Locost: Fixing the handling

I have been really looking forward to this phase of Locost ownership: the development! With the car on the road I can now make changes and feel the difference in the engine and the handling almost straightaway. Back in my previous job I used to do this in a high tech racecar simulator, but I felt the real learning would always be on the road in my own car. Here goes!

Super scary to drive

I have put approximately 100 miles on the Locost since it first hit the road. These have been spirited drives to get to know the handling and deal with any reliability issues, as well as a few commutes to work. In short: I have been dodging winter showers.

The car has been really struggling with poor straight-line stability. Once its in a corner it feel “okay” but in a straight-line it would dart in and out of bumps, and generally wonder across the road, even though I was holding the steering dead straight. This has been really hindering my enjoyment and generally slowing me down. I can’t push the car without holding on to the steering wheel for dear life, which doesn’t seem entirely right!

With this in mind I put the car up on axle stands and reviewed the setup.

Setup “A”

For the first time in pretty much ever I am going to put my cards on the table and show you my setup. If for any reason you decide to use these numbers, you can do so at your own risk.

I’m not racing anyone other than myself, so there is very little value in these numbers other than in comparison to other setups that I might choose to run on the car, or you might choose to run on your car, so enjoy!

VariableFrontRearUnitNotes
Camber-1.1 / -1.1-1.0 / -1.0degAt design ride height with setup pins in place
Spring Pre-load-3.0 / -6.04.0 / 4.0 mmAt the damper, full droop
Castor4.6n/adeg
Toe In-22mmDelta front to back across the axle over 300mm
Tyre Pressures1818psiCold
Spring Rate200120lbs/in
35.0321.02N/mm
Motion Ratio1.6701.128Wheel / Damper
Spring Rate @ Ground71.7194.3 lbs/in K/MR^2
12.5616.51 N/mm
Track Width14401390mm
Roll Stiffness227.30278.40Nm/deg0.5*K*w^2
* (pi/180)
% Roll Stiffness Forward44.95%
Weight245295kgExc. Driver, Full tank, Wet
% Weight Forward45.37%

So there you have it, all my numbers for the world to see. But which of these could be causing my instability issues? Well three of them stood out to me as potential causes for concern: the front castor, the front toe-in and the roll stiffness distribution.

Front Castor

So what does Castor do and why should we care? Here is a picture I stole from suspensionsecrets.co.uk. Some of their written text is a bit dodgy but their pictures are great!

That looks about right!

Increasing Front Castor:

Castor is the angle from vertical made by your upper and lower ball joints, as viewed from the side of the car. Increasing this angle moves the steered axis further forward of the centre of the tyre contact patch on the road, which is approximately where the lateral and longitudinal force is applied; this measurement is known as the “trail”. This causes the steering effort to increase for a given amount of load on the tyre, and the self centring moment to increase.

Why would that help my issue? Well I have straight-line stability problems and increasing the self centring moment means the whole upright assembly, and steering is less prone to wander in response to external steering inputs, like pot holes and bumps. It wants to go in a straight line.

From my experience working with small lightweight single seaters, albeit virtual ones, I know that 4.6deg front castor is very low for this weight of car. I remember Formula 4 cars running in the region of 7 to 10 degrees. In fact the weight on the front axle is very important when picking a castor angle, the lighter the car the more castor you have to run for a comfortable steering load.

The Downside:

I actually reduced the castor down to 4.6deg back in the summer of 2020, and for good reason. I have been running in the region of 7deg for a long time and was happy with the steering feel but I was really concerned about the additional negative and positive camber it induced at high steering angles.

If you look at the following pictures you can see the masses of inside wheel positive camber at medium to high steering angles. With the amount of camber gain that my suspension has its actually quite difficult to control camber, especially when the car pitch’s forward and backward.

But as you might have guessed, I have reluctantly put the castor back into the car as the steering lacks self centring and ultimately stability; its all a balancing act.

“How do you adjust the castor on your car?” I hear you ask? Well its really easy. The upper wishbone can slide backwards and is held in place by shims. This moves the upper balljoint backwards and increases the castor. It should be clear in the picture below:

Front Suspension with toe/camber plate and damper pins attached

Eventually I plan on changing the front suspension geometry to be a bit more of a compromise between pitch and roll, taking into account additional camber induced by castor.

Bonus Round: Bump Steer

Having increased the castor back up to 7deg I rechecked the bump steer and it was very very high; my notebook says +3mm toe out over 64mm of axle bump. I had to shim the outer tie-rods downward by about 10mm to remove any bump steer.

