Wide wheels, camber, castor

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peakrock
Posts: 135
Joined: Thu Sep 27, 2018 4:52 pm
Location: Wreningham

Wide wheels, camber, castor

Post by peakrock » Fri Oct 08, 2021 3:23 pm

Triking with wide front wheels

I've been meaning to post this up for a while, the recent Castor posts have prompted me to finally do it..!

Regular forum readers will no doubt be familar with my desire to make a Triking build as difficult as possible. Instead of being content to build a standard Type 3, which is a perfectly usable and great vehicle to drive, I decided I must have things such as; twin petrol tanks, a Type 4 pedal box, umbrella handbrake and my own unique tail section. This thinking also extended to the front suspension where I decided to go for the high performance suspension with wide (for a Triking) Type 4 front wheels with 4.5 inch rims.

Joking aside, I wanted to make the Triking handle as close as possible to my probable rose tinted memories of the Lotus Elans and Europa I owned in the distant past. I remember they had an impressive ability to soak up road imperfections whilst cornering hard without being thrown off line. The standard Triking suspension is fine but the awful state of some of our roads can give it a hard time and the impressive results of the high performance suspension as documented by "WestEndGuzzi" and "Doverhay" convinced me that was the way to go. Wider wheels also seemed a good idea though Al did say that this combination had not been done before...

Standard Type 4 front wheels have a wider hub than a Type 3 and earlier Trikings and the wide front wheels use the same Type 4 hub. They need a longer stub axle than a Type 3 so it seemed sensible to start with a Type 4 upright on my Type 3 build. The Type 4 upright is taller than a Type 3 upright and is built for a different suspension geometry, as the Type 4 chassis wishbone inner mounts are dimensionally different to a Type 3. Having said that, the wishbones for the Type 3 high performance suspension are different to a standard Type 3, being longer in order to accomodate the greater length of the Hagon gas coilover shock. The upright is also subtly different in that the top balljoint mount is angled to ensure full articulation of the ball joint itself. With all this in mind it seemed sensible to start with a Type 4 upright.

The Triking was built and after MSVA and registration, road testing could commence. Initially things seemed ok but there was a nervousness at higher speeds which was disconcerting. Various things were tried such as more - then less toe in, neutral to negative camber, different tire pressures etc. We experimented with increasing the caster angle, initially by using adjustable top wishbones and then a special pair of wishbones were fabricated so that the caster angle could be increased up to 12 degrees but this didn't really work, the effect of such an extreme caster angle was that the Triking became extremely twitchy at higher speeds. In a conventional 4 wheeled car, extreme caster angles can cause oversteer which is exactly what the Triking felt like it had. As an experiment, I swopped my wide wheels for standard Type 4 wheels with 4.00 x 18 Mitas tires and went back to standard Triking suspension settings. It felt much improved though had less grip and response as you'd expect, but pointed to the wider wheels having a significant effect on the steering response. This led to a close examination of my front tires which had been something of a "bargain buy". They both had a significant flat spot, probably due to long term storage and not being inflated that hard - it being a Triking. This proved to be a red herring but I replaced them anyway with 130/70 x 18 Michelin Road Classics, a great tire but probably the widest tires yet fitted to a Triking.

After some discussion with Al I decided to change the uprights for a Type 3 high performance upright but with the Type 4 backplate and stub axle as I didn't want to junk the wide wheels and tires or the mudguards. These uprights improved things considerably and at standard Triking settings of 4mm toe in, 1 degree of negative camber and 4 degrees of caster it cornered on rails and was showing an impressive ability to soak up bumps and potholes and stay on course. The only thing was that at 50MPH and above the steering was super responsive, not twitchy or feeling like it wanted to dart off into the hedges but very direct - fine on a track car but not relaxing on long drives. At this point though, it seemed safe to assume that the wide wheels were the cause of this as everything else was now standard and well proven.

