Aerodynamics and Reality - MD Readers Respond, Part 2
Aerodynamics and Reality - MD Readers Respond, Part 2
Page 3 of 3
Aerodynamic drag force is proportional to the square of the speed, but it is also proportional to the product of the frontal area and the drag coefficient. The drag coefficient represents the aerodynamic quality of the shape irrespective of the frontal area, which is why you have to multiply the drag coefficient by the frontal area before you can know the drag force on a given object moving at a given speed. When a rider sits upright, the drag coefficient and the frontal area will both increase, and it is reasonable to estimate that the drag force at a given velocity could increase by perhaps one-third as compared to a fully tucked position behind a small fairing. If the drag force increases by one-third, then the quantity of energy spent on overcoming aerodynamic drag, over a given distance at a given velocity, will increase by that same proportion, whatever it happens to be. The total quantity of energy spent over that distance will not increase by nearly as much, because only a portion of the total energy spent is spent on overcoming aerodynamic drag. (The specific distribution of the total energy spent, between overcoming aerodynamic drag vs. friction within the engine and drive train, varies with the speed, since the mechanical friction does not increase as the square of the speed.)
Acceleration is proportional to net force, net force being the difference between the drag force and the force that is applied to the bike at the contact patch of the rear wheel. When the drag force is small compared to the applied force, proportional changes in the drag force yield an insignificant proportional change in the net force. When the drag force is high, at speeds approaching the limiting speed where the applied force and the drag force are in equilibrium, the proportional change in the net force can be as great as one-half of the proportional change in the drag force.
In other words, if the estimate of one-third happens to be realistic for the increase in drag force when the rider sits upright, then the decrease in acceleration at very high speed would be no greater than one-sixth.
But is any of this relevant to street riding? It depends on (1.) how fast you like to ride, (2.) how important acceleration at high speed is to you,
(3.) how important comfort is to you, (4.) how powerful your bike is, and
(5.) whether you care about how much you spend on fuel. The bike that I ride most often, a '97 CBR1100XX, provides as much acceleration as I ever want, even when I am riding at unmentionable speeds. With the ConvertiBars that I installed, I sit almost upright, mostly behind a Givi screen.
Whatever loss in acceleration there is, I compensate for that loss unconsciously by opening the throttle more. The actual penalty for me, then, is fuel consumption. If gasoline prices get to the point where I find myself choosing between riding uncomfortably and riding slower, I will choose to ride slower.
Sportbike sales is about "cool". Ask any successful sportbike salesman. Buyers come into showrooms looking for the most current, successful race bike in street-legal trim. The percentage of sportbike buyers who take advantage of track-days or high performance riding schools is very small in comparison to total sales. When Supermoto-style motorcycles take on the "cool" of MotoGP bikes in the world of racer-boy peer pressure, their sales will rise. For now, "MotoGP-style" is the "cool" every sportbike buyer wants.
These photos were from 1978 and show me on a 400 KTM 2-stroke enduro spanking most of the Formula One bikes at a WERA race through the city streets in Terra Haute, Indiana. Sitting upright and riding like a modern Supermotard bike, I eventually finished 3rd in Formula One and I won the 410cc Superbike class on the KTM. An upright riding position is not an impediment to going fast under many riding situations.
That’s Arlund Crump behind me on # 146 and I believe that is “King of Club Racing” Ed Key on the # 300 Honda 750.
In my experience, leaning forward from a standard upright seating position increases the weight on the front end, which seems to increase stability in corners. The degree of benefit that is provided by forward lean seems to vary from bike to bike, possibly due to the fork's rake angle, and possibly due to the bike's suspension settings and weight distribution front to rear.
Many supermoto bikes (Tuono, etc.) position the rider considerably closer to the triple-tree than is the case for standard bikes. The forward seating position on supermotos might negate the benefit of forward lean in corners.
Under these circumstances, forward lean would not be an issue of aerodynamics because the benefit is felt at slower speeds, when wind blast has little impact on performance.
