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| Q&A: Going Downhill with David Rogers (858 Posts)
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Back to ross the boss
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On 5/24/2005
David Rogers
wrote in from
United States
(66.14.nnn.nnn)
That road in Kimberley is nuts! I don't doubt that you bent a DH there. When John and I raced that road, we were on Comp II’s/Gumballs and had no problems bending axles.
There are a few ways to make axles stronger and more resistant to bending. You can change the material to one that has higher yield strength (the amount of force a material can take before bending), you can increase the size of the axle, or you can do both.
I’m not sure what type of Titanium you are using, but don’t count out steel. Some examples of yield strength: A grade 2 Titanium has a 40,000 psi yield strength…Grade 5 Titanium 120,000 psi (can be heat treated to obtain higher strength)…1018 low carbon steel 55,000 psi…..4142 pre-hardened steel 130,000 psi….Grade 8 hex bolt (good truck kingpins) 150,000 psi.
Titanium has great corrosion resistance and a high melting point also gives you good strength with a substantial weight savings when compared to steel. For me, the weight, corrosion resistance and the melting point of my axles is not something I worry about. If you made 10mm axles from steel with the same or better yield strength than your Titanium, your only penalty (or advantage) would be a heavier board and wallet.
You can spend a bunch of money to get trick axles...trick material, machining, heat treating, straightening, finish grinding etc. But for me…if it’s not broken, don’t fix it…the stock CompII axles have been treating me well for years.
If I had a problem bending axles, I would try to solve the problem this way:
Step 1: Change to a higher yield strength material for the axle. Leave the size the same.
If that did not work...
Step 2: Change to 10mm.
If that did not work....
Step 3: Change to 10mm with higher yield strength material.
Needless to say, John and I have not tried 10mm axles yet.
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Kriptos and 3dm...
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On 5/22/2005
Flavien
wrote in from
United States
(69.109.nnn.nnn)
Thank you Roger for all that...
But I won't go to Barrett coz' I've lost my landy last week...
I'm so pissed off right now
So the money for barrett will go in a new board
Thank you anyway
I'll try this setup next week as soon as I get my new landy
Flavien
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speed suit and drafting - minimum mph?
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On 5/21/2005
peters
wrote in from
United States
(24.18.nnn.nnn)
Does someone here know what minimum speed is req'd before a speed suit really helps shave time, and also, what minimum speed before drafting strategy makes sense? I'll be doing mostly flatland riding, so its all push+pump speed, roughly 12-18 mph for long distances, not slalom/sprints. My bike-racing friends swear a speed suit would help, but is there any real benefit at sub-20 mph?
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aero
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On 5/21/2005
roger
wrote in from
United States
(71.105.nnn.nnn)
Catching someone draft at 50-100 feet away! Must have been some fat ass with a dirty tuck you were reeling in ;)
The distance between riders seems to have an exponential effect. I don’t start to notice any difference till I get within 20 feet, at 10 feet is pretty strong, then a huge pull at 5 feet. Do you really feel a draft at 50 feet, or is that just a psychological effect of becoming a missile locked in on a target?
I’m guessing that two riders teaming together (rear rider bumping lead rather than flaring) will leave the same draft/air turbulance behind them as a single rider going the same speed (as if a single rider could). Have you found this to be true? But if the 2nd rider flares or dirties his tuck to stalk the leader, or passes, then there is a nice big pocket behind them for a 3rd rider. So how the hell do you guys stay so close without increasing the size of the draft behind you and without touching?
While on the topic, one more question. If I am stalking a rider and try to pass, their slipstream just slows me down to their speed and I end up returning to their draft. So I figured out that I have to have more distance behind them to build up enough speed to punch through their slipstream. Is this just me? Do you guys have to back off and get a running start on a pass to complete it too?
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barrett wheels
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On 5/21/2005
roger
wrote in from
United States
(71.105.nnn.nnn)
Flavien,
While 3dm Avilas are fast and get good traction, they hook back up very aggresively on rough roads and are less predictable. I like they way they hook back up quickly and popping you out of a turn - but they are a risky wheel that will look to pitch you over the board rather than pop you out. Put a radius/chamfer on them for sure - mine have chunks missing where the lip folded under the wheel and got torn off. Kludy also rides them (and prototypes) at barrett, otherwise they are rarely seen on the this road.
