Race car suspension Class

Ranger Mike

Missile07 --Thanks for the kind words..pls. see post # 116 on page 8
When using a very trick 4 link rear suspension using roll over steer adds a big advantage on the dirt track surface. It allows the rear to roll into the corner without breaking traction. To review, when the rear rolls, if the outside wheel base grows longer than the inside wheel base we have roll over steer. This is because of the different in 4 link angles. I think if you start with left upper link at 13 degrees uphill, the rt upper link at 17 degrees uphill, and both lower links at 5 degrees down hill, it is a good base line. Make sure the rear end is straight and has not become bent thru wrecks and crashes.

missile07

We have been trying to figure out a way to properly measure at full drop the difference in the right & left. At full drop the left side is leading 3.5 inches more than the right. For the past couple of race nights we have shortened the right side bars up to 1 inch. Driver seems to think its helping him from the center off....but I am almost convinced that this is a band aid fix for something else.

Ranger Mike

i am n Kentucky today...will be back at home base tommorow. I think you have a roll understeer at center off. I agree that there could be something else off. Is is possibel to post the rear end settings on the 4 links?
What are the scale readings at each wheel? Have you read tires with pyrometer lately?

missile07

Well I dont have the settings right in front of me but I can post what I know...

Bar lengths

LR top 16"
LR bottom 12"

RR top 16" shortened to 15.75"
RR bottom 12" shortened to 11.75"

Bar Angles @ Ride Height

LR top 24-26 degree (chassis manufacturer recommendation)
LR lower 5 degree
RR upper 18-20 degree
RR lower 0-2 degree

As far as the wheel rates I would be lying if I said an exact number but he was running 120 lbs of LR bite & about 55% rear. The last night we were able to race I did pyro the tires and the RR was about 10-15 degrees hotter on both of the final two runs. Car hasn't finished out of the top 5 since June but just missing something.

Ranger Mike

not having my notes with me, i think you got just a tic too much roll understeer. My guess is to take 2 to 3 degrees out of top right upper link and read tires..if the temp drops a few degrees and car is better...may have found some of the problem...small changes ...dont do anything dramatic....

THORPE

Thanks for the reply. I'll start over on that anti dive and work on other factors as you suggest.
I'm goin way back and check more into roll center too. I moved mine down and to the right for last night. I got more turn in. I was carrying the left rear going in. I stiffened up both front springs and it help.

From what I'm starting to understand, lowering my RC even lower in the front will counter lifting the LR correct?

Ranger Mike

Thorpe ...great...now your getting it. When you dropped the ft RC you lengthened the lever between the CG and RC so stiffer springs were required to counter the body roll. You got more turn in becuase you used the weight transfer to plant the right front tire better and this gave you more cornering ability.. Excellent. The Lft rear was lifting at turn in BEFORE YOU CHANGED SPRINGS as expected because the right front was loading more and thus unloading the left rear. Also you reduced the " jacking effect" caused by the tire contact patch and RC that tries to lift the left front; hence left rear as well. Montitor the tire temps,,,LR spring change may be in order too? Your camber build should be reduced as a result as well which is a good thing so tire temps should tell you this too. Glad to hear about the change in handling..keep it going!

Ranger Mike

I was in a manufacturing facility last week and saw the 2013 Dodge Viper chassis. That car had huge Anti Dive. On the front the upper A-Arm mounts were more or less parallel with the ground but the lower mounts were significantly angled. Rear was like this as well but not as dramatic. If you think about it Detroit thinking is to have the car go straight when you mash the brake pedal so anti dive throws a lot of caster into the front geometry when in dive. The motion of the lower A-Arm moves the bottom of the spindle forward with upward movement and this adds caster which makes the car go straight under braking. Since the arc of travel is “ straight” up as a true race cars front end would be, this adds to spring rate somewhat. Anyway, that’s the rationale as far as I can figure it...

THORPE

Thanks for the info. I've learned alot from this thread. Mostly you.

Just to make sure I have my head right, when u say possible lr spring change you mean stiffer correct?

