johngwheeler

I run 32mm Vittoria Voyager Hypers as road-tires for my CX bike, which I selected based on their low rolling resistance for this size of tire (17.7W @80psi for 35mm - not sure if 32mm is less or more).

I now have a spare set of 28mm Conti GP4000S II which I bought for a bike that won't fit them (doh!), and was wondering whether I should put these on my CX bike instead of the 32mm Vittorias.

A couple of questions:

1) Will I notice much of a difference in acceleration / speed between the 28mm Conti and the 32mm Vittoria? The Contis are about 100g lighter per tire, and the rolling resistance is c.5W less (12.2W @ 80psi vs c. 17W @ 60psi)

2) What is the subjective comfort difference likely to be between a tire at 60psi and 80psi? (I could try inflating my Vittorias to 80psi (their max.) to test this, but it's still a higher volume tire than the 28mm)

3) What handling / cornering / braking differences would I see between 32mm and 28mm?


Thanks!

John

HTupolev

What will 100g on each wheel do? For an average roadie, it might take on the rough order of a tenth of a second longer to go from 0mph to 20mph, or add 10 seconds to a 1000m climb.

As far as how they roll.

First, you can't use bicyclerollingresistance numbers as a direct expression of losses on your bike. An 80PSI tire will have different hysteresis loss on a bicycle ridden by you on the road, than it will when loaded to 42.5kg against a steel drum and then locked in place.

Second, the data you're looking at is for the 37mm Hyper, not 35mm (no such tire exists), and either way you're not looking at data specific to the tire you're comparing. A 32mm tire - assuming similar construction - will typically have greater rolling resistance at a given pressure than a 37mm tire.

But, the GP4000SII will almost certainly save you some watts, as it's a supple road tire whereas the Hyper is a touring tire.

Handwavy "answer":
If we *do* roughly scale the pressure by tire width and just take the bicyclerollingresistance data raw, you're looking at saving about 10W for the bike... which for a typical roadie is worth somewhere in the vague ballpark of a third of a mph, if you're doing around 20mph. Whether that hypothetical difference is noticeable or not depends on what you consider "noticeable."

(If I had to guess, I would estimate that the real-world difference would be slightly higher, both because the bicyclerollingresistance testing method and data theoretically goes slightly easy on stiff tires, and because suppler tires should perform better as suspension at any given pressure.)

Having higher volume doesn't make a tire softer at the same PSI. The primary reason that wide tires "are more comfortable" is that they can work better at low pressures.

But as far as the question goes, it depends on several factors.

First, different tires will have different squishinesses at the same PSI, because the tire itself is contributing stiffness. A Vittoria Corsa at 90PSI is going to be softer at 90PSI than a Marathon Plus.

Second, it depends on the riding surface. On silky-smooth asphalt, my 25mm Bontrager R3s at 100PSI feel just as comfortable as my 53mm Rat Trap Pass ELs at 35PSI. On a horrific road surface with more cracks than concrete, the latter stays pretty plush while the former gets pretty rattly.

60PSI vs 80PSI will usually be a noticeable difference, if you're making the comparison on bad chipseal. Although, it's also a bigger difference in inflation pressure than I'd normally expect to see between a 28mm and a 32mm tire. (If I'm running a 28 at 80PSI, I'd probably run a 32 at around 70PSI for the same task, maybe very slightly lower.)

With similarly-made tires at the same pressure, not a whole lot.

With similarly-made tires at slightly different pressures adjusted for width, the 32mm may feel very slightly more confident when cornering on dubious roads.

