it's all about the sound, man...

Just kidding, i've never had the chance to ride on nice carbon wheels.
but they do sound cool and look sharp.

They make a significant difference. Especially if you use fast tires on them. I would only use carbon pads on my own wheels, but they'll brake fine without them. The big difference in the pads is that you'll pick up small metal shards in your normal pads that will transfer to the carbon braking surfaces (only applies on tubular wheels....if you're renting clincher wheels, it makes no difference at all). As bad as it may sound, they're not your wheels, so spending $40 on carbon pads to protect the rental wheels is a nice thing to do for the rental company, but it won't help your performance on a flat course. Also, you can probably go for deeper wheels than 404s. Crosswinds don't affect wheels nearly as some people claim. I run a 1080 in the front and a disc in the rear for every race. Even if you've never ridden deep wheels, an 808 in the front and a 1080 in the back shouldn't cause you any trouble. Deeper = faster after all.

agree with Gator... I wouldn't rent 404's as you won't see as much increased speed as you would with 808's or 1080's. Heck even a disk is not that bad. People have a phobia that cross winds and disc/ deep wheels don't mix. When in actuality the rear wheel holds up quite well in cross winds with a disc/ deep dish. Its the front wheel that is all over the place. Mostly due to the uneven weight distribution of the riders position. Unless, you weigh 130lbs 808's will be fine and I would even go with a 1080 or disc in the rear.

CF brakes are over rated. I own my own pair of 808's that I race on and don't bother changing the brake pads. Really, how ofter am I going to brake in a race? Had them for 1 1/2 years now and are just fine.

I've got the same question, thinking of possibly renting for IMCdA. Is it worth it for a hillier course? Do they even rent for 650 wheelsize? I definitely don't want to buy wheels because I want a new bike in the next few years (sadly will not appear before CdA) and it probably won't have 650s. They're not so common anymore.

I've rented from Race Day Wheels several times, all different configurations. Did it make me faster? Who knows? But it does has a placebo effect? Yes, I thought I was faster. And for Florida, I'd get as deep-dish as possible: disc rear, 1080 front. There's nothing vertical on that course to pull those wheels up-and-over, and you're experienced enough to handle any crosswinds. FWIW, if you've never rented from RDW, Marc sends you the wheels about a week in advance. All you'll need to do is swap out our cassette and maybe tune the rear derailleur just a bit. Wheels have new tires and tubes and include a valve stem and disc wheel inflator with pre-paid return FedEx ground.

[quote=UFTriGator](only applies on tubular wheels....if you're renting clincher wheels, it makes no difference at all).[/quote]

Not arguing, but Zipp now has clinchers rims with carbon sidewall brake surfaces, which I still think is a bad idea (heat will eventually de-laminate the surface).

IMO disc wheels, actual or disc cover (i use a disc cover on a 808 as its lighter than an actual disc), is better regardless if the course is hilly or flat.

In a cross-wind the disc wheel actual cuts through the wind where as a non-disc wheel gets pushed by the wind due to the spodes spinning so fast they act like a solid mass catching the air and being pushed.

For a first IM with a planned pace of 17-18? I'm guessing maybe 5-6 minutes difference.
Calculation:
Tri-Talk testing showed 67 seconds improvement over an Oly.
http://www.tri-talk.com/Top20.asp
IM is 4.5 times the distance of an Oly, so 67 seconds improvement for an Oly would be 5-6 minutes faster over an IM. BUT, aerodynamics is more important at higher speeds. Not sure it's going to be that big a difference at 17-18mph.

Ironmon, Race Day Wheels DO have rentals for 650s. The don't have as many options available, though. My wife got them last year(IMFL) for her bike, I think you may have to go with the 404's on the front and back.

[quote=jeffgre]Ironmon, Race Day Wheels DO have rentals for 650s. The don't have as many options available, though. My wife got them last year(IMFL) for her bike, I think you may have to go with the 404's on the front and back.[/quote]

Thanks for the info! I do have race wheels that are Shimano 16-spoke carbon wheels, so I wonder if the 404s would be enough of a step up to make them worth it for me? I am alternatively looking at disc covers, ala: http://www.wheelbuilder.com/store/aero-disc-covers.html

Anyone have thoughts on these?

[quote=dkhartung]
IM is 4.5 times the distance of an Oly, so 67 seconds improvement for an Oly would be 5-6 minutes faster over an IM. BUT, aerodynamics is more important at higher speeds. Not sure it's going to be that big a difference at 17-18mph.[/quote]

I thought it was interesting that RDW's website says you'll see a greater time savings if you're a slower cyclist, because you're going to be out on the course longer. I thought it was the opposite, the faster you go the greater savings you get from aerodynamics, whereas a 15 - 16 mph cyclist will see lower returns.

@ Ironmom. The discover is the way to go, especially if you already have a carbon wheelset. Some independent test show that a disc cover (depending on which wheel they are on) is actually faster than a regular disc.

IMO, it wouldn't be worth stepping up to a 404. Go with the 808 if your going to rent. You could even just rent the front wheel only I believe (you used to be able to).

If you skeptical about the handling of an 808, most LBS's will let you take a short ride on a slightly used 404/808 to try out. Or you could email Marc at Race day wheels and talk to him about renting a set in the non-peak tri season at a discounted rate to try out. That may be your best route to go now that I think about it.