Inliners International Forums General Discussion Ford TIRE SIZE - LESS IS MORE

My 300 SIX powered altered has a chassis tag dated 9-10-01. That's right, one day before the 9/11 attack. As I reflected on the fact that my front line racer is now nearly 19 years old I realized a lot has changed in that 19 years.

When I built the car I was racing quarter mile. Now almost all of the tracks have switched to racing the Super Pro cars to eighth mile. When I built the car track prep was marginal. Now at most tracks the track surface prep is so sticky you can't walk across the track starting line without walking right out of your shoes.

So I considered how my car was built and needed to make some changes. It has 4.30:1 gears in it. This is a typical gear set for a typical bracket racer that wants to keep the engine revs low to save on valve train parts. Optimal gearing for fastest e.t. would have been numerically higher but needing a more expensive valve train and other upgrades for a higher RPM band. Additionally, since I'm now doing the eighth mile the engine is running even less RPM going through the lights. This has led me to conclude that I need a smaller slick.

A smaller slick is lighter.
A smaller slick has less rotating mass / angular momentum.
A smaller slick is more aerodynamic.
A smaller slick gives a higher effective gear ratio.

I was using 12 x 31.5 x 15 slicks. A obtained a pair of 10.5 x 28 x 15 slicks to try. I guess I did not at first realize how much would have to change to use these tires. For instance,

The 4-bar instant center pivot point
The ride height of the chassis
The driveshaft angle
The pinion angle
The anti roll sway bar link
The wheel width and offset was different between the two sets I have
The wheelie bar height

So I have made the required changes to my chassis to run these tiny tires. I am waiting for a longer set of wheel studs to put the car back on the ground. I have had to go with longer coil-over shocks and new axle housing brackets. I'll post results as soon as we can get the car out. Stay tuned

FTF

Some of that I kinda get / understand , But not the change in pinion and drive shaft angle. why would those two change? because to me those should not be moving I would'nd think??

I didn't consider it either. But if you are going with a tire that is, say, four inches shorter then that means you have to raise up the back of the car TWO inches with respect to the axle centerline to maintain the same ride height. Doing that lowers the back end of the driveshaft relative to the surrounding chassis tubes and tunnel so you have to be sure there is still adequate clearance to everything. Depending on where the theoretical pivot point of the rear axle is - the instant center - the pinion angle may also change. Fortunately in my case there was still adequate clearance for the new driveshaft location when it was cycled through its new arc of travel and the pinion angle looks to still be close to optimal. The instant center will change because the back two 4-bar pivot points are now 2" closer to the earth while the front two are unchanged, effectively raising the instant center of the rear suspension. I think I may actually want the I.C. higher as it will cause a harder initial hit to plant the smaller tire. The shock rate needs fine tuning too since I put on longer shocks with more travel and a little softer spring rate.
It all becomes a big fascinating juggling act. Stay tuned...

So not really to much to worry about on a leaf spring car? Or a rigid rail or dragster set-up?

Every car would present some challenges with a significant slick size change. I can't think of any car that would present no challenges.

It's the cheapest and easiest way to quickly change the effective gear ratio, but if you don't have the rest of the car set up to handle the change, you can get yourself in trouble quickly too!!

We would drop from a 14 X32 X15 on a dragster we ran, to a 13 X29 X15 for certain shorter tracks. The slicks were mounted on wheels that interchanged and we're make adjustments with timing and a two step chip in the ignition.

FTF,
Would have going to a Stocker size slick say a 9x30x15 saved from having to make so many changes?Just went and did a quick look at Mickey Thompson, their drag radial slicks are a few pounds lighter than the bias ply slicks.One of their Pro drag radials is 30" tall and 10.5 wide at about 23 pounds.

this is a topic that came up on another site , where when doing a burn out the car would swing to the right But launch straight and run straight. Someone said to check the tire run out(growth) and to swap it to the other side to see if the car would swing the other way.
Does this even sound like a true deal?

Sure!! We had a stick shift Camaro, (Richmond/Doug Nash 5 spd.) that would veer left out of the water box, but ran straight as an arrow. It was something that popped up with a new set of tires. We swapped the tires from side to side and the car veered right. Still ran straight when making a pass. Video revealed that one tire grew a full inch more at high speed. Given the increase in radius, that would amount to a big difference on roll out. The spinning slick catches traction and jerks the car in one or the other direction. On a pass, the tires are mostly "hooked up", so there isn't a violent change in direction due to the difference in speeds.

twisted6, I know alot of guys that always check the tire diameter when mounted on the rim to make sure they're the same or close.Don't think it is as bad with radials as it is with the Bias ply slicks

Thanks now I know a little more and what to re check if I get his same issue with my new tires. I ran into the same problem with a set of used tires. And I didn't even think about the tires making that issue. I was looking at every thing else shocks, springs, I was even thinking it may have been something to do with my line locks not holding one of the front tires. I never had a issue like this before after I installed my Cal track bars. I know I could get the car to do that with the old slapper bars depending where I had the rubber nubber siting ? how close to the spring I could make the car walk either way. I just Never thought about the tires.