With a 32mm Voyager Hyper vs a 28mm GP4000SII... I'd probably bet on the 28mm GP4000SII feeling better. It's a suppler tire, and the tread compound is probably better.

johngwheeler

Thanks for the very complete answers! My confusion with the Voyager tire sizes is due to how they on sold on at least a couple of UK web sites (Wiggle & Planet-X):

Vittoria Voyager Hyper Folding Tyre | Planet X

"These Vittoria tyres use a deep section tread and large carcass and as such their cross section varies from their standard named sizing. The tread width measurement is given below with the ERTO size for reference.
ERTO 32x622 = 700x32c = 32mm
ERTO 37x622 = 700x35c = 35mm
ERTO 40x622 = 700x38c = 38mm"

I bought some 32mm and "35mm" Voyagers from Planet-X, and the box for the "35mm" does indeed say 37-622.

My experience with the 32mm is that they possibly slightly larger, so it seems odd that the larger sizes would inflate smaller. Depends on the rim-width, I expect. I haven't used the 37s yet, but I was planning on using them either on my commuter, or as a (dry) gravel tire.

trailflow1

Put them on and find out. Why guess or speculate ? you can do a real world test.

johngwheeler

Wise words :-)

I was debating whether I should try to return 28mm Contis and replace them with 25mm for the road bike, or try them on the CX bike. I'll probably keep them and experiment!

wphamilton

5 watts per tire, if you really get that, yes you will notice. 100 grams, it's possible you might feel the difference in some ways but as to a performance improvement, doubtful.

WhyFi

Pretty much this, I think. I think that heavier rims/tires affect handling feel (how willingly a bike will tip-in or transition from side-to-side) more than they affect the effort of forward progress.

f4rrest

If you ride the CX bike off road, keep the larger tires.

johngwheeler

I have even larger tires for off-road :-) But the 32mm Vittorias are fine for quick detour over unpaved roads or grass.

johngwheeler

It sounds like the difference in speed or acceleration will be minimal, but I might notice some difference in handling.

I guess I'll just have to experiment!

Chandne

You probably will bit only on longer and steep climbs. I do notice my sealant-filled tubes (S-Works Tarmac) on one long/steep climb that goes about 5,000 ft total...this sealant thing was done recently. The shop guy put a bit too much in there (long story) and I can definitely feel it. I jump on the BMC and I'm quicker on that same climb. On flats and rollers under 2 hours (ride time), I can't really tell though my times are just under the times on my BMC. I need to change the tubes or take the sealant out. I'd say 90% of the time, you will not actually feel the 100g difference.

Abe_Froman

I've had both 40mm hypers and 25mm gp4000s.

As others have noted...the speed difference, if there, is miniscule. The hypers for their size are really fast and also quite light/thin.

The gp4000 also has a great ride. You cant go wrong with either tire. In all honesty though, i've heard the 28mm gp4000s run big, nearly 31-32mm, so they might be basically the same size regardless of labeling. If it were me, and only 1 set of tires will be be used and the other will just sot on a shelf, i'd put the gp4000s on, but i'd be more than happy with either.

Barrettscv 

I also use the 700x32 Vittoria Hyper, mine is installed on a carbon fiber Cyclocross bike. It's a superb tire.

I have the 700x28 Vittoria Corsa G+ also.

The 700x28 is faster, mostly because I keep it above 100psi and I use the Hyper below 85psi. The performance difference is greatest on super smooth pavement. The Hyper is just as fast on some of the rural chipseal roads in my part of Illinois. The Hyper is also just as fast as the smaller tire on urban roads that are damaged by potholes and badly repaired. I also can ride the Hyper on any road, including gravel, and not worry about flats. The smaller tire will flat and is more easily damaged.

If you are lucky to ride on smooth pavement, the 700x28 will be clearly faster. If you ride on typical roads, you may not see as much of a benefit.

johngwheeler

This is a great answer; thanks! Much of my riding is on somewhat poor quality paving, particularly if I venture further afield from the main road, so it does sound as though I'm not "missing out" with the 32mm Vittorias.

I like the flexibility of findng a smooth-ish off-road section and not feeling cautious about exploring.

I've also read that the Conti 28s can expand to 31mm, so as you say, there may not be that much difference between them.

I guess I'll hang on the Contis, and try them when I feel like a change or my current tires wear out.