Kryptos 85 reds are a safe and easy wheel for Barrett cause of their big radius lips. Some of the white ones will blister, melt, and will not at all survive. Krypts are fairly common on the road. Lots of abec11 - the bros can tell you about that.
I don't know about putting krypts up front and avilas in the back. Huge difference in traction and how they hook back up and you may find the back wheels ending their slide much earlier than the front or other weird things.
Make sure you have plenty of shoe to brake with.
Roger
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Hook Up With Dudes
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On 5/20/2005
Chris Chaput
wrote in from
United States
(66.116.nnn.nnn)
I agree, that two riders can go faster than one, without having to actually touch. What I'm pointing out, is that if the guy behind the first one stands up or flares too much, it creates a big hole in the air that a trailing rider can use to his advantage. Two guys working well together can leave Mr. Solo in their dust. I have an idea! How about another analogy! We'll call this one the "Two Guys in a Cow Costume". If the head rider's costume is like a bullet and the trailing rider's costume is like the end of a torpedo, together they make a nice LONG aerodynamic shape where a lone ride is a shorter and slower. If you can get a smooth teardrop shaped object to three times longer than it is wide, it's bette than Mr. Short Rough and Stubby. I sure am glad all of this gay discussion is not happening in my forum!
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Wheel Size
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On 5/20/2005
David Rogers
wrote in from
United States
(66.14.nnn.nnn)
Lemon8: "They" did not pick 76mm...."they" probably picked 3 inches and converted to the nearest millimeter...other wheel makers followed "their" lead.
A 70mm wheel gives great performance and can be raced. The white 70mm Krypto and the 70mm Flashbacks were raced quite often at events a few years back. Most racers opt for something bigger now that we have more choices.
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Hands off my stern!
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On 5/20/2005
David Rogers
wrote in from
United States
(66.14.nnn.nnn)
Sorry to disappoint you Chris, but playing “grab-ass” is NOT required. Two riders can and do go faster together WITHOUT touching each other. Sounds like you need to come out to the Dump Rd. some Sunday for a little refresher course...ha ha. Yep...you can bring that cheezy Landy Lid (can't call that a helmet) and your speed suit...we don’t care how dorky you look. Oh!...no more boat ANALogies…pleeeeease!!
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ass
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On 5/20/2005
Andreas
wrote in from
Sweden
(83.226.nnn.nnn)
I realy love this sport.
But grabbing my best friends ass at 80km is not going to be a hit
Thanks for the tip Chris now I´m of to touch some ass!
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My Boat Analogy
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On 5/20/2005
Chris Chaput
wrote in from
United States
(66.116.nnn.nnn)
This isn't a scientific explanation, but jus a way to visualize whats happening. If there are a bunch of similar motorboats going full throttle out in a rough sea, the guy in the lead is battling the hardest to maintain speed. He smooths out a path that someone behind him can take advantage of. When the second boat pulls right behind the first boat, he has to back off the throttle to avoid running into him. If he isn't directly behind, he'll either be on his own and going the same speed or he may get caught behind the wake which is slower. If they're going the same speed and the second boat tries to go around the first boat, he will likely stall on the wake that the first boat is creating (that's the bad dirty air). On the other hand, if the second boat starts a run on the first boat from the right distance behind, he can use this head of steam to dart around the turbulent wake which will "slingshot" him out into the lead. He should time this move so that the finish line appears before he could be passed back.
Two riders can move faster than one, provided that the guy in the back is "pushing" the lead rider along instead of standing up and/or airbraking which creates a big hole in the air for a 3rd and 4th place rider to take advantage of. This is tricky as not everyone is comfortable with another guy touching his ass!
If it's a two horse race, the second guy should catch the first rider, back off with about 3/8 of mile left, get a run, slingshot past with 1/8 mile left and be way out front at the finish line. Small miscalculations means that he may have a small lead at the line, and bad timing or luck means that he never got past or he made his move too soon and ends up getting passed last.
If Dave's got a run on you, just reach out, grab his thigh and pull him behind you. He and John like that a lot. Right Boys?
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Wheel size
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On 5/19/2005 Lemon8
wrote in from
South Africa
(196.25.nnn.nnn)
Do you guys know why they chose 76mm as the size to use for a race wheel and not 77mm, 78mm or 79mm? I know different wheel sizes for different courses. Is 76mm the minimum you can go for a good race wheel and performance?
