confused on backspace

I have been reading all of the threads that seem to suggest a 5" to a 5." backspace on a 9" wide rear rim. I am looking at doing a 17" diameter and I got under the car to measure and this is what I found. It appears that the limiting point for the tire going 'inboard' is the top of the shock. If I measure from the top of the shock out to the outer edge of the body it measures 12 1/4". If I subtract 1/2" from the outboard to allow me to fit under the 'lip' (which I will roll) and I subtract 1/4" from the inboard (just to be safe), that leaves me with a 11 1/2" space to work in. The front of the mounting surface of the drum is 6 1/8" in from the outer body point (mentioned above). If I subtract the 1/2" mentioned above that leaves me with 5 5/8" from the mounting surface to the outboard side of the 'envelope' which leaves 5 7/8" to the rear of the envelope (as I mentioned above the envelope is 11 1/2" wide). If I am using a 9" wide rim, then I need to reduce those two measurements by 1 1/4" (9" is 2 1/2" less than 11, split in two is 1 1/4) leaving me a measurement of 4 3/8" from the mounting surface to the 'front' of the rim and 4 5/8" to the 'rear'... that would be the backspace number 4 5/8". Based on that, it looks like I could mount a 275/40/17 tire which has a cross section of 10.9" (fitting in my 11" envelope). Does anyone see a problem with that???

I just don't see how a 5.5" backspace would work? On a 17" rim on the drivers side, does the mounting spot of the shock fit 'inside' of the rim or will it hit? It looks VERY close. I assume it will fit inside an 18" rim. With the backspacing aI am talking about 4 5/8" it should be OK since it looks like the mounting point is 4 7/8" behind the mounting surface.

Thanks for your help...

What year are we talking about?????

HA! That definitely would be important wouldn't it.
1972 Pontiac Ventura - should be identical to the nova

For a second there I felt like I was back in math class with the teacher up front on the board throwing all the numers out and I was in the back of the class with that glazed look LOL

Ok, dont forget, a 9" wheel is actually 10" Help?? :) JR

You have to factor the tire, not just the rim.

http://img299.imageshack.us/img299/8655/tiredimesions.jpg (http://img299.imageshack.us/my.php?image=tiredimesions.jpg)

I may be missing something here, but why not just move the shocks inboard?

This is the best compiled info i have seen yet!!!!! http://www.2quicknovas.com/6874.html

There's also some good info here http://www.stevesnovasite.com/forums/showthread.php?t=107616

Well, I measured the fronts and I am seeing the same thing. It looks like I can do 245/40/17's but the BS would need to be 3.5". On the forum (and the links above0 it sounds like other people are using 4.5-4.75 BS on the fronts. It I did that the tires would hit the frame BIG time on full turn. Do drum brakes on the front not 'stick out' as far???

For the rear is looks like I can do 275-40-17's but need a 5.00" BS on the DS and a 5.5" on the PS. I built a fake tire out of cardboard to test things and this looks like the best situation for the back.

ONe of the links has this as the last post. If this is correct, that would explain my issues. This would bring the front to 4-4.5 and the back to 5.5 which is what everyone is saying. Are 9" rims really 10" wide??

User Project Nova informed me of this EXTREMELY valuable tip on a rims backspacing.

When you order a rim the actual width is 1" wider than what you order. For example, if you order a 8", it's really 9" from outside lip to outside lip. Backspacing is measured from the back of the hub surface to the outside of the back lip of the rim, which is a 1/2" past the tire bead. Your goal is to center the tire in the wheel well.

What this all means. After you figure out your BS you need to add the rim lip thickness . I know, . When I measured my car 5" BS would center the tire perfectly. The rims I ordered have a 1/2" lip front and back. So I ordered a 5.5" BS rim. I hope I explained this good enough to understand?

anyone help?

Yes,on a traditional rolled lip wheel a 17x7 advertised is actually 8" wide.18x8 advertised is actually 9" and so on...

Are 9" rims really 10" wide??

Um yeah.. I posted that below, didnt believe me LOL. Ok.. JR

"Ok, dont forget, a 9" wheel is actually 10" Help?? JR"

I'm really not one for "rolling", "hammering", "bending" sheetmetal. Why not just measure it to fit. Leaving 1/4" is not really enough wiggle room. You will probably get a little side to side movement as well. I usually leave at least 1/2" to 3/4" of room on each side of the tire. You also have to account for tire height as on many of these cars, if you get too tall a tire, they hit up in the wheelwell.

