Hi all

Iv just been considering the caster angle of the front upright i am designing and have got a little stuck...

I have a tyre radius of 312mm (215-45 R17) a KPI of 8 deg and a scrub of 10mm from the center of the tyre contact patch.

my inital thought was to apply equal caster to KPI to try and cancel out the worst of the camber change effects of having the 8deg KPI however after a bit of thought and a bit of trig i think this may give too much trail due to the radius of the tyre. (results in a second)

Now i want a reasonable caster to allow for the fact that the car is mid engine 40F 60R % weight distribution at a design weight of 600Kg but i also dont want the steering to be too heavy.

Caster to Trail Results:

8deg caster = 43mm Trail
6deg caster = 32mm Trail
4deg caster = 21mm Trail

as you can see the trail is moving at a factor of about 5.5mm per degree of caster

the question is how much trail / caster do i want???

thanks Tilly

I'm going through the same exercise and considered:-

1. On a lightweight car the effects of trail and scrub will be less critical than on the heavy saloons most data is available for
2. Cortina's ( which were lighter than modern saloons but heavier than locosts) had around 32mm trail combined with 75mm scrub.
3. There is merit in minimising kpi and maximising caster exemplified in the SAE designs
4. Bicycle forks give a clue to the answer with trail from large caster minimised by offsetting the axle in front of the steering axis

My problem is how much can I advance the front axle? A 9 degree caster gives a 42mm trail on my front wheel. I can reduce this to 15mm by advancing the axle 27mm but that makes the upright very heavy cos it has to be wider.

Alternatively I can advance the axle 8mm without adding weight to the upright leaving a trail of 34mm.as a compromise this sounds good to me as with a kpi of 3 deg and a caster of 9deg I will be adding neg camber on turning the wheel which takes pressure off the front SLA suspension design.

Dunno how it works in practice as I am currently destructively testing uprights to see how thin I can make the material - in this respect i have just bent the 20tonne hydraulic press so it is fair to say my upright is a bit over-designed!

Cheers!

quote:

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Originally posted by v8kid
I'm going through the same exercise and considered:-

1. On a lightweight car the effects of trail and scrub will be less critical than on the heavy saloons most data is available for
2. Cortina's ( which were lighter than modern saloons but heavier than locosts) had around 32mm trail combined with 75mm scrub.
3. There is merit in minimising kpi and maximising caster exemplified in the SAE designs
4. Bicycle forks give a clue to the answer with trail from large caster minimised by offsetting the axle in front of the steering axis

My problem is how much can I advance the front axle? A 9 degree caster gives a 42mm trail on my front wheel. I can reduce this to 15mm by advancing the axle 27mm but that makes the upright very heavy cos it has to be wider.

Alternatively I can advance the axle 8mm without adding weight to the upright leaving a trail of 34mm.as a compromise this sounds good to me as with a kpi of 3 deg and a caster of 9deg I will be adding neg camber on turning the wheel which takes pressure off the front SLA suspension design.

Dunno how it works in practice as I am currently destructively testing uprights to see how thin I can make the material - in this respect i have just bent the 20tonne hydraulic press so it is fair to say my upright is a bit over-designed!

Cheers!

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Second column, five down.

http://www.shedworks.eu/tips.html

190/535/13 Dunlops on the front - 535mm dia believe it or not!

scrub radius was 72mm.

Thanks for the dimensions Designer its much appreciated.

I looked at these earlier ( I think it was on your site it looks familiar!) when I was trying to make sense of all the different info out there and for my purposes, but not everyone else's purposes, disregarded them. I want scrub because it gives feedback to the driver on what the wheels are doing. I'm no Lewis Hamilton and I need lots of feedback! Also those cars are designed with noise, vibration and harshness very much in mind. Not qualities that first come to mind on a Losost!

Also kpi on a road car is more a packaging issue and is an unwanted side effect of using struts whuich are used due to cost or chassis construction considerations - we are using SLA suspension.

I'm still not sure I've got it anywhere right though I suppose there is only one way to find out
Cheers!

Sorry about point 4 - Dunno! Don't think so but not sure - works on bicycles!

Dunno.
If this helps: the Venerable Triumph upright used in *many* lotii Including serious racers of some success :-) has a 9 degree KPI as part of it's casting and Lotus' Castor setup on those ranged from 3 degrees on Mid engined chassis to 5 degrees on front engined. Why are you all over the map on this?

That was on hard xplies - different kettle of fish now

Nonsense!
Radials effect Toe in / alignment setup.
Tough to make progress when buried under old wives' tales.

Interesting arguement can you expand on it for us lesser mortals?

Sorry bare for my ignorance but im not sure i understand ither of your posts if you could enlighten me in simple mans terms i would greatly apreacate it

many thanks

tilly