Tire opinions needed......

lateapex911

lateapex911

Super Moderator
I'm finally getting to corner weight my car, and adjusting ride height has proved difficult.

What are you ITA RX-7 guys running for ride heights? I've been recommended to get the car to the 5" minimum, but theres just no way. Tire contact is a huge problem at any height approaching 5".

I'm running 205/60/13 Hoosiers, based on their recomendation that that size fit the required 7" rim the best.

I note that Hoosier builds a 225/50/13 that has similar width dimensions, but is an inch smaller in diameter, which would help tremendously with clearances.

I run Revolution rims, with a backset of 4" (near as I can measure!)

But Hoosier recomends a rim of 8 or 9 inches, although the dimensions they list are from measuring on a 7 inch rim!

So, the questions:

1 What tire sizes are you guys running?
2 What ride heights have you been using? Are these equal side to side? front to rear?
3 What wheels and backset?
4 How close to 50% cross weight can you get to?

Thanks for any and all help!

------------------
Jake Gulick
ITA 57 RX-7
New England Region
[email protected]
 
Originally posted by lateapex911:

So, the questions:

1 What tire sizes are you guys running?
2 What ride heights have you been using? Are these equal side to side? front to rear?
3 What wheels and backset?
4 How close to 50% cross weight can you get to?

Thanks for any and all help!
Click to expand...

1. I ran both 205/60-13 R1's and V700's, I also tried a 215/50-13 r1.

2. Ride heights were equal side-to-side (with me in the car) f/r had some rake. I measured my ride heights from fender lip to bottom of the rim. I did this because it was independent of tire size. (the suspension geometry would be the same regardless of tire size) Easier for me to duplicate with tape measure. I ran 19.25" from fender edge to bottom of the 13" rim front, 19" rear.


3. Wheels were 13 x 7 Diamonds with 4.5" backspace. This would explain your tire/fender interference. Before you try that much offset make sure you have adequate clearance between the wheel/tire and front lower spring perch.

4. Cross weight was no more/less than 49.8-50.2.

** What are you running for suspension? I felt that to further develop my car, I would have needed to go stiffer in the rear to allow the low ride height. Stiffer in the front to keep the f/r balance happy. Upgrade shocks to handle the stiffer springs...it was competitive for my needs, so I left it alone. 375#F, 200#R, Tokico Illumina 5's, GC panhard bar, no rear ARB, 29mm adjust. front bar.

[This message has been edited by Quickshoe (edited October 08, 2002).]
 
I'll add my .02 (for free). Being free, you get what you pay for (GRIN).

One thing to consider when running a specific tire (along with your considerations above) would be to also look at your rear end gear and tire size.

We run Hoosiers... (period). And we only run 225's. We adjust the size (45 vs 50) based on the track that we are running. I.E. I will run the 50's at VIR/Lowes. I like the additional top end on the longer straights.

Road Atlanta, we run 45's. We have played with both tire sizes and found what "runs" best.

As for suspension (a whole other topic), ride height will not vary by "much" with the difference in tire size. We have the car setup for the 45s and we don't modify the ride height for the 50s. That may be wrong BTW...so don't use it as the rule.

At my level of experience, I have not noticed a difference in the handling from one tire to the other.




------------------
Balz
#67 IT-7
www.balz.myip.org
"I live my life one Apex (seal) at a time"
 
Thanks for the response guys!

SO....people ARE using the 225s as opposed to the 205s! An epiphany! Have I been running the wrong tire??? AArrrg!

How do you think they fit on the 7" rim? How much pressure do you run? I've been aiming for 42 hot with the 205s.

Balz, what gears do you run? My current gears are either the stock 3.90, or 4.875s.

Quick- my current revolutions clear my front strut (Ground control) but the tire rubs the lower perch! (I actially had to add 1/4 inch spacers to the front, AND grind the perch) Finally, is backset measured from the hub mating suface to the inside of the rim where the tire mates, or to the exterior of the inside edge ??


Thanks

------------------
Jake Gulick
ITA 57 RX-7
New England Region
[email protected]
 
Jake, get rid of those 3.9's and keep the 4.8's in there, I use to run those every where and loved it.(LRP& NHIS &WGI)
 
Jake...he is 10000% correct...

Ditch the 3.90 for either a 4.88 (what I recommend) or a 5.12.

I use the 4.88 and it is honestly one of the most NOTICEABLE dramatic changes I made to the car yet.

It changed the car big time!

You can add spacers, just watch your track width...you have to not only be legal on the width (I think in IT), but you also have to watch to ensure that your wheels don't protrude. From the sounds of it, they dont.

I am running Circle Rims (13x7) and have had some "minor" rubbing when I run the 50s. The 45s dont rub at all.

