Design clearance between chassis and road at full bump?

Anyone care to comment on what would be a suitable design clearance between chassis and road at full suspension bump travel for a car to be used on road and track.

For example, if a car had 100 mm of ground clearance, and a suspension travel of 60 mm in bump from ride height, the chassis would be 40 mm from the ground at full bump. I am talking here purely of the damper travel, spring rate does not come into it at this stage.

I understand that this clearance will vary depending on the actual profile of the road being driven on. I see some cars do "bottom" in certain circumstances, and you see people winding up static ride height or spring rate to try to prevent this, but in the case of a clean sheet design it would seem incorrect to design so that the clearance would be 0 mm.

Cheers

Fred W B
 
Fred here in Aus the design rule is 100mm clearance at rest with driver, your very rarely at full bump to worry about it. unless you travel in Queensland on the Bruce Hiway where a 4WD is more applicable

cheers John
 
Hello Fred
it's advisable ( if rules allow that) to leave minimum 40 to 50mm at full bump specially because you have on any small roads a curvature to drain rain water on both sides of these roads ; so when "bumping" down at maximum with wheels on each side of road you have probalby less than 20 mm left on the middle of fllor pan !
Just being lucky not to meet some higher stone or unidentified object !!

Have in mind too that a suspension travel of 50 of 60 mm will occur very rarely
( during hard breaking or unexpected jump !!) and if it happens it will be only for some fraction of second if your Spring and shock absorber ratio is well evaluated !

With car like you are buiding , to run higher than 100 to 110 mm will give to the lateral car "silhouette" a very ugly look and wheels will be totaly "offset" from arch wheels .
So this is a "pain" surely but it's advisable to run very hard compression ratio on dampers and as much as possible strong springs to limit suspension travel ;in addition if you succeed in finding a good suspension setup ( camber , toe ,etc ) you will have a nice looking car and a safe drive with big compromise to comfort and inside noise
This is why some homebuilders adapt lift systems when building low profile looking cars
 
Take the tires off the rims, set the suspension at full bump and check that you still have ground clearance. It is bloody embarrassing when you get four flat tires and the chassis is 'grounded', makes a mess of anything underneath.
 
Thanks for the responses guys, picture below is with chassis ride height at about 100 mm at the front axle. I made the bottom of the nose a bit higher than that, and I can still adjust where the body sits by changing the height of the lower side pods slightly.

Cheers

Fred W B
2body%203.jpg
 
Last edited:
First thing is, that body looks fantastic. 100mm is the norm for most of these cars, if you are running 15" wheels you will have tyres with a resonalbe profile and as such there will be some small tyre squash as well if you were to bottom the shocks. I would not have more than 50mm of compression. I'm going for 50mm compression and about the same droop in my setup. I also have to allow for the down force in the travel as I have huge wings, so will have very heavy spring rates, you not so much.

Cheers Leon.
 
Hi Fred as stated above you should allow for tyre squash I found my daily has 20 mm at the engine end and 15 at the other end so I have allowed for that, in nz the minimum in ride height is 100 I allowed 60 bump 40 droop plus the squash amount
Regards
Graeme
 

Randy V

Moderator-Admin
Staff member
Admin
Lifetime Supporter
Anyone care to comment on what would be a suitable design clearance between chassis and road at full suspension bump travel for a car to be used on road and track.

Nice looking car!
The others have given good information and food for thought. You did mention Road and Track and I thought I would share a little trick I've used for dual purpose cars over the years. The picture below is of one of my Cobras which I raced and once in a blue-moon took out on the street. I would scale the car for the track and set all my corner weights appropriately, then I would put a hose clamp around the body of the shock as an "adjuster-ring stop". When going to the street, I would jack up the car and put five turns into each adjuster-ring which would raise the car - giving me maybe another 12-15mm of clearance.
When time to put it back on the track, just reverse the process.
By the way, the other benefit of using the hose clamps as a positive stop was that I no longer had to use split adjuster rings where one would jam and lock the other. I was also able to forget about using adjuster-ring locking screws which would invariably get lost or would damage the threads on the shock body if not the cushioned type.

IMG_0900.jpg
 

Dave Bilyk

Dave Bilyk
Supporter
I would like to add that aerodynamics is a further consideration at higher speeds for flat bottomed cars to prevent nose lift. Thus front ground clearance should be smaller than the rear, sufficient to maintain a nose down attitude at speed once the effects of downforce and spring rate at front and rear are taken into account.
I run my 40 so that the clearance at the front of the floor is 4" and the rear 5" for road use.
Dave
 
Back
Top