Choosing The Right Frame For A Trike – And Why It Matters

Bandit 1250 Trike Frame

In the simplest terms, choosing the right frame for a trike can make all the difference.  Vehicles that have high torsional rigidity deliver a superior ride, superior handling and better response to driver input. Improving the torsional rigidity of a vehicle allows the suspension to work more efficiently and predictably. Vehicles with high torsional rigidity will see more travel in the suspension, as the chassis isn’t moving. Considering the suspension uses dampers and the chassis doesn’t, it makes sense why you want the suspension to move and not the chassis.

This is a rewrite done on an excellent lecture given by Robert “Smitty” Smith at the University Of California, Irvine or UCI on making a chassis or frame. Most of the images and the basic concepts are all taken from the video. If you want to see the full lecture you can see it HERE.

[Different Types Of Chassis]

[Trike Space Frame Chassis]

Smitty starts off talking about how he made the models to demonstrate the theory.  He used one x half inch square pine pieces. The pine pieces are available in any lumber yard across the country and are cheap.  All three models are made from the same wood, but more importantly they are made using a similar amount of pieces. He used between 235 linear inches and 238 linear inches. So for a demonstration it makes it as fair as can be.

All the joints are made in the same way.  Each piece of wood is wood-glued to another.  He constructed the models this way so we could compare the geometries of these three basic designs of a chassis as clearly as possible.

[One Half Inch Square Pine Used In All Three Models]

Ladder Frame For A Trike?

The first chassis to look at is the Ladder Chassis.  It is constructed with two long beams with a few connecting cross beams.  This chassis is exactly what you find in heavy duty trucks. When you take the cab off you will see this chassis.

Made from two very robust beams. These side beams are made of three levels of the half inch wood all laminated together with glue to make it much stronger. The cross members are two levels of half inch wood high and laminated together.

[Ladder Chassis]

Now an actual truck frame will not be quite as strong as this model as they would have to remove one of the cross pieces to allow the engine to sit there.  The Ladder Chassis has the main advantage of being very strong as a beam (down its length).  As for torsional rigidity, not so good.

Box Frame For A Trike?

The next structure is known as a Box Frame Chassis or 4 tube design.  It is four long tubes connected by cross beams horizontally and vertically with diagonals on the ends.  These frames can be used in racing as they give lots of room for the driver, engine and other mechanicals.

[Box Frame Chassis]

Space Frame For A Trike?

Lastly we have a Space Frame Chassis.  The definition of a space frame is that all the structural loads are carried in tension or compression, not in bending.  For example, if we go back to our Box Frame and put load on the long top beam, it is going to bow downwards (bending) which is bad.  The same is true if we look at the Ladder Frame. It only has the bulk (physicality) of the beams to support the pressure or weight.

[Space Frame Chassis]

So here we have three structures.  The Ladder, The Box and the The Space Frame.  They are all made from the same materials, all bonded with glue and all take up the same footprint.

Our goal with building a frame is to minimise torsional twisting or put another way, increase the torsional rigidity.

If you look at the bottom of each of the models you will find that they are fixed to the table at three points.  Each of the models have an unsupported or dangling corner.

At the corner that is unsupported we have attached an arm with a spindle on the end.  If we push down on that arm we get to see the Ladder frame actually twist or demonstrate its deflection.

So we want to measure the deflection of each of the models.  We will use the exact same system we use to test a full sized car.  Support the structure on three corners and put weight on the forth and then measure the deflection.  In this case we will use a Dial Indicator to measure the deflection. It measures down to 1/1000 of an inch increments.

So if we drop just two common nuts down the spindle on to the levering arm, we get a deflection on the Ladder Frame.

[2 x Nuts Produces Twist In Ladder Frame]

[5 x Nuts Produces Twist In Box Frame]

For the Box Frame we need to add five nuts before we see deflection.

On the Space Frame we see a very different result.  We see this because it is made totally from triangle and pyramids therefore distributing the lead around the frame.  On the Space Frame we can place a massive 45 nuts before we see twisting in the frame.

[45 Nuts On The Space Frame]

The lesson here is that Structural Rigidity comes from Geometry not necessarily from the type of materials used or the amount of those materials used.  We also will see cost savings by using a space frame because we get to use less materials in the construction of the frame to get a better result.

So overall, if we look at the three frames, the Ladder we can say has 100% torsional rigidity, the Box Frame has 260% and the Space Frame has a massive 1800% increase in rigidity.

Smitty finishes by saying that triangles are an engineers best friend.  I couldn't agree more.

[1800 Percent Increase On The Space Frame]

Conclusion Of Which Frame Is Best For A Trike?

Wow, those differences in the rigidity are amazing.  I can’t see why anyone would want to build a car or trike out of anything but a space frame.  For me, if I am going to the trouble to produce something that can go this fast and handle this well, then I want it to also have the ability to keep me safe not just increase performance.  The Space Frame certainly does all these things plus keeps the cost and weight down.

I love it.  Thanks for the video Smitty and UCI.  Very cool.

Hope this helps you guys when it comes to trikes and how to build them.