“Science is organized common sense where many a beautiful theory was killed by an ugly fact.” -Thomas Henry Huxley
When Elden Racing Cars owner Brian Hampsheir hired me as Designer-Engineer to recapture their glorious former racing car fortunes, I received the opportunity to interview their previous designer. He provided lots of good engineering history and perspective, but he claimed as fact one jaw-droppingly false physics/engineering idea.
He did not understand weight transfer for race cars. In fact he had it backwards. He believed that the more weight transfer side-to-side – the faster a car could corner. I believe he even bragged that Elden’s Formula cars were narrower than all of our competitors.
Weight Transfer Brief
Weight transfer occurs when during cornering, braking and accelerating – the weight (or load carried) on each tyre changes due to cornering, braking or accelerating.
The (weight transfer) load increases during cornering on the outside tyres, during acceleration on the back tyres, and during braking on the front tyres.
Conversely the load decreases (sometimes to zero) during cornering – on the inside tyres, during braking on the back tyres, and during acceleration on the front tyres.
You’ve probably seen a motorcycle or bicycle performing a “wheelie.” A wheelie is an extreme version of weight transfer where all the weight from the front tyre is transferred to the back tyre.
Another common example occurs during high-g cornering when racing cars sometimes lift one or both of their inner tyres off the ground.
A rarer type is when a racing motorcycle lifts its rear wheel off the ground during braking, called a “Stoppie.” Alarmingly, this can occur at any speed — even when exceeding 150 mph or more – though rarely as dramatic as this photo.
Somehow Elden’s former designer had acquired the backwards belief that for faster cornering more weight transfer is better.
In road racing reality (physics) the reverse is true. Less weight transfer means higher performance. The less sideways weight / mass load a tyre has – the better it can resist the load.
Wider Car = Less Weight Transfer = Faster Cornering
The wider a car is (its “track”, the sideward distance between wheels) the less weight transfer during cornering. This gives more traction and faster cornering.
The reason is that instead of just the outside tyre doing most or all of the work – the inside tyres are contributing traction more equally. More weight transfer makes the outside tyres take more load. Less weight transfer allows the inside tyres to help with the cornering. Similarly the longer the wheelbase, the less weight transfer during braking and accelerating.
I wonder if my design predecessor was confusing vertical tyre loads (or pressures) with sideways tyre loads.
Sideways loads are almost purely from weight-mass of the vehicle. We always want to minimize sideways loads (at least those loads towards the outside of a turn).
All cars and motorcycle tyres have vertical loads from the weight / mass of the vehicle. Non-mass vertical loads come from aerodynamics: wings, body shape and ground effects. Tyre traction does increase with increased non-mass (aerodynamic) vertical loads, but it generally decreases with increased weight-mass sideways loads.
This is why we always want to maximize non-mass vertical loads and minimize mass loads.
- Summary of Generalities
Weight/Mass Transfer = Bad (or slow)
Aerodynamic (vertical) downforce = Good (motorcycles don’t yet use aerodynamic downforce – but that’s for another article)
The result of this misunderstanding was the older Elden racing cars were too narrow. The short version is : wider car = faster around corners, narrow car = slower around corners.
Well, that made my first contribution to Elden easy. I increased the track of the cars by several inches and with no surprise (to me anyway) — the otherwise identical cars immediately went around corners faster and had faster laps times at Brands Hatch. (Same engine, tyres and driver in dry conditions.)
So what does that have to do with racing motorcycles?
The same misunderstanding or tradition appears to be going on in racing motorcycles, but in a different direction. With the Formula cars the problem was side-to-side weight transfer was too high. With racing bikes the problem is way too much back-to-front weight transfer.
It couldn’t be more obvious that there is far too much weight transfer – because it is so painfully easy to get a bike to do a wheelie – and lift the rear wheel during braking.
(Take a look at this video of Casey Stoner going off track at Laguna Seca in 2008 because his rear tyre “got light” while braking — and then ask him if he thinks eliminating nose stand dynamics might be worth investigating.)
Once I braked too hard going into Sears Point’s Turn Two slightly leaned over at maybe 105 mph. It hard to forget the odd feeling of the Kawasaki 350’s rear tyre trying to join me up front — without any sliding . . . :-( . . . Yes, I made it around the corner; a bit slow, eyes wide open, but otherwise just fine.
Here’s a photo I took of Nicky Hayden on his MotoGP bike at Laguna Seca this weekend. His front tyre is off the ground – while he is still cornering hard. That means his bike has all its weight transfered to the rear tyre.
While this does not hinder his acceleration much (because the front tyre doesn’t provide any propulsion) this does mean all cornering power from the front wheel is lost; and because he is accelerating this also reduced the cornering power of his rear tyre.
Because a longer wheelbase would lessen weight transfer with the same acceleration – the front tyre could still be on the ground providing (more than zero) cornering power. Within limits, the longer the wheelbase – the more the front tyre adds cornering power while accelerating.
Faster Acceleration and Braking
Consider for a moment — What is the limit to how hard a racing bike can brake? Because 2011 racing tyres are so sticky and disk brake systems are now adequate, it no longer has anything to do with tyre friction or effective brakes.
