Wheelbase and Weight

[dasicvtec]

Hi

Just a question I've been thinking about recently and thought i might throw it out there and see what happens. Ill sum it up and explain what my questions is - Will a car that has the same difference in Wheelbase and Weight compared to a car with a bigger wheelbase and weight, handle similar or what will the consequences of both be ? This question refers to a dedicated race car, a car that would compete in Supersprints and so on.

My inspiration for this question comes from the BYP civic and the SS works CRX (in japan) funnily enough. Ill let you read this exert first -

"Because of the effect the wheelbase has on the weight transfer of the vehicle, wheelbase dimensions are crucial to the balance and steering of the automobile. When turning there is lateral torque placed upon the tires which imparts a turning force that depends upon the length of the tire distances from the CG. Thus, in a car with a short wheelbase, the short lever arm from the CG to the rear wheel will result in a greater lateral force on the rear tire which means greater acceleration and less time for the driver to adjust and prevent a spin out or worse."

Essentially what i gather from that quote and its relation to wheelbase is that the shorter the wheel base, the less time it takes for a force to be exerted on whatever its exerted on, so the rear tires, tires will lose grip in the same manner, but just quicker, so the rear tends to snap when cornering hard which is the main downside to having a shorter wheelbase.

So in my example of cars, even though a CRX wheelbase is shorter my understanding is that because there is less weight there to transfer, then there shouldn't be a snappy type of feel to handling, so it would handle much the same as a car with a slightly bigger wheelbase and slighter heavier weight. I'm picturing in my head how a Go-kart can handle so good but if you compare that to a civic for example, the wheelbase of the kart is a lot shorter but weighs a lot less too.

Do you understand what I'm trying to say here?

In the case of the civic - if the civic weighed 1000kg with a wheel base of 1000mm and the crx weighed 900kg with a wheelbase of 900mm - would the handling characteristics be similar ?

Any thoughts you can provide would be appreciated. thanks

For Reference -

2nd Gen CRX

2301 mm Wheelbase
1674 mm Width

5th Gen Civic Hatch

2570 mm Wheelbase
1700 mm Width

[EG30]

I have an eg which I track regularly as well as an ED9/b16a crx which I use as a daily hack car in stock trim.

the crx a lot more nimble, and understeer much less when compared to a stock eg5 with stock sway bars. You sit lower in the crx, the overall cross section of the car smaller, much better aero also than the eg.

but on the neg side the crx is flex city and need a lot of reo, engine room is very cramped compared to the eg/dc2/ek etc and no where near as much aftermarket and factory type R parts to choose from where you could interchange from the eg/ek/dc2 parts bin. Cable box a pain also. If you are going to build a track car from scratch and willing to full cage it and do the mods to retrofit a hydraulic g/box and clutch it would be a real weapon.

It will change direction quicker than the eg with the shorter wheelbase and ideal for tighter circuits at the expense of high speed stability, which you could remedy with suspension setup anyway.

[trism]

wheel base and length/width imo are more of a concern

if you have two cars, same weight, same width and same overall length, but one with a longer wheelbase, then the longer wheelbase will always handle better, with equivalent suspension type

[geeang]

wheel base and length/width imo are more of a concern

if you have two cars, same weight, same width and same overall length, but one with a longer wheelbase, then the longer wheelbase will always handle better, with equivalent suspension type

That's not necessarily true.
A shorter wheelbase will give you better 'handling' on a slower tighter circuit, a longer wheelbase will give you better 'handling' on a track with long sweeping corners.

[55EXX]

shorter the more nimble the car will be. nimble ftw.

[JohnL]

wheel base and length/width imo are more of a concern

if you have two cars, same weight, same width and same overall length, but one with a longer wheelbase, then the longer wheelbase will always handle better, with equivalent suspension type

All else being equal:

A shorter wheelbase car will react more quickly to driver inputs but be less directionally stable (i.e. will tend to spin more easily / quickly), the car will generally handle more 'sharply', possibly tom the point of being excessicely 'twitchy'. A longer wheelbase car will tend to be more directionally stable (i.e. will tend to spin less easily / quickly), but less responsive to driver inputs and less 'twitchy'.

Still with all else being equal:

A longer wheelbase may have somewhat better braking performance because less weight will transfer from the rear contact patches to the front contact patches when braking hard (i.e. in effect more 'rubber on the road' when braking, since the rear contact patches will share more of the braking load).

When 'accelerating' hard, a longer wheelbase will tend to see less weight transfer away from the front wheels to the rear wheels, so longer wheelbase is likely to be good for FWD traction off the line / exiting corners, less so for RWD.

For a given contact patch size (x4), more weight will result in less grip, less weight will result in more grip.

[redefine]

^ this is correct.

an example with the formula SAE cars at uni, some teams mount the engine next to the driver, rather then behind in order to have a shorter wheelbase. the advantage in this case is that the FSAE tracks are slow, and have alot of tight corners, meaning the car needs that responsiveness

[curtis265]

how do u think it feels to drive an FD lol

[JohnL]

an example with the formula SAE cars at uni, some teams mount the engine next to the driver, rather then behind in order to have a shorter wheelbase. the advantage in this case is that the FSAE tracks are slow, and have alot of tight corners, meaning the car needs that responsiveness

Are they actually shortening the wheelbase, or are they doing this in order to reduce the polar moment of inertia? They may well be doing both because doing either or both will tend to make the car more responsive, and more difficult to drive, espcially at higher speeds...

