Front vs rear wheel sizes for best handling

[purplehazenils]

Hi, maybe this should be in the wheel & tire section but Im really wondering about performance and handling and this is where most of these conversations go down so I thought it might be better here.

I recently read in a different thread where someone commented on the affects of having different tire sizes front and rear on mopar a bodies. If the aim is having the best handling and grip can anyone comment on the affects of having a staggered setup where the front may be for example 17*7 and rear 18*8 versus 17*7 front and back?

Obviously it s difficult subject because so many things affect this like ride height, steering, suspension and chassis but any info or experience is welcome!

 

[273]

The problem now days is the best tires come in larger rim size. 17 was considered the best compromise between diameter and weight but offers a limited tire selection. Width and compound is more important.

PS. Larger over all tire diameter reduces brake and gearing.

 

[MRGTX]

Best handling and best grip aren't necessarily the same thing. On a stock suspension, best handling will likely come from lower traction tires that don't load up the suspension too severely and have predictable break-away characteristics.

IIRC, these cars plow first (understeer) so while you can probably run a 275 in the rear and a 235(?) in the front, you might do better going with a square set of 235 all around...not that anyone would like the looks of that.

Best grip will likely require some modern 18"+ diameter wheels...which almost always look like utter hell on these cars (IMO)...though there are a few exceptions.

The problem now days is the best tires come in larger rim size. 17 was considered the best compromise between diameter and weight but offers a limited tire selection. Width and compound is more important.

PS. Larger over all tire diameter reduces brake and gearing.

I agree with all of this...especially that last part about diameter. Whatever tire you pick, try to maintain the same outer diameter. There are plenty of tire size calculators out there that will help though there is some variation (up to ~.5") between brand/model even though this is supposed to be a standard. In any case it should get you in the ballpark.

Folks keep saying that tire sizes for 17" wheels are becoming limited...which just sucks. While I haven't been looking at tires yet, I had been keeping an eye out on a deal for 17" Mustang Bullitt wheels (SN95 generation) but now I'm thinking that just sticking with old 15" wheels might not be any worse.

 

[340fourspeedDuster]

I just put new wheels and shoes on the Duster last week. After reading in depth about other A body wheel selections, I went with 17" x 8" wheels. I purchased the tires and wheels at the local Discount tire store. These guys were VERY helpful. The first set of wheels ordered were an aluminum black 5 spoke with a 30mm offset. The Discount crew said to take one wheel and test fit on the car (car is 3/4 compete with its restoration, undergoing a Painless wiring harness install at this time) without mounting the tire to the rim. These wheels sat very close to the rear leaf spring and did not fit over the hub of the front disc brake of the 1975 Duster. Round two: wheels sent back, and a set of US Mags 17" x 8" with a zero offset were ordered. These wheels had a larger opening for the front wheel bearing hub of the disc brake. Once again, I took a wheel to the car, and test for fitment. Well over an inch of space between the leaf spring and outer wheel lip. Also fit good on the front. These aluminum 8" wheels are about 9.2" in overall outside width. Tires. The next challenge involved picking a brand, style, and size. The pick was 235 50 17 BFG G-Force. These are a directional radial tire that is rated well for grip. The car had 1.03" PST torsion bars, front sway bar, and offset bushings installed so it should be good for radial tires. The goal is to have a good handling car that is fun to drive. 235s were a little too big for the front, and touched the rear part of the front fender. On the rear, great fit. The fronts were changed to 215 45 17s which gave an inch or so of clearance on the fender. The Duster is equipped with power steering. The pics are shown before any torsion bar adjustment yet, just lowered to the ground. I do have ceramic Hooker headers on the 340, so there is concern of hitting the collectors on speed bumps if the car sits too low.

 

[purplehazenils]

Thanks for the input everyone!
Ive been thinking a bit about this after reading here. So, provided you can adjust over and understeer after choosing wheel and tire size with sway bar, shock and geometry tuning surely having different tire height and widths front and back cant directly have any negative affects on handling besides reduced braking with larger wheels. Or am I missing anything..?

