Ground clearance! How low is too low?

I'm sure I could find a lot of grim photos on the internet of horribly smashed bumpers and cars, but I thought I'd ask if you all have a rule of thumb.

I've been doing more engine layout work on the TVR, and I'm starting to get a clearer idea of where everything is going to end up.  Looking like the oil pan will be about 4" from the ground.  This is similar to the clearance on the TR6 oil pan, and the low point of the frame is about 4.5" off the ground.  It also shows damage from hitting something...

So is there a magic number?

BTW, I've not played with anything suspension-related at all, though the car doesn't appear to be slammed to the floor in its current state.

all else fails a skid plate isnt a bad idea for a low car

Its not low enough until the door handles take road reflectors off.

In all seriousness, a street car needs to clear normal road debris. I use a soda can on its side for guidance, and add an inch or so of safety margin.

Scrub line is a much bigger concern.

In reply to USERNAMETAKEN :

A few points:

1. I’d make sure the oil pan isn’t the low point, especially with minimal clearance like that, even if it means adding a skid plate or cross brace to protect it. 
2. Suspension geometry and travel play a big part in optimizing ride height. As a general rule on most cars, you don’t want the lower control arm below horizontal at static ride height. You want the camber curve to remain optimized and not induce any bump steer if you can avoid it. 
3. How tall are the speed bumps in your area? Seriously, I left my Miata at 4.25” even though I could have gone lower, just to make sure I have enough clearance on the street. 

Look at what the your RMV requires.  As an inspector I have to reject cars that don't meet minimum hight requirements.  It has nothing to do with actual clearance of things it is about headlight hight above the road.  To low means you don't see far enough down the road to be safe.  Compensate by adjusting your headlights and you are starting to aim them flat or up in to the eyes of on coming traffic.

Yes I am MASS approved professional riced out car killer.  

Dirt track modifieds are 5" at the frame rails.

Too low isn't a thing. 

I had to re-read to really register that this wasn't so much about ride height as how low a low spot can be a problem.

Having an oil pan lower than anything else would worry me a bit. I don't know about a magic number, but finding out that you've run out with the oil pan as the first feeler sounds ouchy.

How good are you at keeping things like that in mind when you see a raised manhole cover in a badly deteriorating section of road works?

If sparks don’t fly, you’re too high. 

I'd say ~2.5" is my limit. I've had to weld up tears in the engine and transmission oil pans on my cars before. So maybe 4" if you're trying to avoid that. 

It's going to depend on your spring rates.

But really, what's most important is what hits first. My track car has skid plates all over the bottom due to its history as a rally car. When I lost a rear wheel at Laguna Seca, it rode out the whole thing on the plates and didn't take any damage that wasn't directly related to running over my own wheel. If you hit your oil pan or cats first, you'll have a problem. If you hit a chunk of well supported UHMW plastic that is designed to be replaced if necessary, you'll just laugh and carry on.

Dean, what is the actual minimum headlight height here in Mass?  Every time I look at my TR8 headlights, I think those things have got to be too low.  One night I just missed a deer and it had me physically shaking the rest of the ride home.  I was looking at his knee caps hitting my bumper and the body of the deer crashing into my windshield if he hadn't gone all Matrix on me and somehow missed my car.  I guess jumping over a lowered TR8 isn't that hard.  Last thing I saw was the underside of his belly, legs stretch out horizontally passing directly over my head.  Good thing it wasn't a buck, or it would have been an epic tea bagging.

I believe the lowest point on our MGB is about 2.5" off the road where the exhaust has a flange and a turn that hangs down. It's right in the middle of the car front to back and centered under the driver. If the road has any big humps it can scrape but its usually pretty avoidable. It even lightly scrapes going into the garage  The springs on the car are stiff enough that it's never really a problem and it's mostly just a cruiser anyway. The very short front and rear overhangs mean that driveway transitions are never an issue.

The s2000 is about 3-3.5" off the ground and it's still very useable but I wouldn't go any lower because of the long front overhang. I have been surprised by some bumps that turned out be much gnarlyer than they appeared and have scraped hard parts, but that's usually not an issue on decent roads.

Obviously much more vigilance to road obstacles is required when driving very low cars.

I feel like the headlight issue is a very good point that nobody ever talks about with low cars. IMO, headlight aiming specs do not apply to cars where the headlights are knee height or lower. You will want to aim your low beam cutoff at the horizon or just slightly lower or you will be able to outdrive your lights easily. Also driving at night sucks when your head sits at or below the height of every other car's headlights.

Figure out how much suspension travel you want to have and set the ride height such that you have sufficient clearance under compression.

The lower you go, the more travel you have to remove from your suspension (increased spring rates,) compromising functionality for form. 

If you want to go as low as possible a good compromise may be air bags so you can tune your ride height from the seat.  I always thought of bags as something for lowriders, but my friend has them in an E46 wagon and they work well.

For street cars, I like to have enough clearance that I don't have to pucker or tip-toe over speed bumps, or turning across traffic into a parking lot that has a steep approach.   Has to pass the road kill test.

Wait is the 2.3 a plastic oil pan like some of the other newer engines? if so i def would have a skid plate.

Depends—are you asking me now, or asking high school me? 

I'm right at the point where I think my friend's '92 Camaro looks really good sitting as low as it is, but I also cringe when he drags his subframe connectors pulling into a driveway. 

My friends s10 was a 6/8 static drop that drug the trans case if you tryed to drive down the center of a crowned road. The traction bars would also hang up on mild parking lot entrances. After that I learned that 4 inches seems to work best for something you actually drive.

The original (not the new ones) Fiat 124s had the same problem. Almost any you find still on the road have a dent in the pan and a broken off oil intake. On those cars the oil pan is the lowest part AND it is in front of the cross member, so it bottoms out on any humps, bumps, or holes. Thankfully the broke oil pickup tube does not seem an engine killer on those cars.

For your TVR, I don't think you need a full plate, just a few well built and spaced piece of tube or angle iron till keep the pan off of the ground,

My 96 Impala is pretty low.  It was even lower than it is in this picture because for a while I had 275/40-17s (stock is 255-50-17).

As long as I go slowly over speed bumps, it never gives me any fits.  If I hit them and cause the car to bounce, the frame sometimes hits.

As it is right now with 2" lowering springs and stock tires, it goes through most automated car washes, but you have to drive it onto a 2x6 to get the pads of a 2-post lift under it.  Its low.  It has been this way since 1998 and I have no regrets.

I think it depends what you're going to do with it.  Hopefully not what happened to this poor Alfa.

I like the idea of UHMW/plastic skid pucks.

Midlana has 4" at the back and 3.75" at the front. Never had a problem at the front but the rear has hit/dragged after hitting a dip in the road due to the rear weight distribution and soft-ish suspension.

My standard is being able to get over speed bumps in this area, which translates to running >3.75". Another "rule" is not having to worry about your testicles climbing back into your abdomen if you see a 4 x 4 wood beam (which measure 3.5" on a side) in your path.

yupididit said:

Too low isn't a thing. 

Love it till oil pans cry foul

Goals...

The skid plate protecting the oil cooler on my RX-7 is 7" off the ground.

It used to be 6" but I hit something on the highway one night and bent the mounts, which got mangled enough that it is now a permanent part of the car.

Skidplate is made of 1/4" or so steel plate and weighs something like 60lb.