Frank


------------------
Balz
#67 IT-7
www.balz.myip.org
"I live my life one Apex (seal) at a time"
 
Originally posted by lateapex911:
Quick- my current revolutions clear my front strut (Ground control) but the tire rubs the lower perch! (I actially had to add 1/4 inch spacers to the front, AND grind the perch) Finally, is backset measured from the hub mating suface to the inside of the rim where the tire mates, or to the exterior of the inside edge ??
Click to expand...

I would guess that it depends on manufacturer. The Diamonds I ran are a spun shell about .060" thick, so where the measurement was made wasn't critical. My pansports on the FF are measured from the exterior edge to the hub mating surface.

Re:225's....test...it will depend on the track. I was faster on 205's than 215's. Could have been gearing related(60's vs. 50's), could have been too much tire (rolling resistance) for the low HP. I ran, most often at WSIR. a.k.a. the fastest road in the west. The 2.5mi track has an ITA lap record of over 95mph average. 2 of the 9 corners are top speed/fortitude limited (for an IT car) not traction. For me turn 8 was faster than either straight (125mph or so).

Re:spring perch. If you went with a 7" spring vs 8" or 8" vs 10" would you still get adequate travel before coil bind? This would allow you to raise the perch 1-2" and out of the way of the tire. (Hypercoil offers a 7" spring)
 
Question to the 1st gen 7 folks. When the struts bottom out on the rubber jounce bumper on a Spec-7 could this bottoming out cause understeer (I have understeer on fast corners from the mid point out)on fast corners from the mid point out of the corner ?

Thanks

------------------
Have Fun

David Dewhurst
CenDiv Milwaukee Region Spec-7 #14
 
David,

Yes! Your spring rate went really stiff really fast.

Try going one click softer on the rebound.

In simple terms: In a long and/or bumpy corner your spring compresses with each little bump more than it has time to rebound back to its' position before the next "bump". Softening the rebound will allow it to extend quicker.

Added on edit. Do your spec7 rules mandate a certain spring rate?


[This message has been edited by Quickshoe (edited October 06, 2002).]
 
What a straightforward question with a not so straightforward answer!


I am also chasing down the right mix of tire size, springs, ride height, etc. Get Susko's book. Even if you don't follow his recommendations, it is enlightening, provides food for thought, and gives you an appreciation for how all this stuff works (www.gforceengeering.net).


Back to your questions. I have 13x7 Revolutions with 4" backspacing. Ride height was 6" / 6.5" F/R. Springs were 270 / 110 F / R with a 15mmm rear bar. I started on 205/60-13's, went to 225/50-13's and had .7 sec or so gains at Mid Ohio. I was kind of maxed out on this setup, so I went to heavier springs (450/300) with no rear bar, and Mazda Motorsports turn in spacers. Ride Height went to 5" / 5.5". This was equally fast, but the feel and balance of the car was gone (understeered if you can believe it!).


Changed to the gForce turn in spacers, the ones that are offset. This brought the balance back, but tires were shot by this time (10 sessions), and so was never faster. When I found all 4 tires were corded, decided to go to the 225/45-13 Hoosiers, but alas have not had the chance to run on them. Looking forward to it!

My thoughts at this moment are:

- My springs might be a bit heavy, overloading the tires, causing problems with feel and balance. After my next race on the shorter tires, I will find out for sure and adjust.

- The more the car is lowered, the more important the turn in spacers are. Get the ones from Susko or buy my old ones for cheap!


- I have not yet cross weighted the car - I don't have easy or free access to scales. My plan was to get it close from a feel perspective, then do it.


The message is that there is no simple answer, that it is all related. Tony Duncan who is the fastest RX7 in Cendiv runs the 205/60-13's, and I would be willing to bet his ride height is at least 5.5" in front. But, his set up is much different than what others run.

Rich Grunenwald
ITA RX7, #31 (for sale at the right price)
 
Originally posted by Quickshoe:
David,

Yes! Your spring rate went really stiff really fast.

Try going one click softer on the rebound.

In simple terms: In a long and/or bumpy corner your spring compresses with each little bump more than it has time to rebound back to its' position before the next "bump". Softening the rebound will allow it to extend quicker.

Added on edit. Do your spec7 rules mandate a certain spring rate?


[This message has been edited by Quickshoe (edited October 06, 2002).]
Click to expand...

David-

Quick is making a good point. I would think that he is referring to double adjustable shocks. Also, in these corners that you're pushing from the mid point out, are you on the throttle? Right away, I would have thought that the bumpers were responsible, but if this is occuring under throttle, it would seem that the front end should be lifting, right? If so, I would think (But remember, my engineering degree is from Holiday Inn, and is only in effect the morning after a stay!) that the bumpers were not involved. If these are REALLY high speed corners, you might not have enough power to lift the front end with the shocks rebound rate set really tight. (IMHO)

A question for those who have played with shocks. My setup uses the ubiquitous Tokiko Illumina adjustables...which are not dual adjustable. What's better? The Advance Design shocks look interseting, but they're not cheap! Are they the way to go or are there alternatives?

