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Is there any way to quantify the advantage (in seconds) of a disk (Hed or other) over a Jet 90 rear in a 40K race? I currently have the c2 Jet 90 and I am considering buying a disk if the advantage is significant at a 24 mph average (155 lbs if that matters).
Thanks.
Bump.
no because it depends how windy it is

but a disc a lot cheaper anyway so...

Is there any way to quantify the advantage (in seconds) of a disk (Hed or other) over a Jet 90 rear in a 40K race? I currently have the c2 Jet 90 and I am considering buying a disk if the advantage is significant at a 24 mph average (155 lbs if that matters).
Thanks.

Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter
Is there any way to quantify the advantage (in seconds) of a disk (Hed or other) over a Jet 90 rear in a 40K race? I currently have the c2 Jet 90 and I am considering buying a disk if the advantage is significant at a 24 mph average (155 lbs if that matters).
Thanks.

I created a computer simulation a while back that models this based on yaw/drag data of various wheels around a circular 40K course -- so all yaw angles are represented and weighted appropriately.

I have the following inputs for you:

Jet 90 and Jet Disk
24 mph average
155 lb

Let me know what wind speed you are interested in and I'll run the numbers for you.

-Mike

"100% of the people who confuse correlation and causation end up dying."
That would be very much appreciated. I would love to see the data at 4 and 8 mph. It seems like our early morning races rarely see winds above 8-10.

Thank you again.
Ok. Bear in mind that this is only a model, with the following known limitations:

1) Relies on published yaw data, pieced together from manufacturers and "independent" sources. Also includes seat-of-the-pants extrapolation and interpolation where needed. The yaw data also does not account for any differences in test setup (e.g. tires).

2) Data is on "generic disk" and Jet 90 (not C2).

3) Simulation is tuned to 21.5 MPH baseline, not 24 as requested. (24 MPH would have the affect of lower average yaw angles, so would skew the results toward those with slightly slower wind speeds.)

4) Simulation is for "naked" wheels and does not account for frame/fork/rider interaction. A good rule of thumb would be to de-rate rear wheel time savings by 50% to account for "dirty air" effects.

Jet 90 @ 4 MPH wind: 21.70 MPH and 6.5 degree weighted average wind angle (WAWA).

Disk @ 4 MPH wind: 21.76 MPH and 6.5 degree WAWA.

Jet 90 @ 8 MPH wind: 21.16 MPH and 12.8 degree WAWA.

Disk @ 8 MPH wind: 21.55 MPH and 12.45 degree WAWA.

So, at 4 MPH wind speed, the disk buys you 0.06 MPH and at 8 MPH wind speed, the disk buys you 0.39 MPH. I'll leave it to you to calculate time gaps with that info. ;)

EDIT: Disclaimer: I make no claims as to the accuracy of these numbers. I have no affiliation with any wheel manufacturer.

"100% of the people who confuse correlation and causation end up dying."
Last edited by: MOP_Mike: Sep 7, 09 18:53
Thanks a lot that really helps when you put some numbers to it. Any chance you could run similar data comparing Jet60 vs. Jet90's?
Same assumptions and disclaimers as above. For the old non-C2 Jet 60:

4 MPH wind: 21.42 MPH at 6.64 deg WAWA

8 MPH wind: 20.99 MPH at 12.91 deg WAWA

"100% of the people who confuse correlation and causation end up dying."
Last edited by: MOP_Mike: Sep 7, 09 19:28
Thanks again. Very useful stuff. Wonder how this would work out with the new C2 version...I suspect the differences would likely be the same if the wheels changed (improved) relative to one another.
mike, those speed calculations, they assume that it is a rear wheel with the associated loss in advantage being at the back of the bike and shielded by the seat tube?

Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter
mike, those speed calculations, they assume that it is a rear wheel with the associated loss in advantage being at the back of the bike and shielded by the seat tube?

Nope. Those are for "naked" wheels with the "best" yaw data I could find at the time (which was over a year ago.) You should de-rate any time savings by...oh, say...50% for a rear wheel.

My sim has many flaws. I never intended to publish any results. I just built it to guide my race wheel purchase decisions last season -- which is why it's all tuned to a 21.5 MPH baseline. (I ended up with a Jet 60 C2 front and Jet Disk rear.)

