This information is targeted toward those who will be seeking to experience the upper capabilities of whatever bike they ride or race on the track, not as much for those who are just out for a spirited cruise around the track on the highest Horsepower bike they could find. By "upper capabilities" I'm talking about being within 110% of the fastest Expert Racer's lap times on that same track by a Racer who's using approximately the same Power to Weight Ratio (P/W/R) as yourself. Due to the numerous variables that effect motorcycles on a race track, the P/W/R numbers I'm going to give in this article are not hard and fixed points, they are approximate ranges that I've personally experienced (and observed many other racers experiencing as well) since I started racing in 1996. Brand new tires are of course going to spin later than used tires and in turn will be somewhat of an exception to the ranges I list, this advice is going to be aimed more toward tires that have some use on them already since most people don't use new tires for every session they do on the track. Also don't think you can't ride a certain bike with a certain P/W/R because of what I've written, this is simply a guide to assist you in making more informed decisions about motorcycle's that will be used on a Road Race course (whether for Track Days or actual Racing).
Power to Weight Ratio effects many different aspects of everyone's lives on a daily basis, yet many people are oblivious as to what it is and how it effects all of us. Everything from lifting/dragging/pushing something, to sports, to driving, to the Space Shuttle, are all effected by P/W/R.
Believe it or not, how much Power you have isn't nearly as important as your ability to use it.
All too often with Motorcycle Road Racing and Track Days many people start out thinking they need the most Horsepower (Hp) possible, but then as they evolve into being an experienced racer the realities of P/W/R become much more obvious or restrictive (even if they don't realize it's P/W/R causing it) and many racers will end up choosing to race lower Hp bikes as a result. Why? Because controllable
power is able to be applied sooner and much more effectively, usually resulting in better lap times. Excessive Hp is often times used as a crutch for lesser experienced riders who want to believe their fast - when in reality many are just racing from corner to corner and parking it in the turns. Even experienced riders who have reached a plateau in their ability (a plateau that others have surpassed with the same P/W/R) may think it's due to a lack of power, when most likely it's suspension set-up and/or corner speed/technique that actually needs to be improved. Does that mean that's always the case? No, but those 2 situations are true more often than not.
What is Power to Weight Ratio?
A slingshot is a great example I thought of that describes P/W/R in a way that's relatively easy to understand. A slingshot will produce a certain amount of Power
based on how far it's been stretched, if you pull the rubber tension bands back the same amount every time you'll have a predictable amount of power available over and over again. If you were to make a fixture to hold a slingshot in a fixed position, and provide a mechanism that would pull back on the tension bands the same amount every time, it would provide a situation that was able to be repeated predictably. To show the various effects of P/W/R all you need is some Ping-Pong balls to shoot from the slingshot fixture I described. Take a Ping-Pong ball and shoot it from the slingshot 3 times and the ball will go apx the same distance every time. Now fill one of the Ping-Pong balls with foam to add a little weight to it and shoot it 3 times, again it will travel apx the same distance every time - but it will be different than when the hollow Ping-Pong ball was used. Now try this with several more Ping-Pong balls filled with increasingly heavier materials inside each of them with the heaviest being filled solid with metal. I used a Ping-Pong ball as a constant in this example because the outside size and surface material would remain the same so the effects of aerodynamics wouldn't be a factor.
What you would see as a result of that test (using the same amount of tension on the slingshot every time) would be that the hollow Ping-Pong ball would go a certain distance and as you start to use the Ping-Pong balls filled with increasingly heavier materials you would see them shoot farther and farther, but only up to a point, then you would see the distance start getting shorter and shorter till you get to the metal filled Ping-Pong ball which would go the shortest distance. When comparing this example to motorcycles the engine would be the slingshot and the combined bike and rider weight would be the Ping-Pong ball. The motorcycle is going to have a predictable amount of power available, no matter how much the rider weighs, the difference will be the ability of that fixed amount of power to propel the various weights. The heaviest Ping-Pong ball would represent a very heavy rider (350 lbs for example), in this situation the motorcycle (I'm going to say a GSXR 1000) is only going to be able to do so much with the power it has available. The hollow Ping-Pong ball would represent the lightest rider you can find (100 lbs for example), in this situation the tables are turned, there's now not enough weight to stop the rear wheel from spinning on acceleration.
