Use a variety of approaches in suspension design

Posted on March 5th, 2009 in Suspension,testing,tools by Julian Edgar

Suspension design is great fun, and very challenging.

I am not talking about ‘design’ as in picking which upgrade kit to buy for your car, but much more fundamental aspects like developing a whole new suspension – anti-dive percentage, camber change, longitudinal and lateral virtual swing arm lengths… stuff like that.

I haven’t done it for a car but I have applied exactly the same concepts to human powered vehicles.

When I designed the double wishbone front end for my first recumbent trike, I struggled with setting the ground rules. Like the:

• Position of front upper wishbone mount
• Position of rear upper wishbone mount
• Position of front lower wishbone mount
• Position of rear lower wishbone mount
• Position of upper ball-joint
• Position of lower ball-joint

With each location defined in three planes, that’s 18 variables. Add to that wheel offset and diameter, and inner and outer steering tie rod positions, and you’re looking at 26 or more accurate dimensions needed before you can even start.

I was hoping to use the well-known Susprog 3D software program to do a lot of the design, and obtained a free review copy. However, despite spending hours on it and exchanging many emails with program designer Robert Small, I couldn’t make it work.

Right now I am designing the front suspension for a recumbent bike. I want to use longitudinal links that will allow me to specify the amount of anti-dive that occurs through the travel of the  front and rear suspension. I also want to closely control the variation in trail and steering rake (castor).

The guru in this area of two wheel design is Tony Foale, who has produced a software program to model these types of suspensions. I’ve bought a copy and have been trying to use it.

But blow me down, just the same thing is happening as occurred with Susprog 3D! In this case, I have found at least two problems in the program which largely invalidates its use in my application.

I am in contact with Tony and, as I write this, he is being very helpful in trying to sort out the issues.

But I am starting to think that while using suspension design software sounds really trick and lets you try out lots of configurations, in the real world it might in fact be more accurate and easier to make a full size model of the proposed suspension and then directly measure what occurs.

For example, it’s relatively easy for me to make out of wood and screws a model of the longitudinal links suspension design, move the assembly through its full suspension travel and then directly measure the outcomes. In fact, I have already been doing this in a rough and ready way to look at castor change and get a feel for the amount of anti-dive.

But to get it accurate to within a degree (castor) and a millimetre or two (trail) and (say) 5 percentage points (anti-dive) requires only a bit more work to the model – eg no slop in the pivot points and more accurate measurement of the actual and virtual arm lengths.

And if you directly measure what occurs with a full-size model, there’s no wondering if software is introducing its own tweaks and bugs – what you see is what you get.

I think that if you’re doing suspension design, it might be best to use a combination of sketching, full-size modelling and software modelling – and not rely on just one approach.

4 Responses to 'Use a variety of approaches in suspension design'

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  1. Ross said,

    on March 5th, 2009 at 8:26 am

    Now you know why most motorcycles (and bicycles that have suspension at all) still use telescopic forks for the front suspension, despite their drawbacks.

  2. BG said,

    on March 5th, 2009 at 11:35 am

    A slightly similiar approach I’ve used a number of times is to do essentially the same as what you’re doing with the sticks, except in CAD.
    Starting by fixing one or two variables like the more important pickup points, track etc., then extend the concept by drawing in the arcs of travel of each arm etc., it’s possible to accurately model suspension movement. For example, in our solar car, I used this approach to keep tire scrub less than 0.5mm across the total range of wheel movement, while having an appropriate amount of camber change to compensate for body roll (ie if the car isn’t sitting flat because it’s on a cambered surface, the tyre will still be approximately perpendicular)

  3. Rob said,

    on March 6th, 2009 at 1:24 pm

    I never could get into the whole ‘must have more power’ mentality or the fascination with engine building and brand name componentry that is still alive and well given what I read in enthusiast motoring publications. Suspension design is far more challenging and mentally stimulating it’s just that far too few understand that.

    Julien, try out some multi-linked virtual pivot type setups too.

  4. wayne said,

    on March 9th, 2009 at 7:23 pm

    To get an accurate, but adjustable model, you could make triangulated components from threaded rod, one end a simple bearing, the other (adjustment) end of the rods locked in by a nut either side of a (fixed) spherical bearing. Just keep adjusting until it you get what you want, or disprove the design. I had the same problem with susprog and decided to just make a model.