I have been riding for 38 years ( started when I was 16) and I have owned 50 bikes. I currently own four. I have always gone for high performance bikes. But the trend to make high performance bikes with "aggressive" riding positions have always left me out in the cold. I have tried to convince myself that aggressive riding postures were comfortable and form follows function. But now that I am old, I have realized I am much more difficult to self-delude, I find the comfort and control from a more upright riding position is answer to the question. Big HP or not. I want bars not too high but the right amount of rise to allow me to lean into the wind at normal highway speeds. My Kawasaki ZRX bars are high enough to be comfortable but low enough to negate the human-sail effect at 70 mph. My Kawasaki H1A has lower bars than stock but with about the same forward lean, but certainly not clubman bars. My Valkyrie Interstate gets by with higher bars because of the backrest. Even on my XR650R, I like the stock bars that are a bit lower than the latest dirt bike trend. And for a bike to just throw a leg over and ride around below highway speeds I always liked Dual Purpose bikes. So I guess it's a Goldilocks thing, bars certainly not to low or to high, and footpegs not to high or too far forward.
you are correct me growing up motorcross, i truly don't understand these mfg.'s a sport bike is NOT a street bike, i road in "pain" for a short time on them. eventually moving to a cbr1100xx, hayabusa, and now Honda ST1300, upright riding ,comfort ect.you know the rest. I got tired of stopping to stretch every hour and i will never trade comfort for the small advantage they have over a modern sport-turing bike in real world conditions.
You are SO right but.......why did it take this long to work it all out?? My friend used to ride his Norton Commando quicker (on a twisty tight road) than most average squids manage on todays "super"bikes. Mike Hailwood was no slouch, nor did he need to put his knee down.
Agreed!
Comfort is not inconsistent with control of the bike.
Given the choice of a Hayabusa or a B-King, a GSX or a Bandit,
the bike which allows you to hold your head up without neck or back pain,
will be the bike best suited for real world riding.
I think it depends on how you 'grew up' on a motorcycle. I started out riding one of my dad's old BMWs before I got my first bike (a '99 R6). I rode that R6 for about 15000 miles before getting on any other type of motorcycle, and when I did it was a DR-Z400. I really didn't feel very comfortable with that bike for a long time, and I'm pretty sure it was due to the upright riding position. I finally did get used to it, but when switching back and forth between the DR and the R6 I always noted how I felt like I couldn't tell what the front tire on the DR was doing, whereas the R6 seemed to provide so much feedback from the front.... the front of the DR was just always vague in comparison. After gaining a bit more experience riding several other motorcycles, sportbikes, dual-sports and standards, it's become fairly clear to me that the more upright I am on a bike, the less I feel from the front end, and usually, the less confident I am in the front tire. May not apply to all people, but for me, I just feel a lot better about riding the more front-wheel biased the weight is, which usually means a more forward, less upright riding position.
I feel a greater sense of comfort riding upright. That’s the sole reason. Well that and I never have a hard time seeing through the opening in my helmet, as some report from the world of 6oocc sport bikes. It just may be that I am 54 years old, have arthritis in my lower back, shoulders, hips, and probably my neck as well that makes an upright position on a 30 year old motorcycle better than laying down on a brand new, shiny, sport bike, so much more attractive. But I don’t think so. It is simply the riding position from hell, just doesn’t do it for me. Too soon old, Too late Smart.
This is certainly a conclusion I've reached having had many motorcycles in 22 years of riding. I live in the santa cruz mountains in northern California commuting on a BMW GS which I consider my 'supermoto' and have finally sold my Mille and am resigned that I can go as fast as I care to upright, and faster than the majority of non number plate adorned sportbikes in my neighborhood. My DR350 is my second bike perfect for the tight bumpy and beautiful back roads near my home. I have loved sportbikes but my aging knees have made my enjoyable time spent on them a shorter duration. Apart from some time spent on Laguna Seca I find myself not really missing owning a racing style sportbike. Now bikes like the new speed triple or a supermoto are starting to appeal to me as my next fun bike, something I could put my Helimot leathers back on again to ride!
I definately agree; sportbikes are becoming too extreme for the streets in some ways. For the majority of riding, there just isn't a need for such a riding position. I do find; however, find a slightly forward riding position to be the most comfortable. With that, I always modify my sportbikes with bar risers and I've ever lowered the pegs slightly on one. It would be nice if manufactuers offered their sportbikes with a touring configurement. I don't want to lose the performance of a sportbike nor the protection offered by the fairings but I would really appreciate a softer seat and less agressive seating position.
I have become convinced that riding a prone position sportbike on the street is a distinct DISADVANTAGE to the rider. There are many reasons, but my favorite right now is that the rearward seating and footpegs puts the riders weight well back from the pivot point of the steering. Any loss of traction followed by the back end stepping out is difficult to control because the riders weight is acting like a pendulum. On a motard type bike, the riders can sit very close to the steering pivot. In this case, when the rear end steps out, it has a natural tendency to re-center because the rider’s weight is well forward of the rear wheel. In emergency braking, sandy corners, swerves, rain riding and other street situations, I feel this could be a life saver.
