look after in new wheels… what kind of internal rim width?, etc to avoid making the same mistakes.

I’d say the average new modern aero rim is coming in at 28mm wide and 21mm internal. It’s acceptable for 28mm tires which is probably the most popular size but still not 100% aero optimized. You’d have to still run 25mm tires to be within the 105% rule.

The rims pushing aero boundaries are 35mm wide and thus aero optimized with a 30mm+ tire.

Personally, unless one is racing at a high level, I don’t think one has to worry about the 105% rule at all. It’s truely marginal gains.

There’s no mistake with your current wheels. If you don’t want to ride aero wheels there’s really nothing to worry about. If you have smooth roads, 25mm tires are fast. To me, you decide what tire you want to ride and then work back from there.

Say you wanted to be fully aero optimized for 30 or 32mm tires, then you are going to need something like the Roval CLX II which has a 35mm width in the front. And you need a frame designed to fit all that. That wheel may be fastest with a 28mm tire but then you have the rim wider than the tire. That isn’t comfortable to me (easy to break a rim) and I don’t need to be on the bleeding edge of aero.

I saw a test a German mag did that a bunch of iterations of DT Swiss’s lineup. A well shaped shallow rim that’s wide is the same as a 32mm deep rim that’s a touch too narrow for the tire.

On thing to keep in mind, a tire marked 25c now might be 23mm on that rim. Also, tires marked size don’t really line up with the actual size. I’d look at BRR measurements for a given model and get one narrower than the rims external #

I have the same impression that is why i wanted to give them worthy tires but the concensus here that the tires are from another epoch,
maybe a language barrier

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Shimano’s 105 carbon wheels are 21mm internal and 28mm external. Boyd’s Altamonts, which are aluminum, are 21 internal, 25.5 mm external. I’m thinking something along those lines.

aren’t those wheels too wide for 28mm tires per instance.
running tubeless 28mm tires would fit on those 28mm external? without danger of just getting off the rim and in the process killing me?

I wouldn’t over think it. I had two sets of Racing Zeros over the years and they were fantastic wheels. Campag/Fulcrum know how to make fast and fun feeling wheelsets even if they might lose out on a theoretical game of top trumps based on the spec sheet. They didn’t hold me back one bit, whether riding up mountains, railing descents or winning crits.

I’ve ridden countless wheels from various brands and I currently have some Campag Bora WTOs and it’s the same story - not as wide as some more “modern” wheels but boy they are nice.

OP - just put a 25mm on there for great all round performance. Or a 28mm if you want a touch more comfort (but personally I think it will handle a smidgen worse than a 25mm, YMMV)

The external measurement has nothing to do with the tire size than can be fitted. The key measurement is the internal.

Thus, with 21mm internal, you can safely fit a 28mm tire. You can probably fit a 25mm tire too but it’s on the edge of being too small. The wheel maker will have specs.