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Tech Article - Un-Spring, 101
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FROM: Summer 2012 VBOOST

Un-sprung weight refers to any part of a bike’s mass not supported by the springs of its suspension. In the case of the V-Max (Gen-1 or Gen-2) that would be everything you see, in gray, (in the line drawing above). To be more precise every part below the fork springs, the lower fork tubes, brakes, rotors, tires, wheels fender, fork brace & axle. In the case of the rear, that would be the swing arm, shaft, rear differential, tire & wheel, brake & brake rotor as well as the axle.

The relationship or ‘ratio’ between the sprung and un-sprung parts of a bike has a great effect on the suspension’s performance both in terms of comfort and tire grip. The speed a wheel, that hits a bump, can track the downside of that bump, without breaking contact, is determined by the acceleration force imparted, to that wheel, by the bump, when it compresses the fork springs and the mass of the wheel assembly (the un sprung weight) you’re asking it to accelerate.

The rate a spring drives the wheel down can be easily calculated by dividing the force on the spring (the sprung weight) by the mass of what’s being driven (the un-sprung weight). If the sprung weight over a bike’s front end is 330lb/150kg and the unsprung weight equals 33lb/15kg then we have a sprung/un-sprung weight ratio of 10:1. When we divide the 330lb/150kg by the 10 and we have 10g of downward acceleration.

Trim 11lb/5kg from the wheel, discs and tire and we have a 15:1 ratio and 15g of acceleration… the wheel’s acceleration changes in direct relation to the sprung/unsprung weight ratio and in the that case the wheel is able to track the road 50 per cent faster. Resulting in better tire grip. And what of the ratio’s effect on ride quality and comfort?

Let’s hit another bump; Wham, the front tire strikes an expansion joint in the highway, that has shifted, and is accelerated an inch upwards. With force equaling mass times acceleration, the lower the weight (mass) of the wheel being accelerated, the less the force being fed into the fork springs. Now this upward force is divided by the mass of the bike to give a figure indicating the un-sprung weight’s (you and the bike’s) upward acceleration.

Sticking with our un-sprung mass example of 33lb/15kg, if a force of 50N (N=Newton's, or about 11.25lb or .225lb = 1 Newton, approximate), is delivered through the fork springs, we have 0.33g of acceleration. If we leave un sprung weight the same (and thus the force at 50N) and increase the bike’s front end sprung weight to 440lb/200kg, then we feel only 0.25g of a bump. Obviously lowering the un sprung weight, and the force applied, has a similar effect.

A lower un-sprung mass also has less inertia and thus can be controlled by a ‘softer’ spring. This will also give a plusher ride. Lowering the weight of any part of a bike, sprung or un-sprung, will have advantages in terms of acceleration… that’s because thrust divided by weight equals acceleration, so either adding power or reducing weight will give a quicker bike. Losing 6.6lb/3kg from a 140bhp 628lb/285kg bike is roughly equal to gaining one extra horsepower. This equation also applies to the rider. A very good reason to go on a diet!

Should that weight have been removed from a wheel it will have even more effect on acceleration, as power/HP is used in accelerating the rotation of the wheel (along with tires, sprockets, rotors) itself. Any weight saved on these rotating parts, means less power/HP used in spinning them up to speed and a consequent increase in the power is left to accelerate the mass of the whole bike/rider combination.

As it’s spinning, the gyroscopic mass of your bike’s wheels holds it upright and gives it stability (you’ve probably noticed that when they stop spinning or slow way down the bike falls over) reducing wheel weight makes it less stable. This is bad for straight-line ridding down a bumpy road, but good when you want to ride the bike in the twisties.

At the front of the bike, wheel weight counts for even more when it comes to maneuverability. To initiate a turn the wheel must be moved off center and the less rotating mass you have, the easier (and faster for a given effort) it is to turn the bars.

The further a part’s weight is from the Center of rotation (the wheel axle) the faster it’s travelling and the greater the gyroscopic effect. Thus the weight of a tire has proportionally more influence than, say, the brake discs.

As quality & strength of a bike’s un sprung components is so important to ride quality, handling and safety, manufacturers put a lot of effort and engineering into keeping weight down in these areas and, consequently, after-market replacement parts that are lighter, yet still strong enough to the job, tend to be quite expensive. A replacement, carbon fiber or exotic metal wheel will set you back over a $1000 and will save over a couple of kilograms/2-4lb from both your un-sprung weight and gyroscopic mass. A top quality carbon fender will cost over $150.00 and save some .5lb/200g.

Converting your standard forks to USD is very expensive and requires some re-engineering to make the new USD forks fit and work correctly but this mod does move a lot of weight from un-sprung to sprung.

By far the most economical way to shed both un-sprung weight and gyroscopic mass is to fit lighter tires. As these tend to be ‘race’ rubber, many designed for track use are unsuitable for the road as stability is reduced, not only by reduced weight but also by a profile that is to easily deflected by bumps. Not to mention that these ‘lighter’ tires tend to wear out rather quickly and don’t do that well in adverse conditions… like rain!

When you look at a motorcycle keep this sprung - un-sprung equation in mind, it will explain the reason why you see so many exotic wheels and drilled out, thinner, wave rotors, made of lighter stronger exotic metal, and Billet exotic brake calibers. Most of all it is why, almost all modern bikes have USD forks! It just makes sense to mount the heaviest part of the forks upside down onto the sprung portion of the frame!

Over the years, that I have been doing this, I have heard many people state that Yamaha missed the boat when they designed the VMax. Especially the front end… From an engineering stand point, they did no such thing! The design engineers were given a set of desired ride parameters by their “front office” and then based on proven and known suspension equations they designed the VMax to meet or exceed the desired end product. Based on those parameters and the style of bike the VMax was intended to be they did an amazing job… Remember that the VMax was never designed to be a canyon carver, road racer nor a super bike! It’s sole purpose was to be the “King of the Street” Muscle bike! The Engineers also had to adhere to a specific set of ride ability and safety demands.

And here most of you thought all of these expensive “look good tricked out” parts were to make your bikes look pretty! Now you know that many of them are designed to reduce Un-Sprung weight!

I hope this Tech article has helped some of you understand your bikes a little bit better.

Later/Steve Jasse

Posted on: 2013/11/5 11:26
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Be safe out there and enjoy the ride....

Mike Moore
VMOA Webmaster






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