Rear Tires Track Inboard Around Curves -- How Much?

[cwbrick]

As we all know, when any car, whether it be a slot car or an actual automobile, goes very slowly through a turn, without any fishtailing, the rear tires/rear axle will track a tighter radius than the front tires. Or, to say this another way, in a slow motion turn the path of the car's rear tires shifts slightly towards the inside of the turn compared to the path of the front tires. In the world of slot cars, having some idea of how much rear tire shift there is in the turns helps when either designing a routed wood track or applying scenery alongside plastic track. If there is insufficient room left from the slot to the inside edge of the track/scenery start in the turns, then the rear tires can run off the inside edge of the track and/or run up onto the scenery during slow motion driving, which can result in the car de-slotting. Just how much the rear of the car shifts inboard on slow motion turns is dependent on two factors: 1.) the radius of the turn, and 2.) the car's "wheelbase", or the distance from the guide shoe to the rear axle centerline. Has anyone done a practical study of this by routing a series of slots of different radii in a test track and slowly driving different wheelbase cars through each radius and measure how much shift there was for each combination of turn radius and car wheelbase?

[dinglebery]

Nope, I haven't done what you suggest. That'll be interesting data. I could set up my GoPro HERO5 Session on a turn on my track (from above), take some 120fps video and drive the same car through at different speeds. It'll be interesting to see how speed plays a role.

[Wet Coast Racer]

When I routed my track, I already had a bunch of old Strombecker pieces. So I just pushed cars around those 60 degree curves to figure out the minimum offtrack space needed.

I suspect Scalextric track would have a similar tight radius using their inner curves.

Also, at some point when I was in High School, geometry had become an abstract concept rather than a useful skill ...

[cwbrick]

I now own two store-bought 1/32 slot cars: a Scalextric '69 Ford GT40 and a Carrera 2017 RB13 Red Bull F1. The reason for acquiring this Red Bull F1 car was because I expected it to have a significantly longer dimension from the guide shoe's pivot axis to the rear axle centerline, presumably it would be a car with one of the longest such dimension. I planned to run this car at slow motion speeds around a piece of test track with a number of different routed slots at ever larger radius and record the rear axle inward shift at each of these radii and compare those results with the test I've already run with the "shorter" Ford GT40 car. All of this is an exercise to determine just how much the rear axle (or the sidewall of the inside rear tire, actually) shifts inboard in the turns at slow motion speeds. I want to do this for two reasons. First, so that I know how to close to route my inner-most slot in each turn on the wood track I'm building so as to prevent the inboard rear tire from running off the track or up onto the scenery. Second, to then create a file or chart to share with this slot car community that documents this information by turn radius, by rear wheel track width and by the distance from the guide shoe's pivot axis to rear axle for future wood track builders. Well, imagine my surprise when the Carrera Red Bull F1 car arrived and the distance from the guide shoe's pivot axis to the rear axle is only 1/8" longer than the GT40. Even though the Red Bull F1 car's actual wheelbase is a long-ish 4 3/16" compared to the Ford GT40's shorty 3 1/16" wheelbase, the reason these two cars have such a similar guide shoe pivot axis to rear axle dimension is because the Ford GT40's guide shoe pivot axis is ahead of the front axle by 1/2" while the Red Bull F1 guide shoe axis pivot is 5/8" behind the front axle -- the car's wheelbase is essentially irrelevant for this exercise. It's this guide shoe/rear axle dimension that will define how much a car will shift its rear axle inward in a slow motion turn at a given radius, not the car's wheelbase.

So, this leads me to ask -- who out there has a good feel for which typical 1/32 scale race car has the longest guide shoe axis pivot to rear axle dimension? Is it a different F1 car than my Carrera? Could it be a late 60's NASCAR car Torino/Dodge/Chevy? Any help with identifying a "long" car would be appreciated. But please be sure to measure the guide shoe pivot axis to rear axle centerline distance, rather than the car's wheelbase when you post a reply -- thanks a bunch.

[dinglebery]

How about a Fly Truck
IMG_7942.JPG

[cwbrick]

Ha! Although it would be interesting to race trucks around a layout, I can't quite see myself designing a track to accommodate them. Good "outside the box" thinking though!

[cwbrick]

Could you please measure the distance from the guide shoe's pivot to the rear axle centerline on your Fly truck for me? I would appreciate it. Thanks.

[dinglebery]

Could you please measure the distance from the guide shoe's pivot to the rear axle centerline on your Fly truck for me? I would appreciate it. Thanks.

Absolutely! Here you go...