This was likely the source of a major handling issues I had back in the day while doing autosolo. In fact, the car was very unstable over bumps and would wiggle around when rolling into a corner; the following video kind of shows that.

I’m glad to have dialled this out. I have learnt an important lesson over the years: You can build a beautiful looking well engineered car but if it isn’t setup properly it’ll be dead slow.

Front Toe-In

With the virtual “Formula” cars they always tended to run some amount of front toe-out and rear toe-in. This made them very nimble and they would turn-in to corners easily. If you visualise it from above, it can be seen as the front inside wheel already turning into the corner.

Here is another picture stolen from the internet to help you:

Toe-in and Toe-out

In terms of stability I was always taught to visualise the car in a strong side wind. With toe-out when the car transfers weight onto the outside wheels it will want to turn away from the direction of the wind, this is unstable. With toe-in the car will want to turn into the wind, this is stable. Like a wind sock.

I decided to give toe-out a go back in the summer of 2020, mostly in response to the weird handling I had on turn-in, which was most likely due to excessive bump steer!

Given my stability problems, nimbleness was the last thing I needed, so I reverted back to a small amount of toe-in. I went from 2mm of toe-out, 1mm a wheel, over 300mm width, to 1mm of toe-in, 0.5mm a wheel. With the Ackerman steering geometry in the car this toe-in will quickly disappear with additional steering angle, but in a straight-line it just numbs the steering inputs.

“But how do you measure toe-in Josh?”, I’m glad you asked. With the car on axle stands, and pins in place of the dampers (putting the car in its “design condition”) I take the boots off of the steering rack and place 3d printed shims to lock the rack central; see the pictures below. I then attach “toe plates”, something of my own creation, onto the wheels and use a tape measure to measure across the axle. Adjustments to the wheel angle are made via the track rods. I also ensure the wheels are both running straight relative to the chassis and generally square everything up.

Steering rack with the boots pulled back (lock stops are in black)
White steering locks in place holding the rack central
You’ve seen this picture before- see the toe plate on the right hand side

The same method works for the rear.

Roll Stiffness Distribution

Roll Stiffness Distribution is something that is often ignored or poorly understood by weekend warriors such as myself but it is a core handling characteristic of a car, or should I say balance characteristic. In fact in F1 it was generally known as the “mechanical balance”, as opposed to “aero balance” or “weight distribution”. Its mechanically controlled.

My little Locost has no anti-roll bars so this is purely governed by spring stiffnesses and geometry; we’ll ignore roll centers for now, my front and rear roll centers are at approximately the same height.

Another stolen picture. I just liked this one and its semi in context.

I’m not going to dig deep into the whys and wherefores of how weight transfer works, that is not what this blog is about, but it would help if you agreed with the following things:

  • Increased roll stiffness on an axle, relative to the other, increases the given amount of weight transfer that axle experiences while cornering relative to the other. If the front axle is stiffer than rear axle, then more weight is transferred between the tyres of the front axle than the rear axle (sort of, this is a simplification, lets go with this for now).
  • Weight transfer on an axle reduces its potential peak grip and cornering stiffness, because this is how tyres react to vertical loads, and an axle is just two tyres. I’m not going to dig any deeper for this article.

All make sense? Great! If not, I strongly suggest reading a book like Tune to Win by Carrol Smith. There are loads of really readable references out there.

The final statement I am going to introduce covers an element of the system that I think is generally missed by most. It was somewhat of a “eureka!” moment for me when it was first introduced:

  • The further an axle is from the center of mass the greater the turning, or stabilising, moment it creates around the center of mass.

Basic physics right?

Given all of the above it would make sense that the further an axle is from the center of mass the less force it should produce, or grip, to keep the car in balance. This is the balancing of the front and rear turning moments.

Its not too big of a leap in logic to realise that if your weight distribution is towards the rear (say ~45% of the weight on the front axle) then your front roll stiffness should be higher than your rear and not ~45% of the total like my little Locost.

Some Basic Maths:

We’ll need to do some very basic maths to work out the roll stiffness of each axle and then adjust the front springs to change the distribution. Ignoring the contribution of roll centers and such, the roll stiffness of an axle can be calculated as follows:

I stole this from another website, and they ripped it out of a book, will we ever learn?