I know that my riding style is better served by a Supermoto type bike rather than a Sportbike with fairing and a set of clip-ons. I have ridden and owned both. I feel more in control with the physical reality of having leverage from real handlebars as well as more leverage from an upright posture. If you doubt this leverage issue try a serious tank slapper on both and see the light... I was raised on dirtbikes and am sure I am biased about this but if you put me on a KTM 990 Supermoto and a another guy on an open class Sportbike and send us up a really tight, curvey road I bet the KTM is across the line first... It's just easier to throw the bike around with leverage. If there are 180 mph straights involved then the Sportbike has he edge but really how many "real world" contests involve 180 mph straights..?
While I am the first to agree that modern-day race-track-derived sportbikes are incredibly exciting, the assertion that just riding one of these is a recipe for the quickest way down a public road is probably wrong in most cases. Having spent lots and lots of time on a repli-racer I do agree that there's a certain excitement associated with feeling like you're riding the tip of a missile powered by the hand of The Almighty Himself, hugging the tank and hitting the hyperspace button. And excitement is one of the things that makes riding so enjoyable. So there's absolutely no chance I've given up on sportbikes!
As you suggest, however, I agree one shouldn't fool oneself into believing one of these tightly focused machines has the most relevance to quick transit over real-world roads. I discovered for myself one recent summer after riding the same twisty mountain road back to back on my RC51 and then V-Strom 650 (reasonable approximations of repli- racer and upright machine with wind protection, no?) that the comfortable, upright, in-control feeling of the V-Strom was conducive to a faster and certainly more confident progress on unfamiliar, tight, bumpy, realistic mountain pass roads. The superior view of the road ahead combined with greater steering leverage on the V-Strom gave a supreme confidence level that was hard to match at sub-80 mph speeds on the sportbike. I'm no Miguel DuHamel, for sure, but am fairly experienced with lots of miles on several kinds of bikes, and therefore might represent some sort of average of skill among experienced street riders.
No doubt the faster rider in a group will be faster on any of the bikes available and speedy riders do often choose sportbikes. But many a sporty-bike rider has been passed, I suspect, by a good rider enjoying the comfort, confidence, and better view of the world from the saddle of a more upright machine. This is a trend I hope to expand upon over the local roads this summer. :) I hope it's not just because I'm getting older that I suspect the benefits of most modern sportbikes begin somewhere north of 80 mph and that's just not where most of us spend most of our time if we want to do this for a good long while
The other (I think major) point that you did not bring up is Visual Field. When riding on the track not only are you going much faster you are required to look at a much smaller field. When riding my HM1100s vs. my sport bike, I find it much easy to scan the side roads.
give me a cbr1000 with the ergonmics and wind protection of the vfr800 interceptor, and I would be in heaven A comfortable superbike that could be rode all day
I agree, I like my KLR and feel totally comfortable riding through the twisties on it as fast as I dare.
Of course the leaned forward position is not needed for a street bike.
What makes me ride a sportbike is just down to the fact that I like the style and I rarely ride for any great distance so I do not need more comfort. My second bike is a V-strom which is for sale purely because I do not need more versatility than my TLR gives me.
Just like a Cadillac versus a Corvette. The Cadillac makes more sense but the Corvette just feels right.
Not being a sport bike rider I can only comment on observations made while riding "standards" equipped for touring.
My previous bikes have been a Suzuki Bandit 1200, a Kawasaki W650, a Honda Nighthawk 750 and my current ride, a Suzuki Boulevard M50. Each has been equipped with one or more windshields over its touring life, usually 75K miles or more.
I find that a full coverage shield with lowers provides a bit better gas mileage than more abbreviated shields. I think its because the rider is the most non-aerodynamic thing on a motorcycle. The large shield seems to smooth the air past the rider (and saddlebags if it has lowers) much better than smaller shields. I've consistently noticed better gas mileage, about 2-4 MPG better than with upper body only type shields.
By full coverage I'm speaking of a Slipstreamer Enterprise or National Plexifairing type vs. Slipstreamer Spitfires of Hellfires or police type shields.
In really hot weather they're a bit warm, but if adjusted so there's about a 1 and 1/2" gap over the headlight enough air enters and flows upward to help a bit. In the cold and rain it's nice to have the protection from kneecaps to head.
I live in MN so spring and fall riding make up a large portion of my season -- if I lived in AZ it would probably be different.