Here's a link on a good read http://www.carcraft.com/techarticles/ccrp_0610_muscle_car_performance_wheels_fitment/index.html

Make a tool or get yourself a WheelRite...well worth the time and effort.

I'm really not one for "rolling", "hammering", "bending" sheetmetal. Why not just measure it to fit. Leaving 1/4" is not really enough wiggle room. You will probably get a little side to side movement as well. I usually leave at least 1/2" to 3/4" of room on each side of the tire. You also have to account for tire height as on many of these cars, if you get too tall a tire, they hit up in the wheelwell.

Here's a link on a good read http://www.carcraft.com/techarticles/ccrp_0610_muscle_car_performance_wheels_fitment/index.html

Make a tool or get yourself a WheelRite...well worth the time and effort.


Umm, kinda funny Mike. I havent been here long, but I see the same wheel fitment questions pop up. And I strongly recommend measuring to make sure, and I always see you advising to measure also.

Its just timeless info, measure for YOUR car, your car is different than someone else's. If you are dropping the hammer on an expensive set of wheels, take the extra time to measure your car. Its so simple, buy the wheelrite, make a tool, make something outta cardboard at least, that will work. Just measure it. Its three hours (max) of work to make sure. JR

I completely agree. That is exactly what I did (see the start of the post), but my measurements seem to suggest quite a different backspace requirement than others on the forum. It seems like almost everyone is saying that they actually got away with a 4.5-5" BS and 'wish' they had a 5-5.5. When I measure my car, there is almost no way a 4.5" rear BS would work at all. The front is even more pronounced. It seems like everyone is running big tires with a 4.5"BS. My measurments say that I really want a 3.5". So I did tons of measuring (and built a tool), but I think that my 'mistake' was the fact that I was not taking into account the fact that the rims are really wider than advertised (see a few posts up)

I completely agree. That is exactly what I did (see the start of the post), but my measurements seem to suggest quite a different backspace requirement than others on the forum. It seems like almost everyone is saying that they actually got away with a 4.5-5" BS and 'wish' they had a 5-5.5. When I measure my car, there is almost no way a 4.5" rear BS would work at all. The front is even more pronounced. It seems like everyone is running big tires with a 4.5"BS. My measurments say that I really want a 3.5". So I did tons of measuring (and built a tool), but I think that my 'mistake' was the fact that I was not taking into account the fact that the rims are really wider than advertised (see a few posts up)

Well it sounds like you have got the right ideas for measuring. Now that you know to add an inch for actual wheel size what are you coming up with? I made this lil tool to simulate a tire and rim, I also forgot to add an inch after I was already using the tool and knew to add it, just forgetful. :)

I cant help on the back, my car is really different. But the front is sorta stock, the rails are stock. M-II spindles though. I have 235/45/27 tires on billet specialties 8" wheels. So I set my car at ride height, set the tool up 9" wide. Got the tire dimensions from goodyear, which includes the side wall spread. Made the tire with wire. Then bolted it up and checked the fit with the wheel straight, full lock to lock. Then raised the tire off the ground by the A arm and spun the tool around again. Checked and rechecked, changing the back spacing some times to fine tune the fit. Got it to what I thought was right on the tool and checked it all again. Then swapped it over to the passengers side and did it all again. 17x8" wheels with 4.75" back spacing. JR

Here is the tool. It is set for a 17x7" for the pic
http://img.photobucket.com/albums/v366/Jrouche/Steves%20Nova/215-45-17.jpg

http://img.photobucket.com/albums/v366/Jrouche/Steves%20Nova/17incher.jpg

Not sure what you mean by "the rims are wider than advertised". Rims are advertised from bead mount to bead mount which is industry standard. Measuring from the outside to outside is NOT the advertised width. Measuring backspace is measuring from rim mount surface to the outside edge of the rim (backside & not to the bead edge).
Also note that each tire vendors product may actually measure different than another vendors tire even though they are stamped as the same size. There's enough of a difference that one 245/45 tire may not fit where another vendors 245/75 will, so always check each vendors spec sheets.

I'm not sure I understand - sorry for sounding stupid. When I lay my existing rim down, I am measuring from the point that mounts to the drum (that is the easy one) and then I am measure up to the 'flat' area running around the rim (it is about 1/4" wide). I am not measureing up to the outside 'bead' running about the rim. All of the pictures seem to say to do it the way I am saying, including the picutre earlier in this post. I may have misread your post, but are you saying that I should be measuring up to the 'bead'?