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team drafter
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On 5/13/2005
Andreas
wrote in from
Sweden
(83.226.nnn.nnn)
Tamks for the tips guys ;)
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makes sense to me
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On 5/12/2005
sean c
wrote in from
United States
(69.173.nnn.nnn)
David, thanks. it makes perfect sense, i have experienced the wall of air to the side when passing tractor trailer trucks on the highway. thanks.
sean c
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Draft Bait
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On 5/12/2005
David
wrote in from
United States
(66.14.nnn.nnn)
Hmmmmm….Fluid Mechanics…not my favorite course in school, but I learned a few things. I hope this helps more than it confuses.
The drag coefficient is a combination of the pressure drag and the friction drag. Think of the pressure drag as the difference in air pressure between the front and the back of the rider. Think of the friction drag as the force exerted on the rider as the air “rubs” along his or her leathers as they speed down the hill. I not going to write about friction drag in this response.
The turbulent wake (dirty air) directly behind a rider creates an area of lower pressure when compared to the air in front of the rider. The resulting pressure difference is known as the pressure drag. Since the air pressure in front is greater than the air pressure behind, the rider feels a force pushing him back…or sucking him back if you prefer to look at it another way. The greater the speed… the greater the pressure difference…the greater the drag. In the wake (dirty air), the air is swirling and there is backflow towards the rear of the rider. In addition to the swirling, the air is pushed (or pulled if you prefer) along in the direction of travel and picks up velocity in the direction the rider is going.
When you are drafting the lead rider from a distance, you want to travel in the wake…the dirty air is your friend. Since the wake is swirling (backflow) and moving slightly in the direction of the lead rider, the rider who is drafting experiences less resultant air velocity opposite to the direction of travel… resulting in a lower pressure drag (pressure difference). Think of the lead rider as creating a little “tail wind” that the drafting rider takes advantage of. From my experiences, the draft can be picked up from distances of 50 to 100 feet or more behind the lead rider. The only “wall” I feel when drafting is usually right off to the rear/side of the rider I am passing. I try to move through this zone quickly when passing, and leave a wider than required distance to the side of the rider. This “wall” is caused by the increase in the velocity of the air as it passes around the lead riders body since it is traveling a greater distance than the air passing by some distance to the side.
In response to Andreas on the “team” drafting: you are correct in saying that it is harder to catch two riders that are “hooked up” in a group. When one rider is directly behind another, they help each other to go faster as a group than they could by themselves. The rider behind fills up some of the volume behind the lead rider thus reducing the turbulence and wake that acts on the lead guy…the drafting rider is actually adding “pressure” to the back of the lead rider to help “push” him along. At the same time, the drafting rider is taking advantage of the lower pressure behind the lead rider to reduce his or her own pressure difference (pressure drag). As a result, two in a train are faster than one. Team tactics…hmmm interesting thought…;)
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dirty air?
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On 5/12/2005
david
wrote in from
United States
(67.100.nnn.nnn)
ha, i love it. so i guess the way to draft is to get in the "clean air" right behind somebody. my fav is when i pass somebody so fast that they don't even have a chance to draft me...sean, see you in one week at attack of danger bay 4!!
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drafting
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On 5/12/2005 mikez
wrote in from
South Africa
(198.54.nnn.nnn)
It is called the dirty air and although it does exist it is quite rare to get cought in it because if you are fast enough to come from behind the dirty air into it you are probably fast enough to break through it.
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more drafting talk
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On 5/11/2005
sean c
wrote in from
United States
(158.136.nnn.nnn)
When I think about drafting, it would seem to me that a lead rider is hitting the air and forcing it up and over them. So i understand that there is an air pocket directly behind the rider where there is less air resistance so you're able to go faster. But further behind that lead rider doesn't that air that got forced over them come back down eventually and wouldn't being in that spot cause more air resistance (kind of like going through a wall to get into the draft? The way I think of it is when your following someone (in cars) on the highway and they wash their windshield and the water gets blown over their car and eventually comes back down on your windshield. (or if you rather think of it this way, say you put your thumb over a garden hose. right in front of your thumb doesn't get wet, but a few inches back it gets wet and the water hitting it is more foreceful then if your thumb wasn't there)I'm sorry if this doesn't make any sense.