[dasicvtec]

Two posts from the same dude i found on another forum

http://www.moparchat.com/forums/showthread.php?t=56934

"The first thing I want to point out is that wheelbase does affect terminal speed in cornering. It has to do with polar moment of inerita(the resistance to turning about a vertical axis through the roof of the car), wheelbase leverage on that center axis and weight distribution.

First, we need to look at polar moment of inerita. This is the thing that makes the car hard to pivot in the turn or maybe a better way to look at it is this is the resistance to turning caused by the momentum of the car's mass. It comes from Newton's laws of physics. Objects in motion tend to stay in motion unless acted upon by an external force. In the case of our racecars, the external force is the lateral force the tires apply to the chassis when the wheel is turned. This force varies with slip angle, tire construction, track friction, load force and air pressure. There are alot of variables involved in generating the lateral or side force. Oh, I forgot chassis geometry too. Longer wheel base cars tend to have greater polar moments of inertia due to the weights of the suspension components. The polar moment of inertia is based on the radius of gyration, the distance from the centroid to the mass. In the case of inertia it is calculated using the square of the distance. This can be offset somewhat if the weight of the car is well below the required minimum, allowing selective ballast placement.

Leverage on the polar axis does have an effect on the relative cornering ability. Longer wheel base cars tend to react slower than shorter wheelbase cars. The longer wheelbase cars can be easier to drive and keep in a straight line. The leverage advantage is linear, however and does not make up for the increase in polar moment of inertia that usually accompanies the longer wheelbase. This is due to the radius of gyration thing we just talked about and it being raised to the power of two.

Weight distribution can work for or against you in a longer wheelbase car. The longer wheelbase allows for a greater degree of adjustability due to the distance bewteen the rear axle and the center of gravity. Large radius corners help to reduce the difference wheelbase makes in cornering. This is because the rate of rotation around the centroidal axis is slower. This requires less of the total tire traction to be used up creating side bite just to overcome the additional inertia of the longer wheelbase car. Typically, shorter wheelbase cars are lighter, allowing more ballast to be placed.

Basically, what I am saying is there are two forces involved in turning the car. One is the centripetal acceleration that must be dealt with to keep the car in a corner of constant radius. Then there is the force required to actually turn the car about its centroidal axis, represented by its polar moment of inertia. Is an of this making sense? Cars of equal weight will require the same centripetal acceleration regardless of wheelbase, but the same cannot be said for polar moment of inertia. This is where the difference is.

In theory, shorter wheelbase cars are better from the standpoint of physics. So can you race a longer wheelbase car and still be competitive? Yes! You just have to be better than the competition."

"More or less, what I was trying to say in the previous post is that longer wheel base cars enjoy the advantage of a longer lever arm due to the increased distance from the wheels to the center of the car. This leverage, however, is not sufficient to overcome the disadvantages due to the location of the associated weight of the chassis components, i.e. the rear axle. The reason for this is that the force the weight generates goes up by the square of the distance, whereas the leverage only goes up by the distance.

An example might be good. If you have a mass M located say 30" from the centroidal axis, then it would be represented by 900M or 900 times the mass, while the leverage is 30. Now lets move the rearend 2" farther back. The formula is 32 squared times M or 1024M while the leverage is 32. This is nearly a 14% increase in the mass force while the leverage shows only a 7% increase. In other words, when we increase wheelbase, the math says that the force created by the mass moving rearward increases faster than the leverage effect of the increase. I was just wanting everyone to look at the thing from a purist or mathematical viewpoint.

So how does this work in the real world. Honda built a motorcycle that had the fuel tank and other heavy items as close to the ground as possible to create a better handling bike back in the late 80's. What they found out was that the bike handled very poorly. This puzzled them for a while, until they realized the the bike must pivot around an axis that parallels the ground. By placing the weight as low as possible, they were in effect increasing the distance of the mass from this rotational axis. This led to slower reaction during rotational movements, like the one they use at the corkscrew on Laguna Seca. They determined that the closer the center of gravity and the center of rotation were, the better the bike handled. This is the same thing we are talking about here, mass centralization.

Now, before anybody flames me for implying that the pendulum effect is the only consideration, hear me out. It is an effect as real as any other. But like all aother things in racing, you need to look at what the overall picture looks like. If you have a car that has great weight proportionality and needs ballast, where is the best place to put it? The answer is what Honda found out, as close to the center of gravity as possible. Adding weight to areas farther from the COG increases the effect they have, but there is a very real tradeoff for placing it a long way from the COG. The same thing is true for side to side weight, but the distances are shorter and the effect is much smaller. Think about the Honda example. A four hundred fifty pound bike and very small distances and yet there was a measureable effect.

I liked what was said about trimming the front end of the vehicle. Every pound of front weight you remove is worth more like two pounds. One pound for the rear weight it would have taken to offset it and another for the total loss associated with it."