Drag racing suspension

[walkerbk]

So I have finished my ls1 4l65e swap and love it. Anything over 3k rpm gets a little scary. Went to the strip to show up all the mustangs and running street tires got nowhere... literally spun tires for the first 60ft and then just rolled through the finish. 

 

After buying a set of street slicks I went back. Still could not launch as hard as I wanted to but I was running low 12s.

My thought is coil overs, bigger sway bar, lsd differential and true slicks.

Well I am getting ready to start my next build (63 c10) and am researching what I am doing for suspension and have decided to keep the solid axle and add a torque arm.

After researching torque arms I thought why not put one on the datsun? Is there a large difference since it is a irs? My thought is the torque is still going through the nose of the diff (thus the clunk that I had to fix).

My thought is remove the extra mount over the diff that I added and remove the stock nose mount and replace it with a bar that goes up to my transmission mount. This would make it to where the diff is mounted on the rear and then to the front of the car right under the driver seat.

[walkerbk]

Btw, I have upgraded to poly bushings, springs and struts and do use the car for autocross as well.

[AkumaNoZeta]

Don't the older Miatas do something like that? I want to say I've seen it when we had one on the hoist one time, but it may be a false memory. Here's my thinking on it though, when you have a torque arm on a live axle, the force goes directly to the tires. While on a IRS, the tires aren't ultimately bolted to the differential so it won't have same effect. Also, a shorter torque arm is supposed to give more anti-squat since it's closer to the neutral line so if that the case, and it did happen to work for IRS too, than a short nose diff would be more effective then adding a arm.

I'm not really a drag racing fan myself, so I don't really know what you need to make IRS launch harder since you can really take advantage of the pinion climbing the ring gear. The only thing I can think of is keeping it stiff enough to limit squatting, and finding the proper camber angle to make sure the tires are flat to the ground when you apply power. Wheels and tires are a big part of it, everyone these days are running giant wheels with rubber bands. A smaller wheel with more sidewall gives a better contact patch and also a smoother ride. I used to have 17X9s with 275 cheap performance tires and I got sick of them so I traded for 16X8s with 245 mid-grade all season tires and the car performed better, spun the tires less, rode better, and didn't rub anymore. 

Like I said, I'm not a drag racer so somebody with more experience will hopefully chime in.

[walkerbk]

So miatas use a open style torque tube that runs from the differential to the transmission. It is the same setup as the corvette torque tube except that it is only closed on 3 sides to make it easier to maintain. And it doesn't need as much since there isn't the torque of a v8 in there stock.

From my understanding, the key to a hard launch is to be able to transfer vehicle weight to the rear wheels as fast as possible. 4 link push and pull the frame up applying all weight from torque to the wheels. The torque arm is mounted to the front of the differential and as the nose of the diff rotates up it pushes the front of the car up applying the weight to the rear. 

My thought is that even though it is an irs, the diff still acts the same. Under hard acceleration, the nose of the differential launches upward. This is the reason for the strap over the diff and the reason the rubber mount under the differential separates.

That being said, if I was 100% sure this would work, I wouldn't be asking here.

My design thought is to remove all the mounting on the front and then have 4 pieces of angle iron bolted to the nose going to a torque arm on both sides of the drive shaft. This will also make it easy to make a drive shaft cage.

[AkumaNoZeta]

Here's a photo I found of the torque arm on a Miata. I can't attest to the effectiveness of it. The way I see it, no matter what the diff's behavior is going directly into the chassis itself so I can't see it doing much, not like with a live axle where the diff is a part of the suspension instead of the chassis. I would just use a RT mount to keep the nose of the diff stable. My car has been modified to use a Ford 8.8 and it's still a few years away from being able to test drive or anything, so I have nothing to back up my claims, just keep that in mind. haha.

 

[walkerbk]

Check out this page, toward the bottom. They call it a subframe connector but it looks like it would do the same thing as a torque arm, lift weight off the nose and apply it to the rear for better traction.   https://ls1tech.com/forums/cadillac-cts-v-2004-2007-gen-i/1549239-tuning-irs-drag-racing.html 

Also, I attached two pics of the same thing where they attach the subframe to the rear suspension. Don't know how much upward torque is applied though, I think this design more helps with wheel hop.

