69CSHC

https://www.dragzine.com/tech-storie...the-dragstrip/

Great read !

Quick take. Reducing tire and wheel size about 40 Lbs total, worked out to the equivalent in performance gain of reducing 200 Lbs sprung weight or 2 tenths reduction in 1/4 mile ET.

PS they did this by swapping factory front wheels for skinnies. Fun fact whats considered a skinny is actually a wider tread width than late 60s Cutlasses came with factory new. 6.3 inch tread width vs 4.5 inch tread width.

wr1970

Hold the phone a second. Switching from iron to aluminum wheels and skinnies has to be way better than 60 cutlass set up with 6.3 vs 4.5 the weight alone is a savings . Stock tires are heavier than skinnies MT. I remember 380 saying he went faster with less width rear tire on his race car years ago.

rustyroger

Ideally you want the lightest wheels that will do the job of giving enough traction to handle the power and give proper steering control. You might also save a hundredth or so with skinnier tires due to reduced drag.
I read an interesting article some years ago by a British guy who built rally cars. The cars needed to be strong enough to withstand the hammering they would get during a race, but his job was to avoid overdoing it and increasing weight unnecessarily. He summed it up rather neatly when he said; "You have done a perfect job if the car falls apart just after it crosses the finish line in first place."

Roger.

wr1970

I went faster just getting tire pressure correct for launching. Found out not using enough air can slow you down . Took pictures of launches on same day starting with 11 pounds to 16/17 pounds. 29/10.5/15 MT. Was best at 16/17 pounds .pictures showed to much roll up on front side of tire at the hit with low tire pressure. Never had any wheel spin.

joe_padavano

While I understand the unsprung weight reduction, the reality is that the real performance benefit for a drag car comes from reducing the rotating inertia of the wheels and tires. Same principle as using an aluminum flywheel - less rotating mass can be accelerated more quickly.

dragline

That mustang is pretty streamlined from the older muscle cars. The math and theory are solid but pushing a brick like a 442 or other musclecar down the track may not achieve the same results. Aerodynamic drag may offset the desired outcome. In a quarter mile run maybe blocking off the grill opening or a lower front spoiler may improve et's. Lot a variables when making back to back runs. Weather, fuel consistency, engine temp. Not wanting to be a naysayer but special wheels and tires gets expensive, especially for 2 tenths of a second.

OLDSter Ralph

Lots of generalities and old news. Joe P is correct. The fact that narrow, taller, lighter weight front tires reduced rolling resistance and rotated at a slower speed than stock 14" tires/rims. Look at the older stock class race cars. They ran Cragar, Keystone and other brand wheels that were 15" X 4" to reduce weight, rolling resistance and friction. The tires were 15" X 4" wide and pumped to about 40 PSI.
Rear tires also used lighter aluminum 15" wheels. Tire pressure was a function of traction and wheel width (footprint). Wider wheels required more air pressure to keep the tire tread from cupping in.
I can tell you I made burnouts to get tire marks to be evenly dark across the tread. Light in the middle, add 2 more PSI and do another burnout. Repeat until you got both rear tires evenly dark, then you used the right rear air bag to make the marks the same darkness. If the left tire was lighter, bump the right air bag up 1 PSI.
The cars on 15" tires and wheels raised the center of gravity and helped in the weight transfer from front to rear, which helped starting line traction.
Ask yourself why they ran 15" wheels and tires on drag cars, because the wheels were also available in 14".
All of this was figured out and written about in old "Hot Rod" magazines........about 50 years ago...........back when "engineers" used slide rules and"computer" wasn't even a word.

coppercutlass

I'm gonna say this flat out from witnessing it. On some cars it won't matter. Some of these tests are combo specific . I went from a drag radial to a bias ply with no drop in mph. Despite the fact everyone swears a radial is faster which it is but for me between the 2 it was the same. You gotta take these things with a grain of salt. I'm not denying them but many dynamics are at play that make things vary.

joe_padavano

As with any optimized configuration, all the parts need to work together as a system. That means engine, cam, induction, exhaust, trans, gearing, tire diameter, etc, etc. No one change will be optimized.

wr1970

When I stated i went faster in my other post it wasn't mph. It was better ET.

jensenracing77

In my past experience of off road racing, the term unsprung weight is all about suspension and chassis setup but the rotational weight is included in that. Not sure I ever seen the term used specifically for rotational weight.

