The Locost Suspension
helper
By Darren Jones
By Darren Jones
I made the helper spreadsheet because I am a locost builder, and like all other locost builders I don’t want to waste money on trying out different springs to achieve the desired result. The spreadsheet takes in some values from your car (relating to it’s weight and suspension geometry), and then uses these to calculate a range of spring rates depending on the natural frequency desired by you, the builder. Once you have decided on that, you then enter a ‘real world’ range of spring rates, and the spreadsheet calculates how these will work in practice.
Download the spreadsheet (.xls format, i.e. for MS Excel)
The Spreadsheet
Note: - Despite having been taught in the metric system, all the entries in the spreadsheet are in imperial units, for the simple reason that the springs that you order are measured as such - conversions would have introduced more room for error, so I have avoided that. There are only a few input variables for the front suspension, and only one for the rear (there is no leverage to take into account on a beam axle). You simply have to measure (accurately, and perpendicular to the vehicle’s centreline) the following:
- Rim Width - this is the width of the wheel-rim, from inner to outer bead.
- Lower Wishbone - this is from the centre of the lower wishbone’s pivot (usually the bush-bolt centreline) to the centre of the wheelrim.
- Inner Pivot to shock - this is from the centre of the lower wishbone’s pivot (as above) to the centre of the shock mounting hole.
Shock angle from vertical - this allows the reduction in the spring’s rate (because the spring is inclined with the damper) to be put into the calculations - entered in degrees, not radians!
- Weight - this is the weight per suspension unit - this is not an easy thing to measure, as unsprung weight comes into the equation, as well as weight distribution.
To make my measurements, I did the following - I jacked the car up and removed the ‘lock up struts’ I had made to temporarily replace the shocks. I then jacked up further until a pair of 200 lb/in springs I had lying about would fit under the front of the chassis. I then supported the wheels on either side (approximately removing the unsprung portion of the suspension’s weight), and then let the vehicle down onto the springs. Their compression (3.05 in) then multiplied by their rate (200) gave my car’s approximate weight (610 lbs). I realise that this is not the most desirable way of weighing the car, but it seems to have been fairly accurate.
You then input these figures into the spreadsheet (in the case of the rear, just enter the weight, and maybe the shock angle, although it is usually negligible - all other figures are given to give a leverage of 1) in the blue highlighted boxes on the sheet, and there are a number of calculations made, for increments of frequency in 0.1 intervals from 0.5 to 2.3.
Frequency is normally given as a measure of the sportiness of the ride - saloon cars have a figure of less than 1 while full-on race cars may be around 2. However, for the locost, a figure of around 1 seems adequate. It is also desirable to have slightly differing rates front and rear, to avoid rocking when encountering bumps that are at the resonant frequency of both ends of the car.
For more information, please see the ‘references’ section at the end.
The columns across the page are as follows:
- Wheel rate - the required wheel rate (lb/in) for that frequency
- Vertical Suspension Rate - the rate including the suspension’s leverage, but not the shock angle - i.e. if the spring was vertical then this would be its rate.
- Spring Rate - the VSR above divided by the Cosine of the spring’s angle - this is the required spring rate for this frequency
- Preload Needed (in red) - this is how much preload (at the damper, so it must include shock angle and suspension leverage) the spring needs - quite a large figure normally.
- Preload - this is how many inches of preload that the Spring Rate would need to be compressed by to support the car’s weight - as you can see for lower frequencies it is very long - such low rates are impracticable.
- Actual preload - this assumes that you are using non-adjustable spring platforms, and consequently rounds the preload given above to the next inch below, and multiplies it by the rate.
- Sag - this is how much more the wheel will move when loaded by the car’s weight - this is the remainder from Actual preload multiplied by the overall leverage.
The second stage of the spreadsheet allows you to narrow down the range required to ‘real world’ spring rates - i.e. specifying a range of spring rates (highlighted in green - just click and enter them), and then seeing the sag and frequency this generates. From this you can choose what you think is the best compromise. Those of you lucky enough to have adjustable platforms (and consequently preload) will be able to remove the ‘INT’ part from the spreadsheet to get precisely the preload you want, but you can do that yourselves if you’re that lucky/loaded. The rest of us will have to pick springs by the rate and sag that they will give for integer lengths.
If all your measurements are accurate (and my calculations are right) then you should be able to go for the lowest preload in your desired range, and get reasonable sag without too much - some sag is necessary to stay in contact with the road when it drops away from you. To get a spring’s free length, just measure the length between your shock’s spring platforms (when fully extended), and then add the integer of the preload of the desired spring rate. (For the given front example and 190 lb/in rate, with a shock of 9” between platforms, then a spring of 12” @ 190 lb/in would be the one - that’s what I went for in fact!)
Disclaimer While I have made every effort to make sure that this spreadsheet is right, I can’t be held responsible for any errors it may contain. The calculations allowed me to get the right springs on my car straight off, but this is no guarantee that I have not made some fundamental error that will cause you to spend much money. Please check the calculations and all the measurements you have made and entered. If you do spot any errors, please tell me at djaychela@triac.freeserve.co.uk , and I’ll be pleased to correct the spreadsheet and up-load it again.
References Allan Staniforth’s book “Race and Rally Car Sourcebook” is an invaluable source of information on all aspects of car construction and design, and most of the information here is from that one source. It is thoroughly recommended.