So I have put 4000 miles on my bike this year. No matter how tight the nuts are, the rear axle will spin a little when I brake or accelerate from it being stripped out. Any suggestions? Ive tried jamming small peices of metal in the space between the axle and frame and it only works for about 20 miles . e

I have a Sears stationary exercise bike from the 1970s. I has not been used for many years but is in good shape. We were going to the gym for exercise about 5 days a week before Covid19 but won't for at least a year so I thought I would resurrect this old bike. My wife is thinking about getting a treadmill but she suggested we try this first since we have it, it takes up less space, and cost will be less. Also she has a weak knee that operates better on a bike than on a treadmill. One of the reasons that we stopped using it is because it does not have a good motion. The resistance is felt pads and there is no flywheel so pedaling is choppy at almost any resistance. I would like to add a flywheel effect to this bike to add momentum and overcome the choppy ride. The front wheel is 20 inches diameter with spokes and removable plastic spoke covers. I know that adding weights near the rims will create a flywheel effect. I have not been able to find any such weights online. From search results it appears that 30 to 60 pounds is the usual flywheel weight on bikes that have them. I think 30 should be enough here. Any suggestions?