Suspensions and Unsprung Wieght

I watched a couple of your youtube videos on suspensions and have a question.

I completed a clutch driven go kart with stub axles and no suspension, but would like to add a torque converter. So I’ll need a live axle, and i thought I I’d add a suspension while I’m at it. The easiest way would be to put the whole rear part of the kart (engine and everything) on a hinge. But I’d heard that a good suspension system minimizes unsprung weight. Having the engine unsprung would seem to be a bad idea. But as you show in your “pitfall ” video, the independent rear whepel suspension looks pretty complex and expensive. I plan to use it mostly off road. It has an 8 hp engine.

Your thoughts?

Answer:

The unsprung weight issue becomes more of a problem if you are looking for high performance applications such as Baha racing and so forth.

The purpose of having a low inertia suspension is to have it return to its relaxed position quickly, so that it is not bottomed out and essentially becoming useless.

The suspension is designed to absorb the impacts that an irregular road surface exhibits.  The trouble is if you are driving along at 50 mph on a road surface in car and then you encounter a corner with a ton of bumps in it, you want the tires in contact with the ground so that the vehicle will react in turning the car around the corner instead of having intermittent road contact and essentially going straight and off the curve.

The suspension weight or mass dictates how fast the suspension is going to react.  The lighter the quicker the suspension will react.   The reaction is for two purposes:

1. Good road contact for optimum directional response
2. Bump absorbtion

We mentioned the good road contact previously, but the bump absorption can be equally important in that the vehicle needs to be able to continue to absorb impacts.  If the suspension is bottomed out, and wont return it essentially is now a solid brick and what hits it now will damage and bend elements in the vehicles such as drive shafts and rims.
On a go kart, the MPH on a rough road situation is not going to be much more than 10 mph.  Suspension response may not be as critical in this  situation.  Having the suspension respond quickly is really the question here.  Is it essential on a go kart?

Depends on how much money you are willing to spend.  If money is an issue, then the compromise of using the total flex rear is acceptable.

On thing of note on the flexible rear suspension is that the engine should be kept as close as possible to the hinge point as possible, this will lessen the Inertial moment that is require to be rotated.    The nice thing about making the whole rear move together is that the drive line is much, much simpler and durable.

The chain does not need to be worried about, because the engine and the drive axel relationship doesn’t change, and so the chain will not lengthen and shorten as the suspension moves up and down.   This is a big plus.

So to recap,

The suspension mass is critical for:
1. Suspension response and tire contact for good vehicle performance
2. Suspension response is important for the next bump, will the suspension be ready, or will it be bottomed out.

Low suspension mass is important for fast moving vehicles that encounter large bumps and repeated hammerings.  Baha vehicles for example are designed for speed, vehicle cornering performance, and bump absorption.

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