We considered lengthening the steering arms on the hubs which would make the steering less sensitive but it would have a big impact on steering lock and reduce it considerably. I decided to keep that option in hand and instead analyse more carefully what effect the wheels were having and what could be done about it. I used an online suspension calculator which enabled me to build an accurate model by inputting the dimensions of the chassis and all of the suspension components. I did this for a standard Type 3 high perfomance suspension setup and for my own setup. The major difference between the two setups was the scrub radius which had more than doubled from the standard setup.

Scrub radius is calculated by taking an imaginary line through the kingpin swivels which will hit the ground at an angle and an imaginary line vertically down from the centre of the stub axle to the ground. Where these two lines meet the ground the distance between them is the scrub radius. On modern cars with ABS the centre of the wheel rim is inside the kingpin swivel line causing negative scrub radius which is desirable as it improves stability under ABS braking, where there can be variable braking effort on each wheel. On designs like the Triking there is always positive scrub radius as it's not possible to bring the rim centre line inboard due to the wire wheel spoke angles and the brake disc. The wider wheels had nearly doubled the scrub radius which older Trikingers may remember is akin to past days when we'd bolt wheel spacers onto our Minis etc to "improve" the cornering. The result of doing that was that the front wheels would try and steer the car at times rather than the driver...!

The only way to reduce the scrub radius was to increase the kingpin angle such that the two lines were closer together. Several hours suspension modelling later I removed the front uprights (I could do it blindfolded by now) and headed over to Al's for them to be modified. The top mount was moved down by 30mm and outboard by 25mm (this is about the same distance out as the Type 4 mount). The mount was also angled a bit more to retain full balljoint articulation. This reduced the scrub radius to 28mm from 60mm and also lowered the roll centre.

Back on the Triking it was now time for some more road testing with things set at standard settings. The steering was more heavy at parking and low road speeds but not excessively so and lightened up as the speed was increased. It had a more damped feel too and on a long cross country drive I became more impressed at just how nice the whole setup felt. I increased the caster to around 6.5 degrees and it felt even better at high speeds - 70MPH feeling safe and stable, apparently as you go faster the effects of kingpin angle are lessened and the effects of caster increased.

I made some minor adjustments and set the castor to 6 degrees, set toe in to approx 5mm and camber to 1 degree negative. Tire pressures were approx 16 psi front at the mo and 40 psi rear. The rear tire is a fat bike tire on a rim spoked wheel and needs a lot of pressure otherwise there is too much sway, I'm probably going to upgrade to a car type rear wheel and tire to get less sidewall flex.

As well as these settings there were the dampers to play around with as well. Initially I set them at two positive clicks front and 1 click positive at the back which works well. I kept remembering the Colin Chapman maxim of soft springs/firm dampers so increased them to 4 clicks positive at the front and two clicks positive at the back and waited for a weather window to test it out again..

A week after the last update to this text and I went out on my test route and came back with a big smile on my face. The front dampers were finalised at 3 clicks positive, the rear dampers at 2 clicks positive, the rear tire was at 40 psi (bike tire remember) and the front tires were at 14.5psi. The overall ride was firm but supple if that makes sense, the steering quite light and stayed that way even cornering hard. On the dual carriageway section I felt happy to take it up to 75MPH.

A couple of months later I upgraded to the solid rear wheel with a car tire set at around 28psi and changed nothing else. It's given the Triking more stability at speed as well as improving the ride from the rear suspension. Currently the front tires are at 16psi and all other settings the same as above. I can take it up to 70MPH + and it feels absolutely fine as well as being nicely responsive when cornering on twisty back roads.

Thanks to Al for being more than willing to modify and fabricate special bits to optimise the setup...

Spaniard
Posts: 62
Joined: Mon Nov 19, 2018 1:06 pm
Location: Wisconsin

Re: Wide wheels, camber, castor

Post by Spaniard » Sat Oct 09, 2021 12:46 am

Very interesting. Change one thing and 3 other follow. A bit of a snow ball effect. Thanks for the info. I love learning about chassis setup. Its fun to try something new and then give it a try and see how it works.

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