Maximum gradient for new driveway

Just remember, it's not just the 11° going into the garage, which I think it would probably be OK not scraping the nose but it's also 11° coming out of the garage and onto The street which I think you're likely to high center the car?
Just thinking out loud..

 

I've often wondered how much engine wear a steep drive would induce. You aren't able to warm up in the garage and you immediately need pretty high torque to pull the car up onto street level, albeit for a very short period. When looking at properties to buy in the past, or parking a classic car overnight in a hotel's deep basement lot, it's something that's always troubled me. Maybe unnecessarily

 

Yes indeed....very unnecessarily.

 

I live on a hill and have a steep driveway up to my garage. I have also just recently repaved the driveway so have had to think through these issues you are dealing with.

Despite the slope, I experience no problem driving the 308 up the driveway. I would expect other 308s might have problems if they had been lowered slightly or had a deeper front valance. My 308 is stock height with the smaller front valance. A few mm of height difference in the car could change everything. Additionally, I creep up the driveway, fearing that hitting the driveway with some speed might cause a slight compression of the springs and the car to bottom out.

I regraded the driveway as part of the repaving project and was able to substantially reduce the severity of the slopes (despite the obvious; not being able to change the height of the street or garage). The important thing to realize is that the driveway should not be a flat plane that abruptly changes slope at the sidewalk and garage floor. Instead, it should be a carefully shaped “S” where the slope meets the sidewalk and garage floor smoothly. The driveway is steepest in the middle. The actual measured slope is irrelevant; it is the slope transitions that matter.

So, key to success is getting the grading right. I would be very explicit to your contractor about what you want related to clearance for a low car. My guy did a great job and actually lowered my driveway about a foot in the first 10 feet of run. A huge improvement, although the middle is now substantially steeper. You might drive around and try entering neighbor’s driveways about the same slope as yours. See what works. Personally inspect the grading before paving begins and ask for changes if it doesn’t look right.

I was going to try to measure slopes and give you numbers, but it seems too situational to really be useful. I can only give you a qualitative response.

 

Great post Brian!

 

+1 -- it's not the slope (unless huge), it's a big change in slope over a distance less than the wheelbase distance that's trouble. I've had to pass on houses we wanted to buy because of this.

 

I have a very steep driveway. The wife scrapes her MB Nose & Exhaust all the time.
My 308, when driven up/down at a steep angle to the driveway has no issues.

 

Just use a tape measure/ruler and basic geometry to get the figure for your 328.

I calculated the front approach angle of my stock US 308 without the deep spoiler to be 20 degrees.

 

That would be my concern more than the approach angle... these cars sit low.

Perhaps an idea to go steeper out of the garage and then flatten gradually at the top?

 

It's not the slope, it's the sudden change in gradient.
If you bring a steep slope to level in a single sharp angle, you're likely to have issues.
But if you round off the join over a distance, it might work better.

Also, check your municipal codes.
Minimum and maximum gradients for a garage and driveway are specified in my city's building codes.

The guys who built my driveway did a good job.
The issue here isn't the gradient of the garage, but joining the level driveway to a steep street.
They did a gradual twist over about for or five feet of "dishpan", so that the driveway itself is flat, but it matches the slope of the road at the join.