Bernhard

I agree 100%
What works for one engine and weight car will not work the same for another car.
The aluminum flywheel is a perfect example weight, HP. engine rpm, track conditions, tire all have a roll to play.
Trick parts like light weight internal transmission parts and riffle drilled axles are a perfect example of parts that have potential to reduce ET and increase MPH
Do they always?
Reducing weight and tire foot print generally produce positive results.
Et and MPH gains always come with a reduction in wallet diameter

69CSHC

Absolutely, I literally meant it in the sense of physical appearance and contact patch. Our stock base tire package weighs 40 lbs per wheel. A suitable skinnies aluminum rim combo is 20 lbs. So yes that's a huge difference obviously, even this Mustangs 18 inch skinnies are 5 lbs less than our factory wheels.
I believe that ! So much so that I am very tempted to put my fronts on my rear and go with a skinnier Torque Thrust set-up, up front. My biggest concern is I already have wheelspin issues with my current 235/60R 15s on a 8 inch rim width as it is...

Still my goal of running that ideal pass according to a computer program has me leaning that way.

Great point, 8 years prior to that article Hot Rod ran a similar but far more extensive test on a 1970-1972 Chevelle, (the proverbial brick you mention) They reduced size and weight both front and back. In the end even though they took off twice as much weight as Dragzines Mustang test, they netted 1/2 the ET gain.

https://www.hotrod.com/articles/ligh...ls-comparison/

Definitely, the Dragzine article backs both of you up. Weld Racing credits the size reduction etc far more than the unsprung reduction.

PS thanks for the posi traction info, hopefully one day I may be able to implement those techniques.

I believe that also. But its still worth giving it a shot in my opinion. To me you cant really lose, at a minimum the skinny or the like, likely matches factory drive-ability anyway ? Which is good enough for me...

coppercutlass

275-60-15 in the rear . You want some bite. skinny tires on the front aint fun . if you are talking real skinnies. I run a 165/85/15 in the front on 15x3 centerlines . about the skinniest radial you can run in one of these cars if ! the front end isn't too heavy. For gains honestly I would look else where. My buddy with a 72 cutlass went from steel wheels to nice light cragar super tricks which are LIGHT and light front runners with no changes. If you have a car that moves out fast with a quick moving chassis I can see the weight really affecting it but on a slower car where the front end rise is slow ( specially if its not set up to move fast) by the time the front end travel is maxed the car is already moving out and has pushed past the point of lifting the heavy rotating mass.

69CSHC

I hear you copper, i'm thinking skinnier but not real skinnies. My current fronts which i would throw on the rear, are Firestone 205 70R 15s on a 15 x 7 . That's the narrowest tire that will fit that rim width by the book. The thread width is 6.3 inches on this rim, so almost 2 inches wider than base stock. Rims are Vision Wheel 141 legend 5 Gunmetal, 20 lbs each. Tires weigh 19 lbs, so i'm technically 1 lb less than stock base wheel weight up front. 39 lbs vs 40 lbs. The thing is its my understanding that the disc brake conversion added up to 12 lbs per wheel area ?

If I then go 15 x 6 American Racing Torq Thrust VN 105 up front, we are talking 13.15 lbs. Then add Coker BFGoodrich Silvertown 165R 15s, All Season. Which weigh 17.75 lbs each. Thread width is 4.6 vs factory 4.5 so that's awesome. However Its a little short at 25.4 in diameter ( current fronts are 26.3 in diameter ) and the real deal breaker is that its max load is 1168 lbs for the tire and my car is heavy. Wheel wise this is about as light as I can get the fronts from my research and still maintain total street drive-ability. I drive my car as often as possible etc. It gets bumpy in my neck of the woods so you don't want anything that's on the delicate side. But again the 165R 15s max load is a deal breaker...

Ughhh my thing is a part of me so wishes that the car was bone stock so I could really see what its absolute baseline was. But i am not crazy enough to revert back to single exhaust and drum and drum brakes to find out. The 20 HP gain from dual exhaust is negated in my opinion due to unsrpung effect. Its like I am making passes with an additional 2 passengers...

Whether the unsrpung effect is mostly rotating mass, additional inertia, contact patch, or what have you. I would be reducing everything regardless to attain less weight.

Bernhard

Bernhard wrote:
Copper is right there are better places to spend your money if performance is your only goal.
We went to Centerline rims and skinnies for the look they were very popular in the late 70's and 80's the car picked up a tenth with the narrower liter tire combination.
How the car presents is also important