[nullbound]

A close friend of mine has ran multiple 240z's in the 10's/11's with Small Block Chevys. His cars were updated with poly busings, solid front diff mount, welded diff, roll bar, tokico springs, tokico cartridges, 15x8 welds on the rear with 26x9 slicks, 15x4s on the front.

One of his can be seen in this DNI video from 2009: https://youtu.be/DXj573SiU58?t=2m49s

He technically needed a full cage for the times he could run. And he could have benefited from strengthening his frame rails and adding subframe connectors.

[walkerbk]

I have everything you just mentioned except the welded diff and the roll bar. I am running a street slick ment more for autocross so I know I am losing traction with them, but I was running the same times with my touring tires. I know I am putting down around 350-400.

Definitely wish I had them slicks like that though.

[Chickenman]

Longg time Road Racer Auto crosser and Hillclimber. And the problem is mainly your tires. Tires built for Autocross do NOT launch well at all at a Drag Strip.. The sidewalls are way to stiff for Drag style launches. Get some dedicated Drag tires and save the Autocross tires for when you run Autocross.  I've Hillclimbed a lot and believe me, you can easily lose a second on a run with Autocross tires. It's usually a slow roll on with lots of clutch slip that works well. Start to spin an Autocross tire and they just go up in smoke. Some are much worse than others. Hoosier DOT Autocross tires don't like being spun up at all. Difficult to get them to bite again when they spin up. 

And put a Posi in that diff. While IRS does not have the same inside wheel unloading issues that a beam axle has, it's not ideal at a Drag Strip. And it will help immensely at Autocross. 

I don't think the extra complexity and particularly the extra weight of the Torque arm setup on an IRS is going to help that much. Biggest change will be getting dedicated Drag slicks.

Eventually you may have to decide what you want to do with this car. Autocross and Drag Racing are two diametrically opposed Chassis setups. Nether is really compatible with the other. You will always be in a compromise situation until you decide which way to go.  That being said. I have seen IRS suspension cars launching VERY hard ( Wheels up hard ) at Drag Strips with Stock IRS suspension. No torque arm Wizardry used. However, that was on proper Drag Slicks and with a Posi. 

You could weld the diff. But it's not an ideal solution. Can be wicked evil nasty on the street if it rains. Be fore warned. Been there done that!!

 

 

 

[walkerbk]

Well I definitely plan on a dedicated set of drag slicks but I have to increase funds first. I have the set of street slicks, daily tires, and it is going to take some talking to the wife to get a 3rd set of tires.

I don't have a goal for the car. Built it to be reliable and I love going fast. Obx lsd is in the future with a set of wolf creek racing axles and coil overs. Should be about 2k next time I have the money. I am also currently seperated from my car. I had to move up north for a while and couldn't drive the Z up and the truck. I should have her back around Christmas 

[Bernardd]

I had consistant 1.6sec 60ft times with a r200 open diff on hoosier 26 x 8.5 with my 72.  

[260DET]

Know bugger all about drag racing but with a properly set up IRS why is a LSD needed? It's not going to do any harm but LSD'd should only be necessary when going around corners.

[nullbound]

For drag racing you want both wheels spinning off the line, so you want locking diff, spool, or welded diff.

[SleeperZ]

I'm thinking I got my best 60' times by not spinning, but I ran an open R200 when I was drag racing.  My best was 1.7 seconds.

[Namor]

This is headed a little off-topic but here is a podcast where Shane Tecklenburg answers a question about wheel slip.  He says pretty much any tire is going get the best acceleration at around 8%-14% wheel slip and more like 20%-40% for a slick.  Counter intuitive but it's reality.  

 

[SleeperZ]

Interesting, sounds like he's got a lot of direct experience, but I don't understand it at all.  And he may be right about the slip producing the best acceleration.  However he also said if you have no slip you have no acceleration, which is clearly wrong.  All you need is a frictional force to apply torque, slip is not required to apply that force. 

 

Again, he is likely right since we are discussing tire grip and not all of that is purely frictional.

[Namor]

I think that was just a way he had it explained to him that kind of helps understand, I don't think he was truly advocating that it is scientifically accurate.  A lot of guys find ways to understand why things do what they do without getting overly complex.