Level of Chassis Relative to the Level of Hub Centers

[Aurora]

When you build a car that utilizes tires of different diameters front to rear, do you design it so that the chassis is level when sitting on the bare wheels (rims)?

Or do you design it so that the chassis is level when it is sitting on the tires?

I've been designing my cars so that the chassis is level when the car is sitting on its tires. For example, if I want the car to have a minimum of 2mm ground clearance under the chassis, it will have a minimum of 2mm clearance beneath the chassis under both the front axle and the rear axle - even though the rear tires are larger diameter. This requires raising the rear axle relative to the front axle, to compensate for the larger diameter of the rear tire.

If I build the chassis so that it is level when sitting on equal diameter bare rims, the chassis will have a forward slope, or rake, when the tires are mounted. The bodywork can be positioned so that it is level relative to the ground, but the chassis itself will be raked.

Does this matter one way or the other? I suppose a sloped chassis will possess a slightly higher center of gravity than a level chassis.  

[Andrew Rowland]

I prefer level on the tyres. I don't build on bare rims. Real cars (generally) appear to be run with flat chassis I believe although i guess suspension settings might have distorted that on race day...

Andi

[ken]

I generally try to make the chassis level to the track and with a shade above the minimum track clearance for cars that I will be competing with, this keeps the guide parallel to the track and the CG low as possible. I use everything from wheels of different diameters to Slot It 18 and 19 mm spur gears when I set up my jig. BUT and it is a big but, you need to have room under the body for the taller upright and bushing which can be difficult in half tonners.

[Aurora]

Thanks for your input guys. I always wonder about this when I see folks building their chassis, and when building my own. When viewing photos of various chassis under construction, I frequently see chassis that are resting on the bare rims when mounted on the chassis jig. However, I usually cannot determine from the photos how the builder is setting the ride height. Are the bare rims being used, or spacers, or what? Nor can I determine if the front and rear axles are level relative to each other.

The reason I ask is because I wondered if it would be easier to build the car using the bare rims, if it didn't really make much difference one way or the other. I've been building cars to be level when they are sitting on their tires. This requires some careful measurement and setup on the jig to account for the difference in diameter between the front and rear tires.

After thinking more about it, the ability to keep the mass at the rear of the car as low as possible seems to be worth the extra effort to make the car level on its tires.  

 

[Deleted]

With the fashion for grinding tyres on expensive machines it would be almost impossible to keep to a particular level, as the grinding is done once the model is finished. But generally, if it really matters, I would use turned discs to represent the wheels and tyres if there's a difference twixt front and rear and just similar ones if Fr. and Rr. use same size tyres. I used to use PP GPT sets for half tonners, which are definitely "bigs'n'littles, so I made a set of discs the same size as those,

Martin