Love them Cal Tracs!!!

One additional thing I remember from a vehicle dynamics standpoint is that the torque reaction of the engine acting on the rear axle results in forces that tend to load the drivers (left) side tire downward and rearward and the passenger side (right) tire upward and forward. This results in the often seen tendency of the left front tire to lift more than the right front, and the body tend to roll right. That is why some performance cars use staggered rear shocks, one on the front of the housing and one on the rear of the housing.

This effect can play havoc with the yaw stability.

I never had this type of issue until I started using Used tires.in place of just buying new. I was just doing the cheap way out to make a event. If I just did a burn out with out the line locks I had no issue. My chevyII always did straight burnouts it never walked either way after I installed the Cal tracs. and the only thing different was the used tires. That is why I said we will see what happens when I put my new tires on the car. So the next time I get to hit the track we'll see.

SUNDAY JUNE 14 - FIRST COMPETITION THIS YEAR

First TT:
60 ft -1.197(all time best of 1.189)
1/8 e.t. - 5.697(all time best of 5.6078)
1/8 mph - 117.62 (all time best 120.10)

Second TT:
60 ft - 1.196
1/8 e.t. - 5.709
1/8 mph - 117.10

First Rd:
dial in 5.69
60 ft - 1.194
1/8 e.t. - 5.700
1/8 mph - 118.19 into a 13 mph headwind

Second Rd:
dial in 5.69
60 ft - 1.189 (tie - new all time best)
1/8 e.t. - 5.674
1/8 mph 115.33 on the brakes
broke out
If I had stayed in it it would have run a 5.639. All-in-all a pretty good start to the year.

If I translate that 5.639 into a quarter mile time it would be about a 8.81 quarter mile time. But that is assuming I would have had enough RPM capability to carry me through the entire quarter. I shift off a timer. With the big tall old tires my 1-2 shift was timed at 2.6 seconds into the run, at around 6800. Over the weekend I gradually reduced the shift time down to 2.1 seconds. At 2.4 seconds the revs were around 7500 which is beyond my comfort zone for a mere bracket race 300. I suspect I'd be doing well over 7000 going through the quarter mile traps. I'm content to stay around 6800 so the valve train lives indefinitely.
But it is comforting to know it will rev to 7K+ without floating the valves.
I'm running a roller cam with 185# of seat pressure with lift at the valve around .660/.680.

I discovered a couple of additional advantages to the smaller slick set further inboard. I'll add to my original post.
A smaller slick set further inboard now aligns my tires in the same track groove as all the rear engined dragsters I race.
A smaller slick set further inboard now puts the tires completely inside the wheel tubs, eliminating splattered water and rubber on my helmet visor during burnouts.


On an entirely unrelated note I put the engine in my nostalgic Logghe altered together and fired it up in my spare time. There is a Nostalgia Comp race coming up in July for that beast so we're good-to-go. I have to get the engine in The Frenchtown Family Truckster RPU out of winter storage mode for the same race. My neighbor drives that one.
Stay safe. Race safe.
FTF

a Big thumbs up to you French. you should have ran the other way from that head wind you mite have hit 131 LOL just kidding GREAT JOB.

Here's a video of Sunday's first pass.


https://mail.google.com/mail/u/0?ui=2&ik=00a8ad44e3&attid=0.1&permmsgid=msg-f:1669621087338001293&th=172baf886c48738d&view=att&disp=safe



We got rained out the following Sunday. Try again this one.

French
the link did not work. It seams to be tied in with your google mail

OK, I'll work on that - it was texted to me,

Okay Cool.

ONE ADDITIONAL UPDATE:
I went racing yesterday. This past week I made some fine tuning adjustments to the rear of the chassis - 4-link and shock setting.

That was it!
I went six rounds all the way to the finals where I red lighted.
The car is on rails now. woohoo

Killer DEAL.

FTF, Congrats on going rounds. How were the 60's after the adjustments?

Over seven runs the sixtys were all between 1.200 and 1.209. The altitude density varied from 2800 ft to 3100 feet. (The actual track altitude is 660 ft.) So, a little slower weather wise.