My two bits worth,
I agree with your statements. As a Buell XB12Ss Lightning Long rider and supermoto aficionado, I can attest to the fact that you don’t need a multitude of plastics to be an aggressive, fast rider in the “twisties”. Many of the naked and supermoto bikes available in today’s market compete favorably with modern sportbikes in typical canyon carving. I am also a track day rider, in which I ride a Honda CBR 600. At track speeds is where you see the aerodynamic differences between naked bikes and a fully faired sportbike. I have tracked both the Buell and the CBR and have found that lap times are close, but the comfort of wind protection behind fairing of the CBR makes for a safer and more enjoyable time. I tend not to tire out as quickly on the CBR due to the wind protection. I am not a fan of fighting the wind resistance on the Buell when exceeding triple digits on the track.
Agreed! I love my 2001 Yamaha FZ1. Does everything well.
I am a long time sport bike riding enthusiast. As the years began to catch up with body, I found the crouched over position was less appealing.
Luckily, the thought of looking for a more comfortable riding position coincided with the appearance of the Triumph Tiger and the Suzuki V Strom.
The BMW GS was already a likely choice but at a much higher cost. I purchased the V Strom DL1000 and found a bike that was upright, comfortable, and still blazing fast in the twisties. It was like getting a new lease on my poor body. The Tiger and GS are certainly included. I would hope that other manufacturers would take notice. Honda’s Varadero would also be a great choice if it were to come to America.
I agree with you about the aerodynamics of street riding. Until one gets to very illegal speeds, there is little need to crouch over the fuel tank. I ride a 2000 Triumph Sprint RS with a Sport-Touring riding position. The bars are elevated a couple inches above the triple clamp and the footpegs are slightly rearset. However, it works well for a slight lean into the wind at highway speeds, but no real strain on my back or wrists. (Please note, however, that I do exercise regularly and maintain a trim physique.)
If I could afford a second bike or newer sportier bike, it would be a Triumph Daytona 675. From what I have read, it offers real sport bike performance and real world street performance and relative comfort. On the other hand, a supermoto style bike would be a lot of fun and would rate well as a commuter for its control and visibility over the traffic as discussed in your recent feedback column.
I totally concur with your reasoning’s, and it is funny that it took this new Supermoto craze to bring it to light more. As a lot of us “aging” riders who have been riding on sportbikes for over 25+ years have come to find out, we still can have a whole lot of fun. I say it has come full circle. In the 70’s and early eighties we were riding around on much more upright “superbikes” that at the time were state of the art even with there flexy flyer chassis. Then like most sportbike nuts, we migrated to the latest and greatest slowly, myself as a example from the GPz line to the Ninja 900, then to the GSXR/CBR and so on for over another decade, then slowly as you realize with a bit more wisdom, that “hey, nice to be a bit more comfortable again like the old days, but with the modern weaponry “, so some of us looked for those GT class bikes like the VFR, or Bandit, but then found those neutered compared to top of the line sportbikes when it came to suspension, power, or quality of chassis.
The next step was to take a superbike and attempt to make it more comfortable with handlebar risers from the likes of Heli-Bar or LSL, a reasonable fix to the bike, but still we suffered with limited seats comfort [yes, aftermarket seats like Corbin or Sargeant do help, but still not like the seats of the one piece designs of the early 80’s] or a look that just wasn’t like the original. Finally now, consumers have lots of fun, comfortable, choices in motorcycles to choose from. If your just looking for a city/canyon bike, any of the modern sport standards [naked bikes], Supermoto’s, or adventure bikes offer upright ergo’s, are confidence inspiring around town or in your local canyons, some are of course even still fun on a track day, and all will embarrass that same full blown superbike rider in any location in the real world other than top speed. Some of course make good practical commuters to use as that excuse to sell the wife on the idea too. My personal current “perfect” bike I own and ride, the new Suzuki B-King. Definitely won’t run with Supermoto’s in the tight stuff, but is still amazingly fun for its size, agile, top quality suspension and chassis, comfortable ergo’s and seat, and un-neutered motor make it both VERY entertaining to ride, yet also a bike you can ride all day. Now I just need a Supermoto bike to complement it and all will be good in the world!
I think there's no question that you're right about riding position having little to do with going fast on the street.
But, my reality is that I'm so used to leaning my bent over torso forward and to the side going through turns aggressively, sitting up to do it seems foreign, uncomfortable and not at all confidence inspiring.