IMG_8431.JPG IMG_8432.JPG IMG_8433.JPG

[cwbrick]

The trucks really are quite long aren't they? I see it measures 119mm, or 4.68", from guide shoe pivot to rear axle C/Line. I really appreciate your measuring this for me. Would you be willing to make a similar measurement on a couple of your longer cars? By chance, do you have any NASCAR or Trans Am cars from the late 60's/early 70's you might measure on my behalf? If so, could you identify the car's make and model year (i.e.: '69 Ford Torino, etc.) and brand (i.e.: Fly, Scalextric, etc.) so that I might track one down and buy it to continue with my testing of how much the rear axle shifts inward on turns vs. the car's shoe to rear axle length?

[dinglebery]

I would, no problem but I don't have any of those kinds of cars.

[Wet Coast Racer]

The trucks really are quite long aren't they? I see it measures 119mm, or 4.68", from guide shoe pivot to rear axle C/Line. I really appreciate your measuring this for me. Would you be willing to make a similar measurement on a couple of your longer cars? By chance, do you have any NASCAR or Trans Am cars from the late 60's/early 70's you might measure on my behalf? If so, could you identify the car's make and model year (i.e.: '69 Ford Torino, etc.) and brand (i.e.: Fly, Scalextric, etc.) so that I might track one down and buy it to continue with my testing of how much the rear axle shifts inward on turns vs. the car's shoe to rear axle length?

I reckon you might be over-thinking this. Even slot cars with magnets on a magnabraid routed track will tend to let the rear end hang out a bit unless they're crawling along at 2 volts or whatever. Many folks who rout a track don't even use magnabraid, and copper tape isn't magnetic, so if your car's front end doesn't hit the edge of the track then it's pretty unlikely that the rear end will. And probably something like a Slot.it Porsche 962 will have as long a wheelbase and guide lead as most slot cars you'll ever race. By the way, if you plan to race Fly Trucks, then be sure to factor that in for any overpasses!

Anyway, if you give yourself an extra half-centimetre margin more than you think you need on the inside of the curves then no worries, probably. And that won't significantly affect the way you rout your turns. What's the plan? Compass turns, some squeeze sections maybe, what margins do you reckon you're working within?

[cwbrick]

I really appreciate your help and, yes, It's certainly possible I'm overthinking this but the test I ran on my 1/32 Scalextric '69 Ford GT40 on a piece of test track with a series of different radii turns cut into it was truly an "eye-opener". I made this piece of test track and copper taped it to see what the car actually does in slow motion driving because of the advice given on the AC2Car.org website to increase the inner slot-to-track edge distance from 1 1/2" (used on the straights) up to 2 1/2" for turns tighter than 12" radius. When I drove the Ford GT40 car at a slow motion speed around an 8" radius turn, the rear axle shifted inward over 5/8" -- which was much more than I was expecting. As the radius of the turns increased, this amount of shift lessened, but even at a 16" radius, the shift was still 1/4". Before I ran this test, I was prepared to have the 2 1/2" inner slot to track edge in all my turns, but now I know it would make many/most of my turns wider than necessary and give me less track length in my available space. The guide shoe axis to rear axle C/L on this car is only 3 3/8". A car with a longer dimension for this feature would certainly have the rear axle shift inward even more. What I don't know at this point is if the relationship of the guide shoe to rear axle vs. rear axle inward tracking is a proportional one or if it varies non-linearly. I need to get my hands on a few "longer" cars to run on my little test track to see what happens. I'd like to design each turn in my layout such that the inner slot to edge of track distance is reduced on the larger radius turns -- I'm just trying to determine how much it can be reduced for the "longest" style of cars I'm interested in running on the track.

Perhaps there's someone out there with a stable of 1/32 scale late 60's NASCAR, or modern Formula 1 or other cars that have a guide shoe pivot to rear axle dimension (not the wheelbase) significantly longer than 3 3/8", but less than what the typical truck has. If so, I'd really appreciate hearing from you and what kind of car it might be. Thanks.

[dinglebery]

I have 4 types of trucks:
A - a MAN (the one pictured above) = 119mm
B - a SISU = 120.5
C - a Mercedes = 114.5
D - and a Buggyra
I'll get data on the others and report back.

Not to confuse the issue, but a Vanquish car has a differential. I wonder what that would do?


EDIT: I've updated 2 Truck dimensions. Also, I have 2 SISU trucks, one is shorter than the one listed here.

[Wet Coast Racer]

Not to confuse the issue, but a Vanquish car has a differential. I wonder what that would do?

Break, probably!

[Wet Coast Racer]

Perhaps there's someone out there with a stable of 1/32 scale late 60's NASCAR, or modern Formula 1 or other cars that have a guide shoe pivot to rear axle dimension (not the wheelbase) significantly longer than 3 3/8", but less than what the typical truck has. If so, I'd really appreciate hearing from you and what kind of car it might be.

Food for thought, maybe. Absolutely, wheelbase is not the issue - it's a matter of guide lead distance.

But, you also have to figure in the track width of the vehicle. The wider it is between the rear wheels, the more room it needs to get around a corner.