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Author Topic: Dodge Dakota C/BMMP  (Read 37050 times)
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ggl205
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« Reply #150 on: July 02, 2016, 04:12:41 PM »

You obviously enjoy fabricating, it shows in the quality of your work. That being said there are a couple of things in the front end that stand out. You have an extreme amount of scrub radius that is undesirable for the salt & the tie rod length is at least twice that of the A arm & that will induce bump steer. About the last thing you want is a vehicle that won't go where you point it.
  Sid.


Hey, Sid:

I have voiced my opinions on scrub radius before but do agree that there appears to be far too much positive scrub here. The only way short a-arms work without huge changes in camber and toe is to add roll and pitch or greatly reduce vertical travel. This is an LSR car so I would hope it doesn't see roll and very little, if any, pitch. But longer tie rods with short a-arms is not bad. They should help reduce toe changes over shorter tie rods and maybe with a bit of shimming, can eliminate toe change at full bump altogether. I would define full bump for this car at less than 2".

John
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Brickster
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« Reply #151 on: July 02, 2016, 05:50:43 PM »

I know what scrub radius is but how do you measure it? As the steering travels the tire doesn't appear to travel in an "extreme" arc. With the limited travel this car has it didn't have much in the way of bump steer. If it had lots of travel it could be more of an issue.
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1995 dodge Dakota, C/BMMP
ggl205
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« Reply #152 on: July 04, 2016, 08:46:36 AM »

Brickster, this may help.

John


* Scrub Radius.JPG (34.77 KB, 640x480 - viewed 105 times.)
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Brickster
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« Reply #153 on: July 04, 2016, 09:31:49 AM »

I read on this forum that near zero scrub radius is ideal but doing some web searching i find a number of .5" to 2.5" being an acceptable amount of scrub. What's your opinions?

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1995 dodge Dakota, C/BMMP
Ron Gibson
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« Reply #154 on: July 04, 2016, 10:22:20 AM »

Not to be a smart azz, but how many of those sites pertain to vehicles that only run straight line?

Ron
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ggl205
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« Reply #155 on: July 04, 2016, 10:52:00 AM »

I read on this forum that near zero scrub radius is ideal but doing some web searching i find a number of .5" to 2.5" being an acceptable amount of scrub. What's your opinions?



I think 2.5" either positive or negative, is too much for our cars. I am shooting for +/- .500" but will accept up to 1" if less cannot be easily achieved.

John
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kiwi belly tank
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« Reply #156 on: July 04, 2016, 11:08:11 AM »

Amongst other things, scrub radius is desirable to help pull a vehicle into a turn & is used in production & racing to different degrees for different applications.  Thats just not desirable when trying to go fast on the salt surface. Going fast on the salt is not as easy as one might think, over the years I've helped a lot of people get their stuff to go straight.
The amount of times I've heard "how hard can it be to go in a straight line!"
Check out some in car vids at the salt & watch the horizon & you'll be surprised how many cars are not actually going straight. I pointed this out to another liner guy who said he didn't even notice it in the car & not long after that he crashed it.
  Just here to help, not to criticize.
  Sid.
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SPARKY
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« Reply #157 on: July 04, 2016, 11:12:31 AM »

Sid is  so so right  my old car hunted or wandered bad we did several things that helped some the biggest-- getting rid of over 2.5" of scrub radius
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« Reply #158 on: July 04, 2016, 12:18:08 PM »

My spindles are Wilwood units that are based around Mustang II geometry. I got them because they are forged and made for affordable brake options. Am I mistaken or is the only thing that can change scrub are the spindle and wheel offset? It's been said a few times that my upper arms are short, I'm aware of that but that won't change scrub right? The other site was not LSR related.

Brick
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1995 dodge Dakota, C/BMMP
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« Reply #159 on: July 04, 2016, 12:20:44 PM »

I read on this forum that near zero scrub radius is ideal but doing some web searching i find a number of .5" to 2.5" being an acceptable amount of scrub. What's your opinions?



I think 2.5" either positive or negative, is too much for our cars. I am shooting for +/- .500" but will accept up to 1" if less cannot be easily achieved.

John

When I get back to the shop I'm going to measure it out and I'll post the results. I'm guessing I'm close to 1" judging with the calibrated eye.
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1995 dodge Dakota, C/BMMP
manta22
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« Reply #160 on: July 04, 2016, 12:26:42 PM »

If you have lots of scrub radius on the front end, you will know it when you stand on the brakes hard. Any little ripple in the road will pull the wheel in that direction. It makes for scary braking on asphalt!

Regards, Neil   Nashville, TN  (vacation)
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Regards, Neil  Tucson, AZ
kiwi belly tank
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« Reply #161 on: July 04, 2016, 03:37:34 PM »

The pictures might be deceptive but it appears that the whole tire is outside the intersection point, that was the reason I decided to say something. I'm also an advocate of correct Ackerman for the salt too, as that will also produce the same kind of turn in situation if not at least close to correct. A lot of production line vehicles are horribly wrong there, Ford "Enguneeers" had it totally a$$ backwards for years with a "toe in" on turns situation.
  Sid.
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javajoe79
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« Reply #162 on: July 07, 2016, 09:05:23 AM »

Good talking to you last night! Can't wait to see this truck run. You should get a red hat no problem!
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Brickster
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« Reply #163 on: July 07, 2016, 04:49:36 PM »

Good talking to you last night! Can't wait to see this truck run. You should get a red hat no problem!

Really appreciate your time and info, it was good talking to you as well.
I don't think it's going to be easy to get that hat but that's the goal.

As far as the scrub radius goes it has more than I thought, at least 2". So as of now I'm going to run it as is with my eyes wide open knowing the possibility of having to rework the front suspension.

Brick
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1995 dodge Dakota, C/BMMP
Stan Back
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« Reply #164 on: July 07, 2016, 05:05:36 PM »

Not an engineer (not even close).

But years ago I found out a (very) little about scrub radius.

I was used to rolling our roadster out of the garage and letting it roll forward to a piece of 2x4 where I wanted it to stop.  I'd just put on new narrower front tires on a set of neat, narrow Centerline wheels I'd been given.  When the car rolled slowly forward and hit the small block in front of the left front wheel, the steering darted to the left.  I thought -- what if that hit a small obstruction at 200 MPH? 

I then inquired why.  Scrub radius.  "Try to have your kingpin angle intersect the center of the tire patch on the ground."  I did the best I could and so far, all's well.
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