Two other VERY important aspect's of P/W/R are acceleration
and top speed
, the slingshot example is an excellent example of both of these as well. Each of the different weight Ping-Pong balls explained in my example would accelerate from the slingshot at different rates and ultimately each would reach different maximum top speeds as well - the heavier an object, the less top speed it can achieve given a fixed amount of power and the longer it will take to accelerate to that top speed. As a result, if a lighter object is able to accelerate at a rate of only 1 mile per hour
faster than a heavier object, the acceleration difference is equal to 1.466 feet per second!
In bike terms that means if someone can accelerate at a rate of only 1 mile an hour faster than you they will pull away from you at a rate of 1 bike length every 5 seconds - on a straight away allowing 15 seconds of acceleration that equals about 3 bike lengths! In a 9 lap race that 1 MPH difference in acceleration (on just that 1 section of the race track repeated 9 times) means the lighter rider could potentially gain almost 200 ft by the end of the race! Both acceleration and top speed are greatly effected by P/W/R, this is why the antiquated system of requiring VEHICLE ONLY minimum weights in many different forms of racing is being replaced by the much more accurate system of COMBINED RACER & VEHICLE minimum weights.
How do you figure P/W/R?
Power to Weight Ratio is actually fairly easy to figure if you know your bikes Rear Wheel Horsepower (R/W/Hp), all you need is 2 bathroom scales and a calculator. Your going to need to figure out your combined Rider and Bike weight: weigh yourself wearing all your safety gear head to toe, then weigh your bike using 2 bathroom scales making sure all fluids are in the bike (some people like to use a 1/2 tank of fuel in their bike as a basis for figuring weight). As a suggestion for not ruining your bathroom scales, make sure they are both rated for at least
250 lbs each (if it's a Busa or something similar you may need heavier scales) and it's not a bad idea to put a piece of wood on top of the scale to help distribute the bikes weight across the scale (not just focused in a small spot by the tires). Another suggestion is to lay down some wood next to each scale to act as a step up, this will help to stop your scale from trying to tip over or push away. I personally like to have the weights of the rider and bike separate so it's easy to calculate the P/W/R of different power and weight combination's.
Earlier I gave an example of a GSXR 1000 and different extreme's of weight between 2 riders, I'm going to continue with those numbers for an actual P/W/R example. Let's say that the bike has 160 R/W/Hp and weighs in at 410 lbs in race trim with apx a 1/2 tank of fuel, the rider weights (100 & 350 lbs) include all their safety gear. What we need to do now is calculate the combined rider and bike weight, then divide that number by the Rear Wheel Horsepower. Here's the numbers:
410 lbs + 100 lbs = 510 lbs ***** 510 lbs / 160 Hp = 3.188 lbs per Hp
410 lbs + 350 lbs = 760 lbs ***** 760 lbs / 160 Hp = 4.750 lbs per Hp
What's the desired P/W/R range for different skill levels?
There's people out there that understand what P/W/R is, but so many people don't seem to grasp the significance of various P/W/R number's and the massive effect it has on a riders capability
to be competitive. I personally have done a lot of number crunching concerning this over the years and found some interesting trends, trends that are too obvious to ignore, yet some will deny that they are relevant (ironically it's almost always lighter weight riders).
