Max speed difference

[sam.g]

I've often pondered why my maximum downhill speed on the Hors Cat consistently exceeds the max speed on my CSi by 1 to 2 mph? My favorite weekday ride includes a significant hill which I ride both ways and regardless of weather, time of year or phase of the moon, and I always seem to end up with a higher max speed on the Hors. Both bikes have been fit and properly set up by the same Serotta dealer and are within 1 cm in size (55 square for the Hors and 56/55 for the CSi). Both bikes have the same Specialized Pro computers and a mostly D/A grouppo. The only significant component difference is that the Hors is shod with D/A Open Pros with alloy nips and 14/15 bd spokes, the CSi is shod with Ultegra Open Pros with brass nips and 14 guage spokes, same pressure in Michelin Race 2's on both.

Could the DKS suspension make this much of a difference or would the marginally lighter wheels have an input?

Sam

[Louis]

Wheel diameter calibration on the computer?

[dirtdigger88]

are both speedos calibrated the same?

bike weights- a heavier bike will descend faster- though I doubt 2 mph

how rough is the road? are you bouncing all over the place or swerving to avoid bumps in the road

maybe the cat fits better overall and you feel more comfortable on it so you go faster

maybe it is some space- time continuim (sp?) thing- that is my Trekkie sideburns talking there

Jason

[alembical]

It is possible, but the first thing that I would check is the total distance for the trip on both computers. If this is the exact same, then the calibration is not an issue, but I suspect that the Hors computer says the route was just a little bit farther.

Alembical

[sam.g]

Both computers are calibrated the same, I've recently re-checked to be sure.

The slightly slower CSi is the heavier of the two bikes.

Regarding the road surface, average without major potholes or bumps to dodge, not fresh asphalt but certainly not chip 'n seal either.

Finally fit, I feel equally comfortable on both frames for the first 30 or so miles, after that the Hors starts to shine. The downhill is fairly early in my route at about the 5 mile mark.

Sam

[dirtdigger88]

then it has to be the space time continuum thing

http://www.megspace.com/arts/stc/

jason

[dirtdigger88]

how about this

http://theory.uwinnipeg.ca/physics/quant/node1.html

jason

[dirtdigger88]

last best hope

http://www2.slac.stanford.edu/vvc/theory/relativity.html

jason

[sam.g]

Alembical,

Although I've rechecked the wheelsize calibration, I have not checked the total distance on both bikes for the exact same route. My route usually changes due to traffic lights and other embelishments added. Total distance should tell all, but I still like the space-time continum explaination.

Sam

[flydhest]

what are the two max speeds?

Are we talking 14 versus 16 or 58 versus 60?

[JohnS]

Is your position slightly lower on the Hors? Different handlebars and stem length could stretch you out and make you lower.

[Sandy]

It would certainly be more significant (I guess) if the speeds were much smaller, but the why of the consistency of the results is still the question.

Statistician Sandy

[sam.g]

For the same hill ridden hundreds of times, I've come close (39.8 mph) but have never broken 40 mph on the CSi. However I consistently see 40 mph plus, even up to 41.5 mph on the Hors.

Sam

[Spinner]

why not move the computers from one to the other and then give it a try?

[M_A_Martin]

Which wheel is the computer set up to read?

What are the widths of the respective rims? If one is slightly wider, the same tire on both will have a different overall diameter at the same pressure.
At least that's my take on the issue.

[keno]

stop at the crest of the hill and coast down and let each bike hit maximum speed and compare.

keno

[sam.g]

Both computers are set up and placed the same on each frame, and both bikes are fitted with F1 forks by the way. I've not exchanged computers which will be an easy test.

The rims are the same, Open Pros, however the CSi is silver, the the Hors has black. Quick, someone jump in and postulate a differential due to heat dissapation resulting in tire pressure changes between silver and black rims.

However, I'm now convinced the differential must be in the computers regardless of my calibration.

Thanks all,

Sam

[M_A_Martin]

Don't forget your coconuts

[musgravecycles]

Have you checked the bearing adjustment in the hubs...

[M_A_Martin]

And we want to see grease samples...

[Len J]

Both computers are set up and placed the same on each frame, and both bikes are fitted with F1 forks by the way. I've not exchanged computers which will be an easy test.

The rims are the same, Open Pros, however the CSi is silver, the the Hors has black. Quick, someone jump in and postulate a differential due to heat dissapation resulting in tire pressure changes between silver and black rims.

However, I'm now convinced the differential must be in the computers regardless of my calibration.

Thanks all,

Sam

It's either

1.)a slight difference in computer calibration

2.) Bearing differences in the wheels. Either, Quality, lubrication or load. This would be especially tru the longer the distance between brakeing and measuring the Max speed. This would be my vote. I've got several different sets of wheels, and some spin easier and longer than others. Less friction = more speed.

Len

[dirtdigger88]

I would agree with the hub theory- My top speed COASTING with my King hubs vs my Mavic or DA yeild a slightly slower top speed- its the drag from the ring drive- but I am talking slight- like a few tenth that all

Jason

[CJH]

I agree! Switch computers. But try switching wheels as well?!

[keno]

Rather than going through the hassle of switching computers, measure the times going down the hill using a few landmarks, say telephone poles, and compare on the two bikes and the corresponding speeds. Alternatively (or additionally), pick a longer distance, start the computers at zero, and compare the computer mileages over that distance.

keno

[Sandy]

Keno smart! Keno very smart!

Stupid Sandy

[David Kirk]

I like Keno's test.

I also like roll down tests. I use them to check tire pressures and rolling resistance.

Pick a slight grade that goes a 1/4 mile or so onto a flat. Use the same start spot at the top and coast down the hill and mark exactly where the front wheel stops. Do it 5 or 6 times and average the results. Switch gear ( wheels, tires, air pressure....whatever you are testing) and repeat. You'll get real results.

This type of test does a good job of mixing rolling resistance with aero resistance with the differing speeds.........a lot like real life I think.

Dave

[Ken Robb]

that's a pretty small difference but at about 40mph I think the biggest variable would be in aerodynamic drag where a slight difference (nobody has mentioned the butted vs. straight spokes yet) in drag could explain the gap. Narrower bars, slightly better position (not necessarily lower but smoother), q-factor pushing knees out a bit, etc.

[zap]

What about environmental factors such as wind direction and speed, air temp. and humidity.

[SGP]

have you tried to duplicate the results with another rider?
are the clothes & shoes that you are wearing the same on each try?
:confused:

[sam.g]

OK, I'm leaning toward computer differences as the chief culprit. So on my next opportunity, I'll try the roll down test from a stationary point without pedaling as mentioned earlier. To eliminate any differences in clothing and atmospheric conditions, I'll need to pack both bikes in the car and take a few measurements on each bike in quick succession (which should render a leasurely 25 miler into a series of hill repeats). Three downhill runs on each bike is about all the effort that I'm prepared to invest, the hill is roughly 1/3 mile long and has a few 12%+ sections.

From all that I've read, differences in rolling resistance between Ultegra and D/A hubs should be negligible. However, could aerodynamic differences between 14 straight guage and 14/15 db spokes at ~40 mph be measurably significant? Both Open Pro wheel sets have 32 spokes laced 3x.

Sam