 

[GMachineDartGT]

I've experimented with this for 30 years with many many track days under my belt. Besides the correct suspension parts for your application, wheel/tire combo along with low and even ride height make lots of difference when it comes to predictable handling.

That being said, same size tires are not a popular choice among the musclecar crowd so I usually keep my opinions to myself. My current setup is 18x9s with 255 40s all around using Falken RT615K tires. The tires I had before were more street friendly and fast too - Michelin Pilot Super Sports. These are 26" tall so they are close to OE heights.

 

[273]

Thanks for the input everyone!
Ive been thinking a bit about this after reading here. So, provided you can adjust over and understeer after choosing wheel and tire size with sway bar, shock and geometry tuning surely having different tire height and widths front and back cant directly have any negative affects on handling besides reduced braking with larger wheels. Or am I missing anything..?

Most super cars run wider in the rear cause of hp some run different rim sizes but idk if they have different overall heights but can't see that effecting too much.

 

[72bluNblu]

Tire height absolutely makes a difference. The biggest difference is the roll center. The taller the rear tires, the higher the roll center will be. If the roll center in the rear is higher than the front, that changes handling characteristics. Even if the car sits level front to rear, but the rear tires are taller, the rear roll center is higher than the front roll center. You can change your springs and sway bars to compensate, but you have to compensate for the difference.

Next is that tire diameter changes the contact patch and number of revolutions the tire is making. Taller tires make less revolutions and have a larger contact patch, not to mention have more rubber. That changes the tire temperatures and heat distribution. On a street car you'll never notice, but if your planning on competitive racing it will make a difference if your front tires are up to temp and the rears are still cold.

Ideally, for best handling you should be running the same size tire all the way around. If you're making less than 500 hp that's especially true. If you think about it, more of the weight of these cars is up front. Most of the braking is done in the front, and all of your steering is. Absent a high horsepower car, the front tires are more important than the rear. GmachineDartGT has the right idea, same tires all the way around.

But it really depends on what you're doing with the car. A small difference front to rear isn't going to be noticeable on the street. And of course your suspension has to match what your tires are capable of. If you're running modern design tires with softer compounds you need the torsion bars and shocks to deal with the extra grip, otherwise you'll just max out the suspension and get a crap load of body roll. Conversely, if you've installed big torsion bars, good shocks, sway bars etc and then slap on a set of hockey puck BFG T/A's you won't have enough grip to load the suspension and the car will be loose and oversteer like crazy.

 

[GMachineDartGT]

Im not going to turn this into the normal pissing match it usually turns into. I think its better to understand where information comes from.
I dont think anyone here really has any intention of any type of competitive racing in mind, me included, besides the Optima Batteries events. Furthermore, I highly doubt many cars can survive the typical 4 or 5 20min sessions on a track without some type of mechanical malfunction.

That being said, there are many ways to slice this pie, but most decide to go for looks over function. Many of the principles outlined make sense. However, you'd be surprised how good a properly set up C3 Corvette handles despite the hockey puck Radial TAs.....

Fire away, but my goal is to contribute to the forum with real experience not theories.

 

[72bluNblu]

Of course you can have cars handle decently well with BFG T/A's, but not if you set your suspension up for modern soft compound tires. That's not a theory, I've done it. If you put BFG T/A's on my Duster, with it's 1.12" torsion bars and giant Hellwig sway bars, taking it around an autoX course would be like driving on a skating rink, you'd be in oversteer mode the entire time and if there were any bumps good luck picking the cones out of the undercarriage. Just like my Challenger was when I was running 15" hockey pucks with a decent height stagger and 1.12" torsion bars, it was noticeably loose even on the street. Now that it has 275/40/17's on all 4 corners it feels much better. Step the torsion bars down to 1" or even 1.03" and you'd be fine, but that's a reduction in spring rate by almost 1/3 compared to 1.12's.