I suppose that if I were forced upright by having someone surprise me with a supermoto in my garage, it wouldn't take me too long to get used to the idea.
The main factor in motorcycle control is having your legs under your body where they can be used to control both your own weight and the bikes. The situp and beg position with your legs out front so popular with cruisers is the worst possible position for control. All you can do is wiggle your butt with very little input from your legs.
Just wanted to say that, having had a Honda Hornet (naked) and currently a Varadero(trail), I Couldn’t agree more. As a matter of fact, occasionally, some of my riding colleagues who drive sport bikes fall behind at twisty roads.
Nice write-up Dirck. The best riding position for true Motorcycle comfort and control is the position that the fewest number of bikes offer. How sad. Most of us are leaning forward with our legs scrunched-up and heads cranked-up straining our necks. Or we are leaning rear-ward and reaching out in front of us with both our hands and feet towards the controls.
Riders are denying themselves the pure "Joy" that motorcycling has to offer with so many people in un-natural riding positions. Why give up comfort and control to "Look Cool."
I too come from the dirt world and still own and ride a KTM 525. My street bike was chosen based on finding a bike that fit the riding position I felt the most comfortable with and knew put me in total control of the bike.
Too much is at stake to make choices based on others perception of what they think is cool.
Thanks for the good write-up. Let's hope others "get" what you are talking about.
Just a quick thought Dirck - I DEFINATELY felt more in control when I moved from my SZR660 sportsbike crouch to the Moto Guzzi Breva 750, which has a neutral riding position. My freinds also commented that I was straight away riding smoother, safer, and quite surprisingly faster. Over here in the UK there are quite a few people who use their sports bikes as a weekend pleasure devices switching to supermotos. They don't do big mileages, carry big loads, or cary pillions, they just want to have fun. A supermoto does that.
agree. With the rider in a more upright position the front tire carries less body weight and therefore a greater percentage of the demand on the front rubber can be assigned by the rider to cornering forces (rather than friction associated with rider weight). Also the front suspension can be more dedicated to road irregularities than simply to bearing the weight of the rider. Because inertia during deceleration throws all the weight forward both of these effects (on the front tire and suspension) are most pronounced at corner entry – which I think for most riders is the most challenging/disconcerting/fun part of any good ride. So a more upright position is best for cornering – so long as the rider can still sense feedback. My caveat to these comments is that for very high speed corners that allow/demand that the rider use a lot of body weight to achieve extreme lean angles a taller sit might make turn in easier, a hunkered down position allows higher precision inputs. And for all those reading these remarks remember that my best qualification may just be that I just got a good night’s sleep at motel 6 last night.
You nailed it, Dirck!
Much of the sportbike's aerodynamic advantage is lost unless the rider is tucked in with chin on the tank behind the minuscule windscreen -- a small percentage of the time for most who ride them in real-world daily use.
I'm a long-time (since the early 70s) roadracing fan who fully understands the appeal of sportbikes. I even went to the trouble to build a Yoshimura-engined, Rickman-framed-and-faired machine before sportbikes were available from any but a few of the "exotic" European manufacturers. For my efforts, I was rewarded with exciting performance and styling as well as theunfortunate realization that I couldn't ride the thing for more than 30 to 45 minutes at a time without significant wrist/neck/back discomfort (I won't even address the "traffic ticket" issue). More importantly, I also found that it was much more difficult to maintain a "360 degree awareness" in traffic as the "head-down" riding position and the virtually worthless mirrors made all but forward visibility difficult at best.
I still take advantage of any opportunity to enjoy rides on more current sportbikes and am amazed at the readily-available technology and incredible level of performance. While they are certainly better in every way than my early bike building effort and are capable of truly mind-boggling acceleration/braking/cornering, the disadvantages of the racing-style riding position remain the same. OK, I realize that because of my advanced age,what used to be "discomfort," would now more accurately be called "pain."
Anyway, for street/twisties/highway riding, I much prefer the more comfortable upright seating position, greater handlebar leverage, usable mirrors and heads-up attitude of most naked, supermoto, sport touring, dual sport, or adventure touring machines. Their upright rider posture makes them easier and safer to maneuver in traffic by placing riders higher relative to surrounding vehicles. This makes it easier to see over/around traffic, and equally important, to be seen. In the end, I can ride longer and stay more focused on potential threats than when mounted upon a sportbike.
Thanks for helping to get the word out, especially to those who have grown up on sportbikes and haven't yet enjoyed the pleasures of other types of bikes.
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