Horsepower varies based on the amount of work done to the engine, but for the most part the following are the 4 basic classifications of bikes (with some examples) and their approximate ranges of Rear Wheel Horsepower (there will be some bikes which fall out of these groups slightly, usually older models or highly modified new bikes):
Lightweight (LW) under 100 Hp (650 V-Twin's & 2-stroke 250's)
Middleweight (MW) 100 to 125 Hp (up to 640cc I-4's, 980cc Triples, & 850cc Twins )
Heavyweight (HW) 125 to 150 Hp (up to 750 cc I-4's & 1000cc Twins)
Unlimited (UL) over 150 Hp (1000 cc I-4's & almost anything else)
Today at most tracks the Middleweight bikes seem to dominate, as a result many people are quick to say that 600's are awesome. I personally believe it has ALOT more to do with the P/W/R of most of the winning racers on those bikes. Ironically many of the same racers who place well on MW bikes don't show much (if any) improvement in lap times when getting on a higher Hp bike, which doesn't seem to make sense till you factor in P/W/R. The following is an example of some Expert racer & bike combination's:
Modified 600cc MW bike / 390 lbs wet bike weight / 120 Rear Wheel Hp / 160 lbs racer with gear
390 lbs + 160 lbs = 550 lbs ***** 550 lbs / 120 Hp = 4.583 lbs per Hp
Now compare that to the same racer on a GSXR 750 with 140 Rear Wheel Hp and 395 lbs wet weight:
395 lbs + 160 lbs = 555 lbs ***** 555 lbs / 140 Hp = 3.964 lbs per Hp
Now compare that to the same racer on a GSXR 1000 with 160 Rear Wheel Hp and 410 lbs wet weight:
410 lbs + 160 lbs = 570 lbs ***** 570 lbs / 160 Hp = 3.563 lbs per Hp
Based on what I've seen, if you want to be a front running Expert level Racer (on a bike without a really good traction control system) you want to shoot for the 4.25 to 4.50 lbs per Hp range in all categories except for Lightweight - in LW you want to get the lowest possible P/W/R you can. I have a name for the power delivered around 4.25 pounds per Hp and below, I call it "Chaos Power", because once you get truly 'fast' out on the track that kind of Power gets exponentially more difficult to deliver to the ground (especially if the bike doesn't have a really good traction control system). That last example of the racer on the 1000 is a great example of 'Chaos Power', 3.5 lbs per Hp on the track is a waste for anyone genuinely wanting to go fast (other than a Top Expert or Pro Racer which will most likely be using a high-end aftermarket traction control system to help tame that power for them).
For new people to the track who truly want to learn I highly suggest something in the 6.0 lbs per Hp and higher range, this range will see a far greater chance of the tire not spinning under acceleration. In my opinion the 5.0 to 6.0 lbs per Hp general range (this is very dependant on your individual situation and could be higher or lower for you personally) is somewhat of a transitional range where your leaving the safety of not really being able to spin the rear tire very easily and reaching a point where tire spin will be somewhat random. Back when I raced my 1999 GSXR 600 I was around 5.75 lbs per Hp and I personally hated it because tire spin seemed somewhat random & unpredictable for me on that bike even though I was using the same suspension upgrades and tires as my GSXR 750's. The 5.0 to 6.0 lbs per Hp range will provide new challenges if your pushing hard
, including some tire spin on acceleration out of some turns and possible head shake from the front end being light under some acceleration situations.
From what I've seen 5.0 lbs per Hp seems to be somewhat of a general range where controlling your bike reaches a different level (that is if your within 110% of the fastest Expert racers lap times with that same P/W/R). Going much below 5.0 lbs per Hp will allow you to spin the rear tire under acceleration out of some turns (pretty much at will) and even lift the front wheel (sometimes unintentionally) under power. Below this P/W/R your now having to regulate your power delivery more carefully or your going to be learning very quickly what high-siding a motorcycle is all about.
Getting down to the magic 4.25 to 4.5 lbs per Hp range will be great for Experienced Racers who are trying to win at the Top levels of Regional racing, a blend of power with some slight remaining limitations to help them not get too awful far over their head. But going much below that 4.25 lbs per Hp threshold seems to be a major hurdle for most everyone who rides under those circumstances, at that point it's not about going fast anymore, it's about not crashing while trying to go fast!
As you may notice I'm not making suggestions about what bike each individual should get, I'm suggesting a Power to Weight Ratio
. For me on my 2002 GSXR 750 I'm at apx 4.9 lbs per Hp, If I bought a new GSXR 750 with about 10 more Hp than I have now, and I personally lost 10 lbs, I would be at apx 4.45 lbs per Hp. Your individual situation is far more important than what someone else is doing because their overall situation is probably different than yours. You may also find that you personally like a certain P/W/R range based on your personal experience, riding style, and the amount of risk your willing to take. There's not any single P/W/R range that's going to be perfect for everyone, instead what I've given here are some basic categories where the majority of riders & racers at different levels will find different degree's of difficulty.
Hope this helps out with choosing a bike for the Track!