Tricycle steering geometry - the spreadsheets Why? There have been a few 'rules of thumb' developed over the years for practical steering linkages to give reasonable Ackermann steering. The best-known is the 'point the track rods at the rear wheel' rule, but this is only occasinally anywhere near correct, and is useless for many linkages commonly used on trikes. I couldn't find anywhere a simple program which could quantify errors for a given linkage, and could be used to optimise the design, taking account of all the relevant geometry. So I wrote these spreadsheets. You enter the variables, and the program gives you the deviation from ideal Ackermann at various turn radii. You can change most elements of the linkage geometry, so as to arrive at a practical design with minimum scrub. These spreadsheets are now used as a design aid by almost every major recumbent trike manufacturer in the world. These manufacturers also add in a lot of experimentation, detail design and development to result in their finished products. Be clear that these spreadsheets are a starting point, NOT an instruction kit to design perfect steering first time. Linkages modelled I couldn't see an easy way to model arbritary linkages, so instead created versions of the spreadsheet for the most common linkages used on trike steering: 'Conventional' geometry, with rear-facing track rods. This sort of linkage is used on commercial designs by e.g. Windcheetah, ICE, Terratrikes and others. It's also often used on cars and the like. Greenspeed-type geometry. Pioneered for trikes by Australian manufacturer Greenspeed, this type has the track rods facing forwards, and crossed track rods. Because I'm such a rubbish programmer I found it necessary to use two different sheets to model this geometry. It's still notperfect. There are particular problems when the 'Handlebar arm initial angle' value is around 90 degrees (in other words, the handlebar pivots and the two track rod pivots on the handlebar are all in line). I'm not sure why. I had to do two separate spreadsheets for the two cases of the pivots being behind (0 to -90) and ahead (-90 to -180) of the handlebar. These are what I've called Greenspeed Type 1 and Greenspeed Type 2 geometry respectively. There's a further 'quirk' that if either of the track rods crosses the handlebar pivot point you get errors. In practice, this isn't a serious limitation in most cases. Type 1 Type 2 'Direct linkage'. In this case, the rear-facing track rods are joined by a single beam. Further linkages are reasonably simple to model, but I do have very limited time to spend on this - and these should cover most applications. If you're desparate for a different linkage for some really compelling reason, and absolutely can't modify the sheets yourself (see below) then email me on peter ---at-- eland dot org.uk and I'll see what I can do. I'm much more likely to help people who appear to have made a serious effort to modify the sheets themselves already, rather than expecting a solution to fall into their lap. Restrictions The main and most obvious restriction is that the spreadsheets model things strictly in two dimensions. It assumes the kingpins are vertical, so they just appear as points when looked on from above... So when you start inclining the kingpins in various directions as discussed in the previous section, you start departing from what's modelled here. The more the kingpins (or the axis on which the handlebar pivots, etc) are inclined, or if the various pivots are at different heights, then the more you'll depart from the 2D approximation. Luckily, experience has shown that the results are usually plenty good enough even with this approximation. I may eventually do a full 3D model, but doing this is FAR more complicated than a 2D spreadsheet, both to produce and to use. For now, live with the approximation, or go away and program it in 3D yourself. How the sheets work Still to be written - sorry Previous: introduction -- Steering home -- Next: downloads