What is really startling is how small the differences are among the various aero wheels (with a disk being the big standout). The non-disk differences are, IMO, so small that they are lost in the noise of external factors and simulation limitations.

EDIT: The sim is really a line integral around an ideal flat circular course at constant power and under constant wind velocity vector. I was curious what kind of yaw angles I would really experience at different wind speeds, given my baseline speed and power. (Yes, in any crowd other than ST, my engineering geekiness would be subject to much ridicule.) ;)

"100% of the people who confuse correlation and causation end up dying."
Last edited by: MOP_Mike: Sep 7, 09 19:30
Mike, so am I looking at this right that would I benefit more overall by going from a Jet60 front to a Jet90 front than by going from a Jet90 rear to a disk, given the 50% reduction in the difference you calculated? Is this still true (even more true) if riding at 24mph?
Mike, so am I looking at this right that would I benefit more overall by going from a Jet60 front to a Jet90 front than by going from a Jet90 rear to a disk, given the 50% reduction in the difference you calculated? Is this still true (even more true) if riding at 24mph?

I think you read the numbers right.

At 4 MPH wind, the numbers say you'd be better off going with the deeper front.

At 8 MPH, you'd be better off going with a disk...unless you use the 50% degradation estimate.

Again, this is all with old, stale yaw data. The latest wheels may differ. I ended up choosing the Jet 60 front over the 90 because I was sure I could use it in just about all wind conditions. If I had unlimited funds, I'd choose a deeper front wheel (or HED 3!) for low-wind days, and a mid-deep, easy handling wheel for windy days. As, JM says, I'd always choose the rear disk.

"100% of the people who confuse correlation and causation end up dying."
Last edited by: MOP_Mike: Sep 7, 09 20:09
Is that 50% a conservative estimate? Even with only a 30% reduction the front option looks pretty good at 8 mph. Bottom line is I already have a Jet 60/90 combo and don't want to buy two more wheels. It looks like I'd be better off with just buying a 90 front, in most cases.
Is that 50% a conservative estimate? Even with only a 30% reduction the front option looks pretty good at 8 mph. Bottom line is I already have a Jet 60/90 combo and don't want to buy two more wheels. It looks like I'd be better off with just buying a 90 front, in most cases.

I can't put a confidence number on the 50% figure. You'd have to poll the true aero experts like J Cobb, A Coggen, R Chung, Chicanery, or Tom A for a better estimate.

Don't overlook the handling consequences of a 90/90 combo. It's been said, and I've even bought into it myself that a rear disk moves the center of lateral pressure rearwards and improves handling. I don't quite buy that any longer. Yes, it does move the CLP rearwards, but it also increases the magnitude of the lateral force by an offsetting amount. Further, the rear disk's location in the frame gives it no effect on steerer tube torque. Nevertheless, it does have a gyroscopic stabilizing effect due to its higher moment of inertia. It's this higher MI that I believe is really behind the improved crosswind handling of a rear disk.

"100% of the people who confuse correlation and causation end up dying."
Last edited by: MOP_Mike: Sep 7, 09 20:22
Is there any way to quantify the advantage (in seconds) of a disk (Hed or other) over a Jet 90 rear in a 40K race? I currently have the c2 Jet 90 and I am considering buying a disk if the advantage is significant at a 24 mph average (155 lbs if that matters).
Thanks.

Ummm...how about just buying a wheelbuilder.com cut cover for the Jet90 and not worry about it? Should be significantly cheaper than buying a dedicated disc.

That's what I use on my rear Jet 90. I even had it trimmed WITHOUT the mounting holes for the hardware. I just tape it in place with 3M electrical tape.

The only issue with a cover is if you need to comply with UCI requirements for some reason...

http://bikeblather.blogspot.com/
Speak of the devil...

What do you think about the 50% figure, Tom?

"100% of the people who confuse correlation and causation end up dying."
Is that 50% a conservative estimate? Even with only a 30% reduction the front option looks pretty good at 8 mph. Bottom line is I already have a Jet 60/90 combo and don't want to buy two more wheels. It looks like I'd be better off with just buying a 90 front, in most cases.

