Rear ends ... open centre, spool or locked

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Jason P
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Rear ends ... open centre, spool or locked

Post by Jason P » Fri Jan 16, 2009 11:18 am

Chasing some info on what would be the best configuration for a borg warner diff for the salt. Thanks in advance

Dr Goggles
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who knows

Post by Dr Goggles » Fri Jan 16, 2009 2:41 pm

Even when the Muslims and the Christians are going on holidays together and can't even remember fighting people will still be debating this question, go to landracing.com and search the topic there....... there is however one thing that keeps coming up whenever the topic is raised and that is the saying "if it spins with a single wheeler change it, if it spins with a slippery,change it"

I do get the feeling though that the majority run LSDs , doesn't seem to be much enthusiasm for lockers.

we run a Ford BW 2.7something "Slip resistant diff" ....it hasn't been to the salt yet, we don't have excess hp and we have a pretty low drag shape....large frontal area and high hp are short cuts to testing the suitability of whatever you choose though I would suspect. The Broughans tank ran 206+ with single wheeler and somewhere between 4 and 500 hp.....

No doubt this will start some discussion but the usual arguments about diffs aren't that applicable coz we aren't cornering. when you are on a surface with low traction the argument is "is it better to distribute the force over two wheels or have one lazy so as to help keep it pointing down the track? some people could drive on marbles and will happily manage tire slip all the way down the track with both rears , others won't .....


see what I mean? :roll:
...few understand what I'm trying to do , but they vastly outnumber those who understand why..

David Leikvold
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slippery

Post by David Leikvold » Sat Jan 17, 2009 1:00 am

Here's my two bob's worth. I seem to remember Matt Saunders explained this to me a long time ago. This only applies to rear wheel drive cars. As we all know, regardless of the surface, an open diff will send power to the spinning wheel and not the one with traction. A locked diff doesn't care, both wheels run at the same speed all the time. The thought is that in an open diff situation with one slipping neither wheel generates any forward force which perhaps helps keep the car straight.
In a locked diff the wheel that still has traction continues to drive the car forward and the car starts to turn slightly towards the side that doesn't have traction. As the loose wheel hooks up again both wheels drive the car and without steering correction the car still drifts slightly off course. This means that maybe, just maybe, a locked diff car would be twitchier on inconsistently damp salt and require frequent tiny steering corrections. If both rear wheels lost traction the stability of the car would then be subject to aerodynamic forces for the time until traction was restored. If the car gets twitchy and the driver backs off the throttle the nose dips and the rear lifts, further exacerbating the instability.

I chose the slippery Celica as my project car partly because front wheel drive cars might not be so twitchy. With the weight of the cast iron V6 right over the drive wheels I should have better traction. With the standard open diff, if my front wheels (either or both) break traction the car just slows momentarily and puts more weight back on the front end, thus helping to restore traction faster and more consistently. But does my car get unstable when that happens? I don't know yet. I won't have any high compression engine braking effect trying to slow the back wheels, they'll just roll along without exerting any force other than downward pressure on the salt. I also intend to have all my ballast as low as possible and just in front of the back axle so that I avoid the pendulum effect and nose lifting effect of lots of weight in the boot. I also intend to keep the front to rear weight balance as close to 50/50 as I can with the weight away from the centre of the car. The transverse engine layout also eliminates the torque reaction body twist seen at the drags. Assuming the corner weights are correct, both wheels always carry equal weight regardless of throttle loading.
I'm not sure how an LSD would work on the salt, I suspect it might get confused with constantly varying slippage and might be slow to react and clunky which wouldn't help on slippery salt. Can anyone comment?
Good, Fast, Cheap, pick any two!

ben james
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Post by ben james » Sat Jan 17, 2009 7:23 am

and the beat goes on :P
ben james dlra#389
moriwaki monster.

teamleader
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Post by teamleader » Sat Jan 17, 2009 10:22 am

I have no practical experience with running a car on salt so I stand corrected if I am speaking out of turn here.
I do believe that there will be more to keeping the front of a rear wheel drive car pointing frontwards than just what type of diff you use. Horsepower at the wheels and variable slipperyness of the salt surface certainly would be factors. Frontal area, airflow management over and around the car as well as steering geometry are bound to affect stability. Things like varying atmospheric density and air movement (wind) will greatly affect stability and we all know that these, like the salt surface itself, are variables that cannot be controlled and that they can and do change constantly and without warning, thus making high speed stability control something of a tightrope walk.
It seems to me that having a locked diff and achieving tyre slip with one wheel and traction with the other is not possible. If you have a locked diff you will either have traction at both tyres or slip at both tyres and the faster you go the greater will be your chances of achieving tyre spin. You need to avoid or at least minimise tyre spin if you can. This is probably where the ability to judge (guess) natural conditions, judge chassis setup and your drivers ability to feel his car play a large part in maintaining frontwards motion as opposed to executing high speed donuts.
I realise this does not answer the original question but I believe that all factors need to be considered carefully and in relation to the overall package.
Then a bunch of good luck will probably be required to get it all as close to right as is possible for each run.