Well - I may have answered my own question. It looks like I should be measuring the BS from the 'bead'

I actually go a step or so a little further when adding and figuring the Rim width Backspacing and Section Width, which is the measurement of a given tire size on a given Rim Width from inside sidewall to outside sidewall at its widest point when properly inflated. I also try to account for Tire Bulge past the rim. I have used this formula (right or wrong) for years with great results on some really tough fitments.

***Note the reason for adding 1 to the Rim Width is because wheels are listed in sizes from inside of the bead lip to the inside of the bead lip. As a general rule when measuring outside to ouside it adds roughly 1 inch***

Section Width - (Rim Width + 1) = Tire Bulge. ***Section width can be found on major tire companies' web sites***

Divide Tire Bulge by 2 for Bulge Per Side (Inner and Outer) and add this to the backspaceor the Dish (amount outward past the wheel mounting face) for the most accurate measurement of how a tire will sit in relation to the axle on ANY car.

Then you can from that find the actual distance on your car to see if YOURS can use that tire. From this you can easily figure where the out side will end up on your car as well. I like a t-square when the car is on its own weight on a frame table or drive-on style lift providing the lift is leveled.


Example below is on my 64 Chevy II SS with stock 5 lug 10 Bolt.
I use a Michelin 215/60-17 on a 17x7 American Torq Thrust II with 4.75 Backspacing.
Their website say my tire measures 27" Tall and is 8.9 Section Width on a 6.5 Wheel. Since I run a 7" wheel my tire is now 9.1 Section Width since per the Michelin website and as a general rule tires grow .2" in Section Width for every .5" of increase in Rim Width.

SectionWidth - (Rim Width + 1) = Tire bulge
9.1 - ( 7 + 1) = 1.1 (Divided by 2 is .55 PER SIDE)

Now add .55 to the 4.75 Backspace and you have a distance of 5.3" to the inside (backspacing side) and if you want front side do:

(Rim Width + 1)-Backspace= Dish to the front (Then add the bulge per side)

( 7 + 1) - 4.75 = 3.25 Dish on the front side of the wheel OR

3.25 + .55 = 3.8 Actual with tire bulge included.

We know this is accurate because the inside measurement 5.3 + the outside measurement 3.8 = 9.1 Which is the actual section width of the tire. I forget Offset and plus this minus that and when I call my American Racing Depot they send what I order with no worries.

Wow! Two pages on how high the mounting surface of a wheel/tire is from the ground when it's on its side.:D
HR&C nice job! I have used the .2 since you mentioned it in a previous thread and it was dead on. This is the one piece of the puzzle you won't find in the tire catalogs.Thanks

You are quite welcome hope it helps. Its a peeve of mine that the wheels and tires have to fit right before it can sit right. I go out to people's garages sometimes during their builds, sometimes as much as 3 times, to help them with tire/wheel fitment and ride height. Getting this right has sold me more wheels (even if I wasn't the cheapest - believe it or not) than advertising ever has.

Also note that each tire vendors product may actually measure different than another vendors tire even though they are stamped as the same size. There's enough of a difference that one 245/45 tire may not fit where another vendors 245/75 will, so always check each vendors spec sheets.

And your not kidding. Pic below, both 315mm tires, goodyear F1 on the right, and Sumitomo. Sure is a difference. JR

http://img.photobucket.com/albums/v366/Jrouche/Steves%20Nova/100_0053.jpg

And your not kidding. Pic below, both 315mm tires, goodyear F1 on the right, and Sumitomo. Sure is a difference. JR

http://img.photobucket.com/albums/v366/Jrouche/Steves%20Nova/100_0053.jpg

Hey JR, if storage is a problem I'm there for you dude.:D

a wheelrite + www.tirerack.com is also your friends, you won't have to do as many calculations, and looking at the section width column on tire rack will let you know how wide a certain tire is.

You are going to have zero room for error with a 9" wheel. On my 73 I have 15x8 Draglites with 4.5" backspacing with BFG 275/60-15 tires on the rear and they are no where near fitting without the back jacked up. I just ordered some 15x8 with 5.5" backspacing today and they should fit spot on, at least alot closer. It seems like each car is different, even the same year model.