The problem is that it makes sense to me, but i have not experienced it in real life. so i guess my question is, if you're far enough behind someone is there a point where it would be more of an advantage to be off to the side instead of directly behind a rider?
sean c
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cHRIS oN SpEeD
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On 5/9/2005 KLUDY
wrote in from
United States
(205.188.nnn.nnn)
CHRIS DOES NOT KNOW HOW TO GO FAST ON A SKATEBOARD, JUST USES STUNT SKATERS. hAAAAAAAAAAAAAAAAAAAAAAH
YAH ,i AM CALLING YOU OUT YOU SHOW BIZ GUY!NO WORRYS i KNOW WE ARE GETTIN OLD BUT YOU CAN STILL TALK TO THE LITTLE GUYS NOW AND THEN!
\NEVER GET SO BIG TOO FORGET .
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aaron W email me
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On 5/9/2005
civilian2b
wrote in from
United States
(140.185.nnn.nnn)
aaron, i shot you an email, hit me back at civilian2b@aol.com
thanks
justin
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bARRET AND PAT
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On 5/7/2005 kLUDY
wrote in from
United States
(152.163.nnn.nnn)
hE DID HAVE A BALL IN MY TRUCK WITH ME!i THINK IT WAS HIM, THAT NIGTH WAS FUSSY
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barret
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On 5/7/2005 jimz
wrote in from
Canada
(24.71.nnn.nnn)
I'm not a big fan of Barret. The road is survival first, then racing. The nearest hospital is where?
Pat and I were there racing IGSA one year and he dislocated his shoulder. The EMT that was there tried to help him put it back in but no go. We had to go to the hospital. Only problem was NO ONE knew where it was. If I remember correctly the EMT was from Texas, he did a great job actually but didn't know the area. Marcus didn't have a clue where it was either, which really surprised me, shouldn’t he? He was kinda shruggin like he had a race to deal with, Pat was stabelized and helping us find the hospital wasn't a high priority. The EMT (sorry forgot his name) and I were reduced to looking for the closest "H" on a map that was dug up from someone’s car. Pat and I drove (thanks for the car will!) the 45 minutes to the "H" only to find it had been closed for who knows how many years and we had to ask directions to the next one. It was another 20 minute drive away. Poor pat's shoulder was out the entire time and he was in PAIN!
Barret's in the middle of nowhere, not only will you need a rough road setup, but you might also want to do your research on where to get fixed should you need it. The organizer just might not have that info!
Jim
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draftng
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On 5/7/2005
Andreas
wrote in from
Sweden
(83.226.nnn.nnn)
John and Dave
When it comes to drafting i know that i have to stay lower than the guy infront and stay on his tail untill it´s time and i know when to keep to the left or right of the goofy or regular footer and I´v found out that not moving you body at all is better than having the perfect tuck and still moving.
But i know nothing about team drafting...it seems to me that catching two guys is much harder than catching one guys draft.
Please help me on this one.
Be cool Andreas
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Who's Theory
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On 5/6/2005
Cliff Coleman
wrote in from
United States
(68.127.nnn.nnn)
Marcus also stated that the races at San Dimas would allow fast speeds. Dave and John, I'll let you and the others do the math.
The .6 mile course is very wide and speeds in excess of 60mph are possible.
The actual length of the course raced was .4 miles.
Here is an example where John won both the Downhill and the Luge. How many MPHs was he going?
Downhill Skateboarding Qualifying Results
Pos No. First Surname Nat Run #1 Best
1 103 John Rogers USA 48.969 48.969
Unlimited Street Luge Qualifying Results
Pos No. First Surname Nat Run #1 Best
1 92 John Rogers USA 48.028 48.028
Th is stated theory about speeds is a little off.
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Randal DH
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On 5/6/2005
AaronW
wrote in from
United States
(24.52.nnn.nnn)
Justin
I have a set of DH trucks that I can spare.
send me an email so we dont take up space on this forum
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Barrett...
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On 5/6/2005
Flavien
wrote in from
United States
(69.109.nnn.nnn)
Hi
I've planned to go to Barrett for the race in june...
I've never ridden there but I've heard that the pavement is really rough...I'm riding a Landy Evo and I would like to know with wheels would be the best for that race... I was thinking to put kriptos 85 in the front and 3dm 75 in the back... Never tried this setup... Isn't it a too 'slow" setup?
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