Scrub line solution?

Has anyone thought of using some kind of small wheels mounted some distance off the ground inboard or behind the rear wheels, where the rear frame is below (what would otherwise be) the scrub line? I'm thinking of situations where you've got a lot more sidewall on the rear tyres than the fronts, and little or no rake.

The idea is, both rear tyres blow out, the car drops onto the small wheels, preventing the frame and mechanicals from hitting the ground, and keeping the car rolling forwards rather than sliding sideways.

I don't think this would be practical at the front, because you'd want to keep steering control, and rigging small wheels that steer the same as the fronts would just be too much, not to mention look awful. But how about at the rear?

I see possible legal/roadworthiness issues, as it would be easy to do this so badly that it's not worth much, e.g. using wheels that aren't up to the weight and impact, or bearings that won't take the sudden rpm: though I suppose standards could be set by one of the hobby organizations. But done well, and from a pure safety standpoint, it should work?

Is there any precedent, and if so what was the consensus at the time? I've heard of trolley wheels under the back of sleds, for getting into driveways, etc.

 

Ummm, why not build the car with no scrubline violations?

 

Well, that's the idea. It's just another way of going about it.

 

I wouldnt want to be skating around on tiny wheels at high speed.

 

Just dont build a RAT ROD, build a real hot rod and the problem goes away.

 

I wouldn't want to be skating around on flat tyres at high speed, either.

 

I'll take flat tires on rims on a well built car over a POS car with skate wheels that was below scrub line ANY DAY OF THE WEEK. Learn to build a correct car, then you dont need to put bandaids on over poor engineering.

 

The lowrider guys used to do something similar IIRC.

 

Who said anything about skate wheels? Who said anything about a Rat Rod?

I was thinking more,

ultralight aircraft wheels, perhaps.

 

Doesn't make much sense to engineer in those big *** aircraft wheels when you could just do some simple planning in the building stage to not have anything below scrubline.

 

You're kidding, right?

 

Those wheels really aren´t big at all - but I rather doubt they´d stand up to having the weight of a car being dropped on them at 60mph.

 

They are big enough to make it stupid to figure out where to put them instead of building a car correctly.

 

True.

Now - about your avatar picture....

 

Haha I knew that would come up. When I am at ride height there are no scrubline violations.

 

The tires listed on a HUGE list on an ultralight sales site shows them from 10"od up to 17.5" od,,,,,, thats pretty freaking big.

 

We use run flat inserts in our FAV's, they can only do 120 but even on smooth surfaces they vibrate and try to shake the vehicle to bits. Make them a pig to drive.

 

The only time I've seen 2 (or 4) tires blowout at the same time is when some fugitive is fleeing and drives over spikes layed down by the cops.

But if you're really set on driving 'on the edge' you could perhaps build some rollers like below into the frame-rails;

Yeah, that makes real sense...

 

Not a bad option to prevent s****ing in a steep driveway but not to solve scrub line issues

 

I agree. A steel roller won't offer any lateral grip, so as regards staying pointed in the right direction it's little better than skidding along on a frame rail. But thanks, BigBlockMopar, for some potentially useful information.

But there might be solid-tyred industrial possibilities. The wheels basically have to last the time it takes to bring the vehicle to a safe stop. They have to be able to take the impact without breaking, and they have to stay round enough to keep rolling for 15-20 seconds. They are not intended to allow you to continue your journey as if nothing happened.

 

I've done a cursory search. There are a lot of suitable urethane-treaded industrial wheels out there. One that'll do the job is likely to be 4-6" in diameter and 2½-3" wide. I suspect that some form of rudimentary spring suspension might bring the load requirement down somewhat.

 

I have some industial skate wheels sat right here I picked up from E bay a while back,
3 1/4 diameter four inches wide from a pallet truck,
they would more than do the job.
But it does not mean I think it's a great idea,
I would just go for some sacrificial blocks of nylon if you must,
How tall are the side walls of these tyres again ?
And if the tyres are truely m***ive, how little pressure does the tyre have to drop to for the frame to touch ?
I dug my trike out start of the week because the sun had been out a few days,
Rears are Mickey T Sportsman's fairly big compaired to m*** of vehicle, can let all the air out and the back end drops an inch or so, sidewall will hold up well enough to get the 1/4 mile to an airline.
Unless the rear tyres come off the rims ( make sure they can't)
the back end won't drop the second a tyre looses pressure,
just how close to the deck are you going with the rails ?

Silly thought,
Citroen hydraulics,
use the stock height corrector valve to compensate for tyre wall height loss rather than load, so if a tyres blows the suspension maintains ride height saving the frame from touching down.
Added benefit of improved ride quality !

 

all you will be doing with adding a little steel wheel is making a low friction frame skate..
Not a good subsitute for correct , or safe Ch***is building..its a short cut, and a dangerous one at that.
If you dont care about what will happen to you or the car or its p***engers when or if you have a blow out..
think about the legal aspect of a law suit when you take out that family in the Mini van that was just minding its own business going down the road along the same path as you and your 4,000 pound roller skate!

you will not be able to control that car if it comes down on 2 small steel or rubber wheels attached to the frame at 45 to 80 MPH..no F-ing way!

Short cuts kill

Back to the drawing board.

 

Thunderace, I wasn't thinking of a specific situation with this.

But I've given a lot of thought to interconnected hydropneumatics. I wouldn't want a pump-driven (and non-interconnected) constant-flow system like the Citroën, though. I'd been thinking along more Hydragas-like lines, only taken a step further with separate bump/pitch and roll circuits operating air and steel springs, respectively. I don't know if that'll correct any scrub-line situation, even with the air-based self-levelling I've considered.

 

Do any MG-TC or Pre War MG drivers give any thought to this? The ch***is were underslung in the rear from the factory.

 

The big trucks that run at Bonneville in the Deisel Truck cl*** have a steel ski mounted to either side of the front axle and maybe the rear to. Never looked. But they work fine to keep the truck up and headed straight if and when a tire blows. Some of those big trucks go a lot faster than almost any of us will ever go.

 

maybe true..but they dont have to deal with traffic either..or some one making a lane change in front of them...or an itersection

just sounds like alot of work for a "patch job"

 

I'm fairly sure a front skid is required on Vintage dirt track race cars as a safety feature in the event a wheel parts company.

 

37Kid, those cars ran skinny tyres with very little sidewall. You can go very low indeed without running into scrub-line issues.

I'm very wary of anything that is intended to skid along the road, because it has the same friction characteristics in all directions: it goes sideways as easily as forwards. An urethane-treaded industrial wheel won't pull 1.2g's of lateral acceleration, but then a flat tyre won't either. It will offer some lateral resistance, though, simply because it is rolling and therefore subject to static friction.

Von Rigg Fink, I don't see how this is necessarily a short cut. That's like saying a roll cage is a short cut, like you should have built a stronger roof. You have a roll cage because you have a flimsy roof - or no roof. If this thing is well engineered it wouldn't be a short cut. I'm open to being convinced otherwise, but I'm not just going to take your word for it. You'll have to come up with better reasons than just that it's different to the normal way of building a safe ch***is.

 

This question makes no sense to me. There are plenty of ways to build a ch***is right down into the dirt without having to resort to engineering in "extra wheels"! Granted, you may get into completely modifying the entire frame and suspension, but is finding the space and correctly engineering an extra set of wheels any easier?? Wtf...