You have to match your suspension to the level of grip you provide with your tires, that was my point. Everything starts from the tires. I spent most of my youth driving a '56 Austin Healey on the street that had been set up for Vintage Racing and race compound tires. Lots of fun, but not nearly enough suspension compliance for street tires, it was loose as heck. It didn't stop me from putting tens of thousands of miles on it, but it needed less spring rate for what I was doing with it. It was always in oversteer mode at the autoX's I ran with it. Or did you not think I did that? Maybe not recently with my Mopars, but I've autoX'd, built cars for and competed in FSAE events while in college, spent numerous days at the track with my sport bike, and had professional EVOC training. I'm sure I'll have more tuning to do when I finally do get my Mopars to the track because I can't push them hard enough on the street to have everything perfect, but that doesn't mean I don't know what I'm talking about or that I'm not in the ballpark.

I agree, at the level most of the people on this board use their cars a small to moderate front-to-rear height tire height stagger probably won't be noticed at all. That's why I said it would depend on the use of the car and wouldn't be significant on the street, it's right there in black and white. Heck I run 275/35/18's up front and 295/35/18's out back, so my front tires are 25.6" and my rears are 26.1". But that doesn't mean there isn't a difference. It does change the roll center, and that does change the handling, regardless of whether or not I notice it running around on the street (and I don't). Pick up any of the books by Carrol Smith and you can read all about it, these aren't just theories I'm making up as I go along. It's information based on the experience of people that have raced at a far higher level than anyone here, myself included.

Do whatever you like. The question was asked if a front-to-rear tire height stagger makes a difference. It DOES. It changes the rear roll center of the car, that's a fact. Will anyone here notice that on the street or doing weekend warrior autoX's, Optima street car challenges or even SCCA CAM events? I don't know. But I'm not going to say it doesn't make a difference.

 

[dukeboy440]

Do whatever you like. The question was asked if a front-to-rear tire height stagger makes a difference. It DOES. It changes the rear roll center of the car, that's a fact. Will anyone here notice that on the street or doing weekend warrior autoX's, Optima street car challenges or even SCCA CAM events? I don't know. But I'm not going to say it doesn't make a difference.

Agreed 100% on this.

There is something else I'd like to point out when it comes to tires front vs rear, not only in height but width as well. The width affects the over track width, as well as the width of the rims/offset/backspace etc. These cars in their stock form, come with what, a 3 inch difference between the front track width vs rear track width.

Now, before I get rolling, let me first define track width, lots of people get this confused. Track width is measured as center to center of the tread between the pair of tires, front left to front right, rear left to rear right. Tread width is measured outside to outside of the tread. Tire width is measured outside to outside of the sidewalls.

According to the 74 chassis manual, A bodies were 59.2 inches on stock tires in the front and 55.6 inches in the back, or 3.6 inches different. so you need to keep this in mind when selecting the rim/tire size. I dont want to get to in depth and confuse anyone but lets just say its a balance between tread with ratio and track width ratio and how you chose your rim size. Because you can change your track width with different rims and tires and close that gap or worse, make that gap wider.

Another thing to keep in mind for ultimate handling, you have to match the tire properly to the rim. For maximum handling, most suspension/pro-touring guys will tell you you want your rim to match the tread width, or perhaps even be an inch wider. However, often times, what do you see, guys with hour glass shaped tires because they're running a tire that is pushing the recommended max width limits of the rim. Tread width difference between front and rear is also important.

Course this wont matter as much on a hobby car or street car but if you're looking for tenths of a second, it becomes a major issue

 

[rumblefish360]

AutoXcuda could be of some help in this though he races his ride, it may not be the best idea for the street. This topic is really not my strong point on cars.

With that said, I did equipe my '79 Magnum with 245/60/15's on all 4 corners. Nothing else special underneath it. 440/HEMI rear springs is the big improvement, which it isn't really. Stock sway bars and upgraded KYB shocks over the stockers or stock replacement style.

It's a good driver but far from a well handling ride. My point is the equal tire size and added width over the stock spec'd sizes are very nice.

 

[purplehazenils]

Some really good info posted here so many thanks everyone! Have a lot of reading to do with reference to what has been said. If anyone has any other input please do keep it coming!