I can't put a confidence number on the 50% figure. You'd have to poll the true aero experts like J Cobb, A Coggen, R Chung, Chicanery, or Tom A for a better estimate.

Don't overlook the handling consequences of a 90/90 combo. It's been said, and I've even bought into it myself that a rear disk moves the center of lateral pressure rearwards and improves handling. I don't quite buy that any longer. Yes, it does move the CLP rearwards, but it also increases the magnitude of the lateral force. Further, the rear disk location in the frame gives it no effect on steer tube torque. Nevertheless, it does have a greater gyroscopic stabilizing effect due to its higher moment of inertia. It's this higher MI that I believe is really behind the improved crosswind handling of a rear disk.

I'm not sure if you quite have the right take on the handling bit...here's my take:

A deep wheel on the front tends to put a torque on the steering axis that turns the wheel away from the side wind since the center of pressure is in front of the steering axis...BUT, what does that do? It actually causes the bike to turn INTO the wind due to the countersteering effect (i.e. to initiate a turn to the right, one actually starts the turn by steering the wheel to left. This causes the CG of the bike to shift down and to the right, initiating the right lean of the bike).

Now then, what happens at the rear with the disc? Well, the wind will tend to tilt the bike AWAY from the wind gust...thereby counteracting the front wheel wanting to tilt the bike INTO the gust. Make sense?

Of course, all of that also doesn't take into account any "splitter plate" effect of a solid disc on the rear of a bike, which can actually help decrease the drag at zero yaw.

http://bikeblather.blogspot.com/
not sure what bike you have- but HED has a program that will tell you this information- for several different framesets- if you give them a call on the phone. I asked about the bike i own- a P3C- and if I recall correctly- i asked= for a 1 hour time trial, at approx 26-27 mph, with an 8 degree yaw- what's the difference-

disc- fastest
Jet 90- about 14 seconds slower

so- basically very little difference over 60 minutes with the above scenario on a bike that shields the rear wheel very well. which lends me to believe that the time difference between bump, flat, standard discs- is marketing hooey. going to a wind tunnel and refining your position could save you minutes.
Last edited by: mlinenb: Sep 7, 09 20:30
Tom:

How does the cover look with the tape? Not to be prissy about it but can you make it look reasonably good with tape without mounting holes? Any problems with this setup? I assume you just leave it on all the time.
Speak of the devil...

What do you think about the 50% figure, Tom?

IIRC, the 50% figure is a bit on the high side...I thought it was that a rear wheel was 60-70% effective (i.e. reduce the time gains by 30-40%)...in fact, Tom Compton at Analyticcycling.com has notes from the Greenwell paper where he states this:

"The drag on the rear wheel is reduced by 25% due to the seat tube."

http://bikeblather.blogspot.com/
A deep wheel on the front tends to put a torque on the steering axis that turns the wheel away from the side wind since the center of pressure is in front of the steering axis...BUT, what does that do? It actually causes the bike to turn INTO the wind due to the countersteering effect (i.e. to initiate a turn to the right, one actually starts the turn by steering the wheel to left. This causes the CG of the bike to shift down and to the right, initiating the right lean of the bike).

Now then, what happens at the rear with the disc? Well, the wind will tend to tilt the bike AWAY from the wind gust...thereby counteracting the front wheel wanting to tilt the bike INTO the gust. Make sense?

Good Stuff! I'll buy that. That's the first time I've heard that explanation.

"100% of the people who confuse correlation and causation end up dying."
Thanks for the info.. I ride a Specialized Transition S-Works and I suspect the numbers would be very close as the frames are comparable aerodynamically , at least from I have seen.
Tom:

How does the cover look with the tape? Not to be prissy about it but can you make it look reasonably good with tape without mounting holes? Any problems with this setup? I assume you just leave it on all the time.

You tell me...here are a couple of "action shots" from last month's local TT:

Yeah...I leave it on there unless I have a reason to take it off...for example, like for a road race I had scheduled last weekend. In fact, I just put it back on there this afternoon...took all of 10-15 minutes to do so.

The setup seems fairly sturdy...then again, it's not like I've ridden it in rain and stuff ;-)

http://bikeblather.blogspot.com/

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