Just a quick question for you bike experts. Duman is thinking about getting wet in some sprint tris this year and was wondering if you think a rear disc will be noticably faster than my 20 spoke 30mm rim rear I have now. My HED jet hubs broke two years ago and Ive been racing on ritchey wheels that came stock on my cervelo since then and I dont think Ive noticed too much of a difference. I am actually faster than I used to be on my softride with Hed jets. I think the cervelo fits me better.
Over the short distance of a sprint tri, I don’t really think a disc is going to buy you all that much. I’d guesstimate that your time savings is going to be well until a minute (maybe something like 15 seconds?).
That being said, I plan on using my disc at all my sprint tris this year. I already have one so I might as well use it. 
Unless it is really, really hilly (weight), or has continuous sharp turns with short straights (acceleration), use the disc. It will be faster, how much faster is a matter for debate, but if you have one, might as well use it. If it’s windy, use a different (lower profile) front wheel, it’ll make a bigger difference in handling, and the disc will be even faster than in calm conditions.
There is a discussion of how much time is saved with a disc on Jim Martin’s site
In the study on the link, his “standard” wheels I assume are round spoked 32 hole rims. He did it over 40k so my sprints will be half that distance plus the fact that my wheels are somewhat aero (16 bladed up front and 20 bladed rear with 28 to 30 mm aero rims) so I figure my time savings will be less than half of what he shows. Maybe 30 to 45 seconds? So for the races I will be doing the disc isn’t really monetarily justified. If I had the spare $400 I’d opt for a Renn but alas I don’t.
Look at the results from last year of the race you aim to do. See how many people finish within a 30 second period around where you expect to finish. Then decide if you’re bothered about the number of places you’ve lost out on by not having a disc.
Duman-
The use of a disc, as opposed to a spoked Ritchey wheel, saves just as much time as a percentage of the total time of your bike split as it does at any distance. The reason that the time savings appear to be smaller is because the race doesn’t take as much total time. So in theory, and taking the time differences into account, the finishing times of a sprint distance race should be bunched much closer together when compared to longer events. This means that the disc is every bit as likely to help you gain an advantage over fellow competitors at the sprint distance as at any longer distance. The real question, as other posters have pointed out, is to figure out whether a disc is appropriate to the course you are racing, ie.- hilly or windy or technical= don’t use a disc, flat and straight= use a disc. There is no denying that a disc will be faster than the Ritchey rear wheel that you are currently using.
RipVanWinkle-
I don’t know why you would refer to my suggestions as “myths?” Is choosing equipment based on what will best suit a given course bad advice? Unless a person is a very competent bike handler and has a bike that fits, giving them proper weight distribution, or is a very heavy person, riding a disc in wind can be extremely difficult. This is why disc wheels are prohibited at the Hawaii Ironman. And disc wheels can weigh up to twice as much as aero spoked wheels, so when racing a course that is very hilly an athlete will expend a lot of energy to get the weight of the disc up the hills. In addition, while climbing an athlete’s speed is low, meaning that the disc is not saving them as much time as it might at higher speeds. The same goes for a technical course. Because you have to re-accelerate the bike wheels rotating weight coming out of every corner, a disc is not always the ideal choice.
duman just say no to swimming. Actually my friends are trying to get me back into te water after a 7 yr break. If you have a disc, use it for all but the super hilly courses. Use it in he sprint. A good idea is to check ebay for a good deal or the classifieds on this site. just remember tris = joining boy scouts, duathlons = joining the Marines
the ritchey ds wheel that comes on a 2003 cervelo p3 weighs 1035 grams (real weight on our gram scale). our current clincher is 1200 grams (i weighed four on the rack, they range from 1195 to 1180 grams). lets call it a difference of 160 grams. our rim weighs 280 as a clincher. i would think the rim on the ritchey weighs between 400 and 500 grams. i doubt the “feel” of either one of these wheels would be different giving the moment of inertia of both. so we are really talking about “dead” weight. 160 grams ain’t much. now if you go tubular our current weight is less then 1120 grams. with tires you are looking at something lighter in the disc. hills are not an issue comparing these two and is debatable in others given aero far outweighs weight.
now, stability: center of aero pressure dictates this. cobb has more on this on his site. to put it short, discs get a bad rap on the stability issue as well.
frank rehnelt, renn multisport
There have been numerous studies that suggest that discs are the best choice out there. In my own unscientific studies, I was always faster on a disc unless the race was entirely uphill with no downhill sections. I have almost always got onto the podium (the few times I have over the past few years) on a disc, I have NEVER been on a podium with nothing less than a trispoke. I have only placed once on a tri spoke, but the other times was on a disc.
Your rear wheel you currently have is within a few hundred grams of a good disc. Most disc wheels use a much lighter rim, so your moment of inertia is towards the centre of the wheel. Couple that with the superior aerodynamics AND stiffness of a disc, and the so-called weight issue is a non-issue.
A word about cross-winds: unless you are in a tornado/hurricane, the rear disc is a non-factor in corsswind handling. The front wheel is much more critical. I would suggest that a deep front wheel be saved for all but the flat, non-windy races. A good all-round setup that is currently available would be this: a Nimble Fly front with as few spokes as your weight will handle, and a Renn disc. Get the deeper front wheel as your budget will allow.
The Nimble Fly is a good lightweight wheel, within grams of the Reynolds rim in weight with a low cost to boot, and the Renn disc is the best disc for the money out there, if not the best disc period.
If the acceleration of a disc is such a downfall for using it on a “technical” course, how come most match sprinters use a disc? Last I checked, acceleration is sort of important for track sprinters, so is top end. MOMENT OF INERTIA!!! Look it up in a physics text. A disc is the way to go just about everywhere. The variety of conditions that a disc is the best choice far outweighs the conditions where it is less than ideal. There is no reason to screw yourself out of equipment that will serve you better 8 times out of ten just so you can be prepared for the two exceptions (and with the “exceptions” the disc is probably not even that significantly worse than a spoked wheel, where as the spoked wheel is significantly worse in the other 8 cases). I have to agree with RVW that some of these points you bring up are “myths” that have no basis in the science available. There are disc wheels available that weigh significantly less that discs of several years ago, and don’t weigh so much more than a lightweight spoked wheel as to make much of a difference.
JM-
Most triathletes don’t have quads and biceps the size of match sprinters. If they did, the issue wouldn’t be whether using a disc was appropriate, it would be whether they could float or run more than a mile. Joking aside, if you read my original post to the thread, I was in support of using a disc for almost every situation. I pointed out that a disc would in fact save a valuable amount of time in even a short sprint triathlon, where there are smaller time differences between competitors. I don’t want to get pinned as being anti-disc, though if I had to choose something as my only set of race wheels, it would not include a disc. That’s just me.