Quick: Good idea on the spring swap..I'll go measure immediatley!

I know I'm asking for trade secrets here....feel free to email me at the below adress if you don't want to broadcast your info across the net...I promise not to race where you are! Besides...the biggest problem with my car is probably me! No worries!

------------------
Jake Gulick
ITA 57 RX-7
New England Region
[email protected]
 
Originally posted by rgrunenw:
What a straightforward question with a not so straightforward answer!


Get Susko's book. Even if you don't follow his recommendations, it is enlightening, provides food for thought, and gives you an appreciation for how all this stuff works (www.gforceengeering.net).





Changed to the gForce turn in spacers, the ones that are offset. - The more the car is lowered, the more important the turn in spacers are. Get the ones from Susko or buy my old ones for cheap!


The message is that there is no simple answer, that it is all related. Tony Duncan who is the fastest RX7 in Cendiv runs the 205/60-13's, and I would be willing to bet his ride height is at least 5.5" in front. But, his set up is much different than what others run.

Rich Grunenwald
ITA RX7, #31 (for sale at the right price)
Click to expand...

Rich-

I agree! No easy answers! I actually do have Jim Suskos' book, and talk with him regularly, but as this dawned on me over the weekend I haven't called him yet. Actually it was his book that brought this up...somewhere buried in it was a reference to the "45 series Hoosiers". My eyebrows raised! I had long forgoten they even made them because THEY don't recomend them for a 7" rim. But as Jim also insists on a 5" ride height, I figured that something was amiss, and went searching on HOW you can get to 5"!.

I do run his turn in spacers, plus the unique front roll bar as well as the adjustable rear bar, the cool front drag link bushings, the tri link and the GT panhard rod. I still need to replace my stock control arm bushings....they're a pain!

All this stuff is great, but the problem that results is that with all the adjustability, you can adjust yourself into a real mess! Hard to test effectively at the races. Especially if you're adjusting for the wrong tires!

Edited for spelling, etc.
------------------
Jake Gulick
ITA 57 RX-7
New England Region
[email protected]

[This message has been edited by lateapex911 (edited October 06, 2002).]
 
I had an opportunity to see my old car today. Sold it to a friend. I measured the ride-height (excluding spot welded seams) front was 5.5", rear was 5.25". I changed my fender measurements in my original post to the correct figures. Both off by exactly 1". Front 19.25", Rear is 19".

Regarding re-adjusting ride-heights with tire size. If you always run 5" with whatever size tire, you have to change your spring perch height (geometry and alignment) when ever you change tire size. I chose to leave my suspension and alignment alone and deal with the 1/4-1/2" my ride height changed with tire size. Too easy to screw up my cross weights if doing this at the track.

My ride heights worked for me. They are LOWER than the minimum Ground Control recommended for a 1st gen ITA rx-7.




[This message has been edited by Quickshoe (edited October 08, 2002).]
 
Really Doofus question...

Where are you measuring ride height???

Frame rails?
Crossbars??

------------------
G. Brooke Carter
# 10 Challenge Car
Calgary, Alberta
 
Quickshoe,
I have a Ground Control suspension on my 1st gen. and have never been able to get it lower than 6.5-7 in. at the rocker. My ride height has been limited by jounce travel at the front struts. With approx. 1/2 of a stock jounce bumper, any lower and the front struts "bottom out" in constant radius sweeping turns. Did you do anything special for your setup? What spring rates?
 
You can always try shortening the strut or raising the top mounting point to incrase shock travel.
 
Originally posted by RacinRich:
Quickshoe,
I have a Ground Control suspension on my 1st gen. and have never been able to get it lower than 6.5-7 in. at the rocker. My ride height has been limited by jounce travel at the front struts. With approx. 1/2 of a stock jounce bumper, any lower and the front struts "bottom out" in constant radius sweeping turns. Did you do anything special for your setup? What spring rates?
Click to expand...

No jounce rubber. Made sure that the coil spring would bind prior to the struct bottoming. As to not damage the strut. I ran 375# spring front, 200# rear. Maybe our tracks are just really smooth.



[This message has been edited by Quickshoe (edited October 08, 2002).]
 
Originally posted by bcarter:
Really Doofus question...

Where are you measuring ride height???

Frame rails?
Crossbars??
Click to expand...

Not a doofus question at all. My ~5" measurements were taken at the lowest point, excluding seam, on the rocker panel at the front door seam.

The rear was measured at the rocker panel where the trailing link mounts.

The set-up sheet that I obtained from Ground control measured the ride height from the bottom of the wheel straight up to the fender lip. They suggested this method as it is directly related to the suspension geometry, and independent of tire size. It's much easier, too. If I were in IT, I would use this method and have a 5" tall block that I would slide under the rocker panels to make sure I'm legal.
 
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