Anyone using springs for friction drive?

I've got my first build going recently. It's a small 2-stroke on an old Schwinn Continental. It's not actually on a rack; it pivots on mounts on the chain stays. For that reason, I designed it to use springs for the scrubber wheel "tension". A shifter-like cable setup pulls to disengage the drive.
The maiden voyage was sweet. It bump started quickly and was quieter (to me) than I expected. None of my homemade parts exploded. It gave me a nice boost at low motor rpms.
But my tire had noticable wear after just 10 miles or so. I knew the drive roller (skateboard wheel) was slipping, and it was too large. I took the diameter down from 2" to about 1 1/4" and coated the wheel with grit. It gripped better and ran in a better rpm range, but still slipped and ground the tire down. So I've got stronger springs to try out. I think I'll get it to work well eventually. I don't give up easily.

So does anyone else use springs? If not, if you crank down your friction drive mechanically, is the tire always in contact with the drive roller even when the motor isn't being used? So what I'm looking for is a friction drive that can be engaged/disengaged with the throw of a lever while riding.

I've never heard of a spring, but I've heard of that shifter-like cable setup pulling the entire engine up that's mounted on a hinge. Then when the engine is dropped, the scrubber just turns toward the wheel, sucking the tire & itself downward. Pretty neat, IMO.

I seem to remember Iride showing a disengagement system he made, in the tread about Weedwhackers and chainsaws. Staton uses quick releases on tension rods.

The advantage of a spring based system would be that, once you get it dialed in, it would be applying a consistent pressure, every time you run it.

However, there are some disadvantages, too.

• You really don't want consistent pressure, every time. You want the least pressure that doesn't slip. And, the least pressure that doesn't slip varies, depending on weather conditions, tire pressure, tire make/model, tread wear, friction roller wear/material, and road conditions.
• If the locking mechanism fails, your scrubber wheel will be pressing against the tire with full force. Meaning that if you have to peddle, you've got a lot of extra drag, because you'll be spinning the clutch (or engine on a direct drive!)

These issues can be overcome, but, you'll need to think about how to do so. Maybe a big adjustment nut to increase or decrease the tension on the spring. And, some sort of interlock you could add to lock the scrubber in the up position. (that would also serve to keep the tension off any linkage when the bike is being stored...)

Another possible positive thing: Bike wheels, and sometimes tires, are occassionally not perfectly round. Springs might even out the tension. Pure speculation.

Yeah, Van. I also thought that spring pressure would be more even in an out-of-round situation. A downside is that bumps in the road make the pressure vary. Not much, I don't think, since the downward force of the springs is greater than the 5 pounds or so of the motor pivoting on the mounts.

Loquin. That's true about the locking mechanism. Mine is a lever I made from heavy aluminum stock (photo). It pulls a steel cable (brake cable) to lift the motor/scrubber off the tire. My springs run from the motor mounting plate to the dropout on one side; from the outboard bearing to the dropout on the other side (photo). I'm using those removable chain links with the screw barrels (don't know the proper name for them) to attach the ends of the springs to the rack holes in the dropouts (photo), so if my disengage cable ever broke I could easily undo the spring tension and keep the scrubber off the tire with some string or something.
I was using a friction shifter mounted on the seat tube, but it wasn't strong enough. The lever would want to creep up, and then the lever "clamp" wanted to twist on the frame tube since the pull is on one side. I made the new lever/clamp out of nice beefy material. It ain't going anywhere.
All is working good so far. I might have to play with the scrubber diameter and material until I get 'er tuned in. Right now the scrubber is made from hockey puck material. It's black rubber; a bit harder than the tire rubber. It seems to grip pretty well (when dry). Boy, I found out that tires sure wear down fast when your scrubber is slipping!
And here's a pic showing the paint job before I put the motor on (photo). It's yellow Krylon X-Metals over bare metal. The one with all the junk in the background seems to show the "metallic" effect best. It's hard to capture the effect in a still photo.