 

[rumblefish360]

The only thing I can add or say that I have done has been just to follow what I have read to as far as I want to take it. In other words, I'm not making a Ralley car. But I will drive it hard once in awhile. With that said, the best upgrades I can suggest without breaking the bank and getting into over the top modifications is simplely to do these parts new and a bit thicker. Street versions. Personal tastes vary.

Thicker T-bars and heavier duty rear springs. Small block cars will like big block T-bars or even the upgraded units for better handling. HEMI/440 springs at the rear.
Increased size/thickness, anti roll bars
Excellent shocks. And excellent shocks will cost the most. Even the basic QA1's.

As mentioned, tire size can be tricky. I have not investigated a whole lot in this area. I have been happy and confident on the road enough with the same size tire in all 4 corners.

Will off set widths and diameters help?
Probably so but I did say I'm good with the same size on all 4 corners and have not investigated with a lot of varying widths. I have run them, just not made comparable bites to make a good suggestion.

I myself like as wide as a tire I can get under the car for best results but do not go to the size of looking stupid.

 

[MopaR&D]

From reading these posts (if I understand correctly), even though these cars need as much FRONT rubber as possible for max effort, if you are going to run different size tires front/rear at least make sure the overall height and aspect ratio are the same... Right?

I can attest to crappy handling with my 215-65-15 fronts and tall 275-60-15 rears (with the rear springs beefed up and sitting a bit higher) on my '70 Duster. The inside rear of the car lifts badly in hard cornering and it feels like the front/rear weight distribution is messed up. I wanted 255s for the rear but the Firehawk Indy 500s got discontinued and there were no more left; as a result like gmachinedart said with the 15 x 8" rear wheels the sidewalls bulge out visibly. Looks kinda cool in an old-school way but sucks for cornering lol.

 

[rumblefish360]

I'd say yes in the tire diameter being equal. The width shouldn't (IMO at least.) be to different.

What I did on my wife's '67 Cuda was 26 X 8 up front and 26 X 10 at the rear. Basic QA1 shocks. New rear leaf springs with a 1 inch over bend.
Front and rear sway bars coming.
Current bushings are rubber. PST poly graphite for the future.
(Loved it in my E body!)

 

[JoesEdge]

I've experimented with this for 30 years with many many track days under my belt. Besides the correct suspension parts for your application, wheel/tire combo along with low and even ride height make lots of difference when it comes to predictable handling.

That being said, same size tires are not a popular choice among the musclecar crowd so I usually keep my opinions to myself. My current setup is 18x9s with 255 40s all around using Falken RT615K tires. The tires I had before were more street friendly and fast too - Michelin Pilot Super Sports. These are 26" tall so they are close to OE heights.

View attachment 1714959563

What wheels are those that you're running? In order to to fit those size tires, what's the backspace or offset? Just curious because they would look on my Duster especially if I can squeeze 275's instead of 255's. Thanks!

 

[72bluNblu]

What wheels are those that you're running? In order to to fit those size tires, what's the backspace or offset? Just curious because they would look on my Duster especially if I can squeeze 275's instead of 255's. Thanks!

Here's the thread on his wheels GMachine Dart - New Wheel Package
( I have it bookmarked, his numbers are in the list of the references I keep to figure out backspaces and tire sizes )

I forget the name of them, but he runs 6.35" of backspace in the front and 5.95" of backspace in the rear with 1/2" offset springs.

For 275's on a Duster you wouldn't need the offset springs out back, but I would suggest a little less backspace even on 18x9's in the front. I run 275's on mine up front with 18x9's and 6.1" of effective backspace (13" rotors and a 3mm spacer). In the back it depends what rear axle you use. With an A-body 8 3/4 housing, BBP axles and the springs in the stock location you'd want about 5.25" of backspace.

 

[JoesEdge]

Here's the thread on his wheels GMachine Dart - New Wheel Package
( I have it bookmarked, his numbers are in the list of the references I keep to figure out backspaces and tire sizes )

I forget the name of them, but he runs 6.35" of backspace in the front and 5.95" of backspace in the rear with 1/2" offset springs.