Answer: The limit is when the rear tyre lifts off the ground and bike control is sharply decreased. As noted above braking too hard with the current short wheelbase MotoGP bikes and the rear tyre lifts off the ground – even at speeds above 150 mph. I’m guessing that even makes Kenny Roberts a bit squeamish.
Same thing for acceleration – what is the limit to MotoGP acceleration? Answer: The point at which the bike wheelies and steering control is lost.
Both of these limits can be removed by increasing the bike’s wheelbase; and the improvement will be noticeable and measurable.
Part of the reason a longer wheelbase works is it lowers the front to back “dynamic center of gravity.”(2)
The “dynamic center of gravity” is different than, and somewhat independent of, the standard center of gravity. The dynamic center of gravity is the standard center of gravity related to the horizontal distance to where the tyres touch the ground. This means that by lengthening the wheelbase you can have the same standard center of gravity height, yet have a lower “dynamic center of gravity.”
The lower the center of gravity in any direction – the less weight transfers – and the more friction the tyres can deliver.
While a short wheelbase may help street bikes’ walking speed maneuverability, and let you park in smaller spaces, a longer wheelbase will allow MotoGP bikes to lap notably faster by accelerating out of corners faster and braking harder into corners.
Of course if you make the wheelbase (or track width for cars) too large other problems may show up. However, I don’t believe we are anywhere near that limit for racing bike wheelbases.
I suspect MotoGP bikes could increase their wheelbase by 20% to 30% to 50% and maybe more without any significant side effects. There could be other benefits because there will be more room to put components. (Does a larger fuel tank attract anyone? or bigger, but lighter batteries for electric racing bikes.)
Take a look at this long wheelbase bike which was called the “quickest and fastest stock bike in the world” improving upon the otherwise identical short-wheelbase version which was merely quick.
I realize this BMW in the photo is intended for drag racing, not road racing, but you might pause and consider – why do drag bikes and cars (dragsters) need long wheelbases?
The main reason is outlined in this article – weight transfer. Imagine a car dragster with 2,000+ horsepower — on the short wheelbase of a street car. How would it handle that power? It would be difficult to prevent a wheelie, just like on a motoGP bike. In any case the driver would not be able to use the full amount of power available to the wheels – just as MotoGP riders cannot always use full throttle or full braking – even when not cornering.
Longer Bike = Less Weight Transfer = Faster Accelerating, Braking and Cornering
There is no doubt in my mind that significantly lengthening MotoGP wheelbases would provide notably stronger braking, faster acceleration and slightly better cornering.
More subtle benefits should be improved control (and rider confidence) when combining braking and cornering, and when accelerating during cornering.
Update: After Colin Edwards tested a longer wheelbase: “I’ve gone back to a longer wheelbase setting … the longer wheelbase has made it probably the best bike I’ve ever been on for the last part of this track.” (1)
Update: I’ve been thinking about how to test whether a wheelbase is long enough. My first guess/hypothesis is – if a bike can easily do either a nose stand or a wheelie – the wheelbase is too short.
My reasoning is that – a nose stand and a wheelie are overwhelming proof of total weight transfer – which is the opposite of what we want.
Formula 1 cars don’t wheelie or nose-stand and they have about the same power-to-weight ratio as MotoGP bikes ~ 1.4 pounds per horsepower. The big differences are wheelbase (and polar moment of inertia which is closely related) and center of gravity height. While center of gravity height has some bike racing tradeoffs, a mildly longer wheelbase does not.
I look forward to when we can all race bikes where we can brake and accelerate as hard as possible and both tyres stay on the ground – especially during cornering. We’ll leave today’s short wheelbase bikes in our dust.
While this article is copyrighted, there is no patent on this concept. This concept of longer wheelbases is free for anyone to use. Free, as in free beer ! Which I’ll be buying for everyone at the finish line when the first road racing bike using this concept wins a major race. For real.
1. Full quote from Colin Edwards:
“It was a reasonable day without being spectacular. I’ve gone back to a longer wheelbase setting I haven’t run since winter testing and it’s similar to what Valentino (Rossi) and Jorge (Lorenzo) have been running. There are some advantages and some disadvantages but I’ve still got a small issue on the front like I had during the last race in Catalunya. I’m not able to turn into the corner as quickly as I’d like, and it feels like I’m waiting on the front before I can go through the corner. But the longer wheelbase has made it probably the best bike I’ve ever been on for the last part of this track. It’s so stable in the fast sections and I’m able to make up a lot of time, so there’s some good and some bad with it.”
2. There is not yet an official engineering term “Dynamic Center of Gravity (DCoG).” This is a term I developed to relate the center of mass to the width and length of a vehicle. While there may be a better term such as “Overturning Moment“, this term has the advantage that it is easy to remember (and makes some sense as it is related to a lower CoG). The lower the DCoG, the more effective a road racing car’s or motorcycle’s tyres are for turning and braking.
Weight Transfer — A good overview from the perspective of Formula 1 racing cars. (There’s only one tiny error – that has no bearing on bike dynamics.)
Street Bikes Too: I remember getting surprised with a road bike cornering wheelie – when I upshifted a bit too sharply with a passenger on the back. (One slight difference between my cornering wheelie and Hayden’s is — I wasn’t sliding the rear tyre on asphalt – the fraction of a second previous to my wheelie. ;-)