More on 20 inch wheel fitment issues for 67 De Ville

[1cerberus4u]

Well, after fussing all winter, I decided on a set of Foose (nitrous) wheels for the 67 De ville.  They came today.

Despite so much planning and conversation (ended up with RimsNTires.com as they "guarantee" fitment and had the package in stock.....

Well the 20 x 8.5 with 4" of back spacing (measured to the actual outside of the inner wheel) fits the front fine.  I'd think that a 4.5 inch Back spacing will work too.

On the rear, I ordered 20 x 10 and these wheels showed up with a 5" backspacing (again measured to the actual outside of the rim) which is just a CH (well maybe 1/4 inch) too tight to the outside, so the fender skirt wont lock in place.

If my measurement is correct, the rear rim is closest to the frame rail at the front side of the wheel (say the 2 o'clock position) and the rim is 1 1/4 inches from the frame rail.

So, in the end, now I need to look for a rear wheel that is 20 x 10 with a 6 inch backspacing (measured to the actual outside of the rim).  This will give me about 5/8"s to either the frame rail or the fender skirt.

Anyone know of some cool wheels with a 6 inch back spacing?

Hope this saves someone time..............

[Sloggie]

I had done some calculations previously for my 67 Deville.  I've also talked to a guy who mounted 245/40-20 tires on an 8.5 inch rim with a 15mm offset (5.34 inch backspace)

What size tires were you planning to mount on your rims?

[EZ]

Check with Coy's Wheels.  They have pics of Cadillacs on their website. They
will help with fitment too.  Thanks for posting this so I don't make the same mistake!

"EZ"

[Hawaiian Boy]

Hey guys, when going with either 18 or 20 inch wheels is it fine with the original drum set up 0n a '66 or do you have to upgrade to disk brakes? Thanks

[73eldo]

Its usually discs that cause the wheel size problems with the smaller sizes.  I would think 18 and 20's should clear just about any brake you could find. 

[Hawaiian Boy]

so your saying it's fine with the original set up on my '66?

[EZ]

Yeah, your '66 will be fine with drum brakes and 20" wheels.

The problem come with trying to fit a 12" or larger disc brake rotor inside a 14" - 17" wheel.  Then you have fitment problems with the calipers hitting the rims.

Reading back to the top of this thread I'm curious if 1cerberus4u ever got his 20x10" wheels to fit?  I would think a 245/40R20 tire would look too small on the rear.  I'd think a 275/45R20 would be a LOT better on this big of a car. But would they fit with a 10" wide wheel if it had a 6" back space??

I'm torn between going with a set of 15" Supremes and wide whites or maybe even a set of triple whie walls or a set of 20" Torque Thrust type 5 spoke wheels.  Decisions decisions.............

[Hawaiian Boy]

Thanks EZ for your help.

[Gary]

Something to consider is the extra weight and momentum generated by 20" wheels. You are asking those old drums to slow a lot more mass that is rotating further away from the hub center.
At the least, make sure that the brakes are in tip-top shape and properly adjusted. Then test them carefully is a safe place so you can get a feel for any change in characteristics, such as stopping distance.
Just trying to keep you safe, ya know?

Gary

[Cadillac Kid-CLC #15364]

Gary,
Any possible difference in mass between stock and 20 inch wheels is far outshowed by the work the brakes do in dissapating the energy of the car.  Actually, with 20 inch wheels and a larger (than stock) wheel/tire diameter, the wheels are rotating slower at a given speed, allowing the brakes to be more effective.
Greg

[Hawaiian Boy]

Interesting takes by both Gary and Greg.  Thanks for the info, definitely goin to weight my options.  1.Old skool look  3in white walls with either hubs or some type of steel wheels painted or not with bullets, etc.  2. New school look 18 or20in wheels. Time to punder?

[The Tassie Devil (Bruce Reynolds)]

Actually, the amount of weight that the brakes will be trying to stop is only the actual weight of the vehicle.

It is the suspension that is working on the wheels and tyres.

The stiffer tyres will be working the ball joints, steering and bushings a lot more, as there is less give in the narrower sidewalls.

The only difference the brakes would feel is if the tyres and wheel was off the ground, and the brake pedal was applied to stop the rotation.

Bruce.

[Gary]

Greg/Bruce
good points.
I was ASSuming that the new wheel/tire combo would be signifigantly heavier than stock. Which means that Bruce is correct that the greater unsprung weight would be difficult for the stock spring/shock combo to control.
But I also believe that this greater unsprung weight, further from the center of rotation would be harder to stop.

"As you increase rotating mass, it's also harder for the brakes to stop the wheel from spinning. This can decrease brake life and increase braking distance. The same principal applies to acceleration, because a heavier wheel is harder to get rolling and acceleration is decreased. O'Connor claims that adding a set of big wheels can increase 60-0-mph stopping distance by as much as 20 feet. That can mean the difference between stopping short and being killed in an accident."
from here: http://www.insideline.com/features/the-danger-of-dubs.html

Gary

[Cadillac Kid-CLC #15364]

I repeat, the brakes are made to stop the car, not stop the  wheels from rotating.
Greg

[pdxmose]

My opinion...

The +20ft number seems like a pretty drastic number.  But the +40lbs per wheel also seems a little excessive.  The example may be based on a worst case scenario where a small car with small wheel/tire and marginal brakes recieves a very heavy 'upgrade' that also includes a larger diameter overall tire size.  A larger diameter can definitely affect braking as it creates a lager lever arm that the brakes are working against.

For most of our Cadillacs, I think that the overall tire size isn't changing much when going from a 235/70/15 to a 275/35/20 (for example).  This means that the braking force ratio isn't changed much.  In addition I think the weight change will be closer to about 20# per wheel/tire combo.

However, adding weight to a wheel/tire combo DOES affect braking as the brakes must remove the momentum from the wheel.  Most of the larger wheel 'upgrades' include a wider alloy wheel where much of the weight is added near the perimeter of the wheel.  But even if you assume 20# added at the outer edge, the affect isn't that significant.  Since the tire acts as a 2:1 lever arm, the added mass is effectively travelling at 2x the speed of the car, so the added momentum would be roughly equal to adding twice the wheel weight difference to the weight of the car, or about 20# * 2x * 4 wheels, or 160#.  So if you are concerned about stopping distance, just don't take any passengers!

Now if you go to a 24" wheel that adds 4" to the overall height, then that could decrease braking by the ratio of the radius increase, or about 15%.  This would be significant, but it's not from the weight difference.

          ...mose