For 275's on a Duster you wouldn't need the offset springs out back, but I would suggest a little less backspace even on 18x9's in the front. I run 275's on mine up front with 18x9's and 6.1" of effective backspace (13" rotors and a 3mm spacer). In the back it depends what rear axle you use. With an A-body 8 3/4 housing, BBP axles and the springs in the stock location you'd want about 5.25" of backspace.

Nice!! Thanks!! I shall bookmark it too. Since my front end has been rebuilt and stuff with bigger T-bars and all that, the limitations of 215's are showing quite loudly when I push it in the turns. So, need a bigger wheel package soon.

 

[rumblefish360]

In the back it depends what rear axle you use. With an A-body 8 3/4 housing, BBP axles and the springs in the stock location you'd want about 5.25" of backspace.

Asking a stupid question here but I'd rather not assume....

Moving the springs in would allow for a 5.5 inch back space wheel. Correct?

Now the question is how much can be fitted with the springs moved in?

 

[72bluNblu]

Asking a stupid question here but I'd rather not assume....

Moving the springs in would allow for a 5.5 inch back space wheel. Correct?

Now the question is how much can be fitted with the springs moved in?

Not a stupid question at all. Actually can be complicated.

Yes, moving the springs in would allow for a 5.5" backspace. More actually, I'd want about 5.75". Peter runs his 18x9's with a 5.95" backspace and a 1/2" offset in the back. But that's where it starts to get more complicated. I haven't seen his car, but I would guess that he might need a little less backspace than that to run 275's and keep them off the springs because they'll hang out over the rims more. Based on the math I'd want about 5.75" on 9" wide rims for 275's. That would give the same clearance to the springs that I have on the 295's on my Duster (not much!). But that's working off the math because I'm running a B body rear axle in my Duster and don't have anything that exciting on my Dart yet.

As for how much fits, it depends on the car. On a Dart a 275 is the pretty much the max with a 1/2" offset. On a Duster, well, it depends on how much you want to cut. I run 295's on my Duster with a 1/2" spring offset, but I also cut almost a full 1/2" off the quarter lips. A 285 would fit most Dusters/Demons/Dart Sports with stock quarter lips with a 1/2" offset. And that should be on the conservative side, my car sits low. With a higher ride height you'd have more clearance because the widest part of the tires would stay below the quarter lips, unlike on my car where you have to figure the clearance from the full section width.

 

[autoxcuda]

AutoXcuda could be of some help in this though he races his ride, it may not be the best idea for the street. This topic is really not my strong point on cars.

With that said, I did equipe my '79 Magnum with 245/60/15's on all 4 corners. Nothing else special underneath it. 440/HEMI rear springs is the big improvement, which it isn't really. Stock sway bars and upgraded KYB shocks over the stockers or stock replacement style.

It's a good driver but far from a well handling ride. My point is the equal tire size and added width over the stock spec'd sizes are very nice.

I agree with what GMachineDartGT and 72bluNblu are saying. Same size back and front. Wide as you can go in the front. My next move is 275/40/17's all the way around. But I need to look at the quality of tires available for the size. I might have to go to 18's to get the good quality tires.

Peter (GMachineDartGT) in your experience what's the pros vs cons on the Falken RT615K tires. vs the Michelin Pilot Super Sports? Sorta sounded like you preferred the Michelin's??

 

[pauly v.100]

I made a small compromise for aesthetics, 245/40 in front and 255/40 in back, wheels are the same 8.75x18 all around.

 

[2 Darts]

Thanks for the input everyone!
Ive been thinking a bit about this after reading here. So, provided you can adjust over and understeer after choosing wheel and tire size with sway bar, shock and geometry tuning surely having different tire height and widths front and back cant directly have any negative affects on handling besides reduced braking with larger wheels. Or am I missing anything..?

In general, putting a larger tire on the rear of an A-body will make it understeer more. The only mechanically compelling reason for having a larger tire in the rear than on the front is for straight line acceleration trials.

FWIW: In addition to adjusting handling through spring rates etc., add in the variable of tire pressures (aka 3rd world chassis tuning).