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Message started by Brian_H on 12/03/2 at 10:05:47

Title: Winter Rally Wheel Spin Affect Odo
Post by Brian_H on 12/03/2 at 10:05:47

Although I'm really curious about this, I'll keep my question simple, and hope for a complicated answer from an expert in the forums.

With snowy/icy conditions in TSD winter rallies, there must be at least a little wheel spin for all competitors, either from accelerating, corners or climbing hills. Although I'm driving AWD, as I'm sure some of you are, I would anticipate some wheel spin if conditions were slippery. This must have an effect on the ODO and could throw off the connection with the instruction odos over 50 or 60km of distance. Since I don't have a programmable odometer, is there something I or my navigator can do to avoid the numbers getting thrown off too much, or reduce the effect of wheel spin?

Title: Re: Winter Rally Wheel Spin Affect Odo
Post by DaveP on 12/04/2 at 08:30:07

Hi Brian,
I am surprised that no AWD afficionados have replied, so let me give you my two cents worth.

I do not think that wheel spin will be a factor on winter tsd rallies.

I used to compete in a rwd car (with a welded diff mind you) running the odo off the transmission and found that odo error was minimal. I assume that brake lockage cancelled out wheel spin. (I used to run with more rear brake bias in the winter to help get the rear end out.) In the early days I ran the odo off a front (non-driven) wheel and on occasion, I found odo slow down due to brake lockage, but again it was minimal.

Secondly, as I recall, most rally sections average thirty km in length (when you can zero the odo), not fifty or sixty. But that may have changed these days.

What you will need however is a navigator who can correct the odo error using a calculator. Chances are your stock odo will not match the organizers odo, and based on the initial route odo check, the navigator will have to correct distances during the entire event.

Dave

Title: Re: Winter Rally Wheel Spin Affect Odo
Post by Brian_H on 12/04/2 at 09:51:16

Thanks Dave,

What you're saying gives me some relief. My navigator's good, my tires are good, and the car ain't bad either. I'm guessing that my Sube's ABS reducing brake lock won't be able to counter any forward wheel spin, but I'm sure we can work it out.

I still wouldn't mind a response from a couple of AWD drivers anyway...

Later

Title: Re: Winter Rally Wheel Spin Affect Odo
Post by OWDLVR on 12/04/2 at 16:59:35

Alright...I'll jump in.

I run an Audi 90 quattro. I've mounted the sensor for our ODO on the rear diff housing.

We do notice the odo being off by .1 or .2 km, usually near the end of a section, when we've done a fair number of Km's already.

My navvie is usually pretty quick to realize that the ODO settings have begun to slip, and he just compensates for it. If he notices one reading is off, he checks it against the next...and can figure out how off we are.

We've only ever made one wrong turn because of it...it was a section where you had to turn at an unmarked road at XXkm. Problem was, there were 3 roads within about 300m. Truth be told though, we were a little confused at that point anyways...so even if we were dead on, we would have had to check it.

I don't find I lock up the brakes much...if I do, it's for what maybe 3 feet at a time? I do, however, spin the rear wheels more...and that accounts for more 'distance' in the ODO. I will probably swap my sensor to the front tranny to see if I notice a difference.

HTH,

-Dave

Title: Re: Winter Rally Wheel Spin Affect Odo
Post by DaveP on 12/05/2 at 07:49:34

Brian_H wrote on 12/04/2 at 09:51:16:

I'm guessing that my Sube's ABS reducing brake lock won't be able to counter any forward wheel spin,

Brian,
The general consensus among winter tsd competitors seems to be to disable the ABS. (I have never driven an ABS equipped car on ice, so I am only commenting from hearsay.) I believe all you have to do is remove a fuse to disable it on a Subaru, but you can check others.

I will contact Alan Ryall and ask him to contribute to this thread. He competed in the Subaru/Yokohama series last year in an Impreza WRX and is well qualified to answer all your questions.

Dave

Title: Re: Winter Rally Wheel Spin Affect Odo
Post by Alan Ryall on 12/05/2 at 08:39:57

I don't know about "well qualified", but I'll try to relate our experience.

The answers so far cover off most of the points about adjusting as you go along. You're really never going very far without a reference point with an official distance, so your navi can correct as you go along - sometimes running it all through the calculator, mentally adding the .05 or whatever. Just make sure you're communicating about the same readings - car odo as is or "adjusted" car odo, or the route instructions. You may find that you are adjusting it for awhile while the navi catches up on calculations. ie you are given 24.5 and you know it will read .1 high and be 24.6. After you navi catches up, remember to stop adjusting. In the heat of an event or after 8 hours or motoring you have to keep on talking !!

We do use a TerraTrip, which takes care of the official to car odo factors. However, it also is affected by drivetrain/wheelspin - in winter or summer. As tempting and as much fun as it looks, I don't you have to throw the car around that much in winter or get a great deal of wheelspin....maybe not more than hard driving on gravel.

AS for the ABS - I have a cut-off switch for it. I turn it off for gravel, but in the winter I like leaving it on in the slippy stuff. I like the feedback it gives as an indicator of how slippy it really is. When I get a little of the pumping I know I'm pushing; and I'll back off "a little".

Hope this helps. See you out there.

Alan

Title: Re: Winter Rally Wheel Spin Affect Odo
Post by Brian_H on 12/09/2 at 10:49:52

Just one last question in this thread. Would two wheel drive vehicles have their stock ODO sensors on the driving wheels or not? Just a curiosity...

Thanks for your 2 cents each.
BH

Title: Re: Winter Rally Wheel Spin Affect Odo
Post by Robert Roaldi on 12/09/2 at 19:20:21

By stock I assume you mean the odo on the dashboard. They are always hooked up to the transmission. Either mechanically connected to a gear running off the final driveline shaft or electronically to a sensor mounted on the final drive.

So, in relation to the above questions, the dashborad odo will always experience wheelspin error regardless of whether they're front, read or AWD. With AWD, there will be much less spin in general, there's too much grip.