Well, that is kind of useless as we don’t have a solid axle with perfectly vertical springs attached to it, but we can turn the Locosts suspension into an equivalent axle by calculating the spring equivalent stiffness’s at the ground. We do this by using the motion ratio between the damper and the contact patch of the tyre. The maths is as follows:

I made this using math.tools/equation/image and paint

Where:

  • K is the stiffness of a spring, in Newtons per Meter, [N/m]
  • x is the displacement of a spring, in Meters, [m]
  • MR is the motion ratio between the wheel and damper, in Meters divided by Meters, its non-dimensional

Given the above equation we can calculate the spring equivalent stiffness’s at the ground and then use the track width (tyre center to tyre center) to calculate the overall roll stiffness for each axle. The distribution is then the front roll stiffness divided by the total roll stiffness (front plus rear, two springs in series).

The End Result:

I’ll reiterate the numbers from the table above:

VariableFrontRearUnitNotes
Spring Rate200120lbs/in
35.0321.02N/mm
Motion Ratio1.6701.128Wheel / Damper
Spring Rate @ Ground71.7194.3 lbs/in K/MR^2
12.5616.51 N/mm
Track Width14401390mm
Roll Stiffness227.30278.40Nm/deg0.5*K*w^2
* (pi/180)
% Roll Stiffness Forward44.95%

As explained, the front is softer in roll than the rear. How did I solve this? I increased the front spring rate from 200 lbs/in to 275 lbs/in, giving the following:

Variable Front Change Unit Notes
Spring Rate275+75lbs/in
48.16+13.13N/mm
Motion Ratio1.670Wheel / Damper
Spring Rate @ Ground98.61+26.9 lbs/in K/MR^2
17.27+4.71 N/mm
Track Width1440mm
Roll Stiffness312.55+85.25Nm/deg0.5*K*w^2
* (pi/180)
% Roll Stiffness Forward52.89%+7.52%

That is a whooping big change in Roll Stiffness Distribution, 7.52%! In the simulator I would usually aim for steps of 4% as these were usually noticeable, but I’m not messing about with the Locost. It was hideous to drive at speed and I wanted to feel confident in the car and enjoy it.

Setup “B”

The changes listed above amount to following setup differences:

VariableFrontChangeUnitNotes
Spring Pre-load-6.0 / -6.0mmAt the damper, full droop
Castor7.0+2.4deg
Toe In1+3mmDelta front to back across the axle over 300mm
Spring Rate275+75lbs/in
48.16+13.13N/mm
Motion Ratio1.670Wheel / Damper
Spring Rate @ Ground98.61+26.9 lbs/in K/MR^2
17.27+4.71 N/mm
Track Width1440mm
Roll Stiffness312.55+85.25Nm/deg0.5*K*w^2
* (pi/180)
% Roll Stiffness Forward52.89%+7.52%

The Result

After all of the above it was time for the fun bit: driving! So how does it feel?

  • The steering weight greatly increased. I can now feel the lateral acceleration build up and fall off with steering input. Its not uncomfortable but it is readable.
  • The car now tracks straight and doesn’t wander over bumps and dips in the road.
  • It still feels nimble and will quickly “take a set” in a corner; its not unpredictable.
  • Its not understeery or too stable, the balance of the car can be changed with the brake on entry and the throttle on exit.

But what does all of the above actually mean? It means the car is finally fun to drive! I went out for a good long session on a sunny sunday afternoon and people were waving me by and giving me the thumbs up. I managed to catch up with a group of Porsche owners out for a pleasant drive and give them a bit of a wake up with my bright red loud viper of a car. It was awesome.

Now I spend my days looking out the window waiting for the sun to shine… keys in hand… just waiting…

2 Comments

  1. Been visiting your blog for some time now as I’ve been interested in getting a Swift GTI Mk2. Finally got one last year and super happy with it. I live in Scotland but the car is back in Portugal – the plan is to bring it here though. I’m looking to stock some spare parts just in case – would you recommend any shops/sellers/contacts to get G13B parts in the UK?

    Thank you in advance

    • Josh

      June 19, 2022 at 11:38 am

      Suzuki have a UK ebay shop and honestly the stock parts are fantastic. When I’m engine building I keep track of the suzuki part numbers and then search them on there. Nothing is listed as GTi but suzuki seemed to use a single parts bin for all of their G-Series stuff, so generally what fits a Suzuki Van usually fits a G13B. I’d be happy to share the parts list.

      Good luck!

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