David Leikvold
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wiggly

Post by David Leikvold » Sat Jan 17, 2009 3:51 pm

Team leader, like you I have plenty of racing experience but no salt time, I'm just wondering out loud and hoping for some useful responses from those who may have more experience. Your thoughts about the locked diff are very reasonable. I was imagining a scenario where traction on the salt is less than perfect with lots of variation in moisture content and thus traction in various places along the course. If the racing line happened to pass through lots of small wet patches, or large ones for that matter, it is quite possible that even though both rear wheels are always turning at the same rate one may well momentarily have a better grip than the other. That loss of even driving force would induce the tiny rear wheel steering effects I was talking about. Reducing wheelspin is what it's all about. I know that traction control is forbidden, and with good reason too, so maybe a data logging system that compared front wheel speed with rear wheel speed could be used and displayed on the dashboard. It would then be up to the driver to make whatever use of that information that he deemed appropriate.
Good, Fast, Cheap, pick any two!

momec
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Post by momec » Sun Jan 18, 2009 10:11 am

My ute runs a 2.5:1 locked diff.
Weight distribution is 50/50 front to rear and side to side.
All up weight on the startline is about 1580kg
Horsepower never enough

I always think of the salt like driving on a loose gravel road. The salt has no camber which helps but traction varies.
I don't mind the car moving around under me a bit and when its loose the locked diff versus horsepower always does this.
Just pretend your on a gravel road at 174mph!
If you like a nice straight ride use an open diff.

Cannot wait. First gear off the line let her go. 100mph second aim for the horizon. 3rd gear settle the car, redline then pick top gear. 5000rpm hit the first stage button and nail it. Nosing over hit the second stage and high speed retard as I enter the mile. Hold it flat..................
Oh also, keep it between the cones. :lol: :lol:

Can't wait Chris
Acme Racing #251

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w3stie
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Post by w3stie » Sun Jan 18, 2009 8:48 pm

So I'm guessing, minimise the distance between the wheels would be good whatever diff you choose. Take this to the logical end ... a motorcycle ???
Why haven't motorcycles got the speed record?

teamleader
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Post by teamleader » Mon Jan 19, 2009 9:30 am

Chris do you get goose bumps and sweaty palms when you go thru that in your mind.
David your point about a computer to record wheel rotation front to rear will rely on having identical circumference tyres all around and I reckon that even if you have time to read all that at speed the driver will never be able to react quick enough and acurately enough to make a positive difference.
I suspect that this is one of those 1%ers that needs to be thought about after you have run a car for a fair while, adjusted all the easier things for stability ( if there are any), mostly have it sorted and you are still looking for better stability. Possibly even then with big variables that are out of our control such as cross winds, air density and salt surface condition the need for locked, limited slip or open could change but how will you know.

nitro-nige
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Post by nitro-nige » Mon Jan 19, 2009 10:02 am

V8 Supercars go round corners with a locked diff (spool).
Speedway sprint cars car rounds corners with a locked diff.

So in theory based on those examples to go straight you'd want an open diff.

Oh wait hang on Top fuel dragsters go straight with a locked diff.

I would have thought Horsepower & driving style would have more influence.
But like Dr Goggles said "if it spins change it"

momec
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Post by momec » Wed Jan 21, 2009 11:34 pm

Teamleader
I've had nothing to do in the last 3 years but go over it my bloody mind and my mind needs a rest. I'd like my body to do it next.
Roll on March!!!!!!!!
Chris
Acme Racing #251

the randy dero
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Post by the randy dero » Tue Jan 27, 2009 2:08 pm

I haven't driven fast on salt (yet) but have seen some high speeds on dirt and gravel. I have also driven on wet grass. Diff type doesn't make that much difference; if the driving wheel (s) lose traction the car will try to turn, usually right as the left loses less traction than the right.
Stephen

John Broughan
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Post by John Broughan » Tue Feb 17, 2009 4:03 pm

Hi guys, a bit late to be joining this discussion, but we ran a LSD at meetings past. We found this to be no problem with the street tyres, but the car was moving from one edge of the track to the other with the land speed tyres. You were too busy steering the car to be able to look at any guages or anything else. We have now gone to a spool, as we have upped our axle size in our little Holden banjo to stop them twisting. This meant we had to remove most of the guts from the center.
I have also added some ballast in front of the rear axle, which should give us a bit more time to look at the scenery as it goes past.
John Broughan
Empty Pockets Racing
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Reverend Hedgash
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Post by Reverend Hedgash » Tue Feb 17, 2009 4:17 pm

Cool John, so you are coming this year? Good luck with the upgrade. Any other mod's? Same class etc?

rH+
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John Broughan
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Post by John Broughan » Wed Feb 18, 2009 3:49 pm

Minimal changes from before. Some ballast, slightly larger throttle body.
Could be many things changed, but then you have no idea which things are helping, and which are not.
John Broughan
Empty Pockets Racing
200 MPH club life member

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