We dont really have a way to measure tire temps on these small scale cars. I may can use an infrared temp guage that we check engine temps with but I haven't tried that before.


I'm going back to review your spring info. This weekend I ran a split in the front springs. Stiffer lf. Equal in rear both stiffer than front.

The rear Rc is a bit higher than front. I have fully independent and adjustable a arms in rear to play with.

Ranger Mike

correct on left rear needing stiffer spring to keep the tire in contact with the track. Typically you will have non equal springs on rear too if you are set up to turn left. You could do it with wedge but a tad stiffer spring will do it too.

John LNZ

Hi Mike. I am a new member to this forum. I am a racecar fabricator in New Zealand. Have found your posts on this site really interesting as I am very much into the physics and the effects it has on racecar set up and construction. I build various types of cars for dirt oval racing exclusively on 1/4 mile tracks. These are spaceframe cars which are classed as "Super Saloons" in NZ and are unique to NZ but similar to a late model with late model front suspension with a sprint car type rear end and tyres. My initial question to you is could you please explain the physics behind moving the LR wheel further out on a slick track as we are struggling for forward bite. I notice a lot of guys are doing this and would like to understand the physics behind it. Many thanks

Ranger Mike

Thanks John..means a lot when we get reports from the other side of the world...and I was the guy who thought the internet was a FAD...
we went thru this on page 26 of this forum with a fellow running a drawf car...wider is better in that you have more cornering ability in that for a given set up you will transfer less weight and the tires can better accommodate the weight that is transferred. Also the Left rear to right front leverage is improved slightly...and thus side bite off the turn with a tad more wedge..read the whole page as we had a good debate on this...
General Rule of thumb - 1 inch lower COG transfers 3 to 4 % less weight. 1 inch wider car transfers 1 to 1 1/2 % less weight.
Assume we have a race car with 66 inch wide rear track and we add 100 weight to the ballast and we locate this weight 20 inch from the center line of the left rear tire. 20" divided by 66" = 37% of the added weight will go to the right and 63 % will go to the left side of the car ( diagonal weight is not in this calculation). Now if we increase the track width on t he left side by one inch we have 21" divided by 67" = 31% right side weight and 68 % left side weight. (from Short track chassis set up by Duke Southard)

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THORPE

Ranger mike:

What do you feel is more important On dirt oval?

Getting shocks to work perfectly with spring rate for each corner of the car

or

Using dampening to control weight transfer.

This is to settle argument.
I feel like proper spring selection should be used to control weight transfer and dampening should be used to control that particular spring

THORPE

To specify:

Using front tie down shocks and a lr with stiff bump and light rebound.

Ranger Mike

The purpose of a shock is to dampen the kinetic energy stored in the spring during weight transfer. Specifically the shock converts this vertical energy to heat energy by creating resistance to movement within the shock. Shocks control the RATE of weight transferred during cornering. Shocks have nothing to do with the amount of weight transferred during cornering. They can affect how quickly the weight is transferred.


So using shocks dampening to control weight transfer is not exactly correct. Using the dampening to control the RATE of weight transfer is correct.
The amount of weight transferred is dependent on the center of gravity, roll axis and roll rates. Where the weight is transferred is dependent on the spring rates. How quickly this weight is transferred is controlled by the shocks. So Thorpe, you are a little closer to the true purpose of the shock than the other fellow..my opinion. And you are right on with the tie down scenario in that you are tuning the chassis regarding rate of transfer after you get the spring rate real close to perfect!

THORPE

Thanks for the info. We just agreed to disagree lol.

What I'm looking for this weekend is s small bit of tire loading control by change in transfer rate.

On a med bank track with decent grip.
I plan to run:

Rf lf Soft.
Lr 2 rates higher than front.
Rr one rate up from front.

Then stiffer bump and rebound in both lf and lr.

From what I understand stiffer lf bump and lr rebound unloads lr tire load sooner on entry and takes cross weight out.
Then stiffer lf and lr bump adds cross weight on exit.

My hope is to be slightly loose entry and slightly tight off witch fits my driving style.

THORPE

Btw this spring setup has been very close with equal dampening all 4 corners