No, it is not possible to convert freewheel into a cassette hub, because freewheel is an old version and it is also cheap as compared to modern cassette hubs. it also contains ratchets within the freewheel but the cassette hub does not contain ratchets, and also freewheel cassette contains internal threading and the hub of these freewheels also contains threads on the drive side, these freewheel cassettes are simply threaded and screwed within the freewheel hubs. it is the old discovery but the modern discovery is called freehub, these are installed in higher-end frames but traditional freewheels are installed in lower-end frames.
These modern freehubs do not contain ratchets but these contain sprockets which are called freehub cassettes. and this new freehub system is simply placed on the freehub but do not threaded on the hub after inserting the cassette freehub contains a lock-ring system, which is tightly held and threaded to give support and hold the cassette on the hub. the main difference between both freewheel and freehub cassettes is that the freehub contains extra spacers and adapters on both drive and non-drive sides of the wheel, these axles would prevent the freehub from bending and deforming but the freewheel hubs do not contain side spacers and the riders weight and load on the bike is exerted directly on the ball bearings due to which freewheel hubs would start bending and deforming.
The other difference is that freehub wheels support large cassettes and sprockets but freewheel hubs do not support larger cassettes, there are certain smaller sizes of cassettes for freewheel hubs. the difference is simply in front of you old freewheel has 7-speed cassettes and higher 11 to 12-speed cassettes are not installed on these freewheel hubs but modern freehubs contain 11 to 12-speed cassettes. all new modern road bikes, mountain bikes, gravel, and BMX have freehub axles because these are much more powerful with greater performance and are able to lift higher weights and their axles do not break and bend on hard and aggressive terrains.
Upgrading From Freewheel To Freehub Cassette:
You need a new freehub and axle and also a new cassette and the old freewheel axle and cassette are not compatible with the new freehub and axles. install the freehub in a cassette and then cassette, lock the cassette with a lock-ring you can insert an 11 or 12-speed cassette, it is your choice. Remove the tire from the rim for upgrading the freewheel to the freehub. although it is an easier process it is time-consuming because you have to umount the wheel from the bike frame for the insertion of the freehub and cassette.
Bearings Of Freewheel And Freehub?
Bearings of freewheel are closely held with each other and the distance between one bearing to the other is very small and negligible while the distance of freehub bearings is larger and these are not closely attached to each other, larger spaced bearings have an advantage for the inner axle and the wheel, because by lifting heavier weights on freehub wheels, bike axle and bearings are not affected but freewheel bearings do not support aggressive rides and heavier weights.
Difference In Length Of Spokes:
When shifting from freewheel to freehub cassette, the length of the flanges of the hub would also change, because both freewheel and freehub flanges are of different heights( shorter and longer). you need to re-lace the wheel and if the length of the spokes is smaller or larger, then you have to buy the same-sized spokes for the freehub to make them compatible with the freehub flanges.
Length And Diameter Of The Chain Links:
The diameter and width of the freewheel cassette cogs and teeth are also different from the freehub cassettes, which means you also need a new chain for the freehub cassette, and also the length of that chain matters, it would be smaller or longer depending on the new cassette.
Difference In Clamp Holes And Bushings Of Front And Rear Fork:
Bushings and clamp holes of front and rear forks of freewheels are narrower and their inner diameter is not wider as compared to freehub cassettes, which means when shifting from freewheel to freehub you need larger diameter clamp holes and bushings of the frame.
Difference In Axle Lengths:
Hub axles are also different in their lengths, road bikes axles are 130mm, and mountain bike hub axles are 135mm, it depends on your wheelsets and freehub diameter, if it has a smaller diameter of 130mm then the road bike hub would be compatible with your axle, but if the axle is larger and wider then 135mm MTB hubs are compatible for your bike axles, you have to measure the length of the axles to make the hub and axles compatible with each other.
Difference In Spoke Holes:
Hub flanges contain 24 holes, 28 holes, 32 holes, and 36 holes for spokes, if your rim is 32 holes then you need 32 hole freehub when shifting from freewheel to freehub, depending on the number of spokes of the rim, and the previous freewheel, you have to buy new hub with the same number of flange holes to make the freehub compatible with the wheel.
Hub Overlock Nut Or Lock Ring( Dimension And Length):
Lock nuts are present on both right and left threaded sides of the hub, you have to measure the length of the freewheel for buying and interchanging a new freehub with the same length and dimensions, a smaller or larger length of the hub is also easily measured through the lockring and overlock nuts of the freewheels.
Spacing Of Freewheel And Cassette:
If the freewheel hub and cassette are of different brands, for example, if the freewheel hub is of SRAM or any other brand company and the cassette is of Shimano company, these both would not work smoothly, the difference in branding would also matter a lot for freewheel hub and cassette, use the same brand for both freewheel hub and cassette to make them compatible with each other.
So if you are interchanging the 8, 9, and 10-speed cassette with a 7-speed cassette bike and freewheels, then it is not a compatible setup because the distance between the clamp holes or dropouts from right to the left hole is 120mm for the 7-speed cassettes and these dropouts are not compatible for larger and wider 8,9 and 10-speed cassettes, which have 130mm of dropouts on their frames.
But it does not mean that all the 7-speed cassette bikes have 120 or 126mm of dropouts there are bike frames that have 130mm to 135mm of dropouts in frames, for these frames you can easily swap 7-speed cassette with 8,9 and 10-speed cassette.
Is 8-Speed Cassette Compatible With 7-Speed Freewheel Hub?
Yes, 7-speed freewheel hubs are compatible with 8-speed cassettes and you can easily insert 8-speed cassettes on 7-speed freewheels, but it cannot be said that these are compatible with each other, because you have to face a little bit of difficulty in the lining of the 8-speed cassette, you can tight or lose the screws of the derailer for correcting the chain line with the 8-speed cassette, and also 8-speed cassette is larger and wider than the 7-speed cassette, while it is possible to insert 8-speed cassettes on 7-speed freewheel hubs.
The incorrect center lining of the 8-speed cassette is maintained by inserting 4.5mm spacers within the 7-speed freewheel hubs. spacers would perform better with this non-compatible setup, but still, it is not a long-lasting setup because you have to upgrade the 8-speed cassette freewheel hub, otherwise, cassette cogs and rim center lining would rub badly and decreases the lifespan of the cassettes, and the rim.
Because the distance between eighth cogs of the 8-speed cassette is also different for 7 and 8-speed cassettes, 7-speed cassettes cogs have a smaller distance and the chain can easily slip into the other cog by shifting the gears from the handlebar, but due to the greater distance in 8-speed cassettes, you will feel difficulty in shifting gears from higher to lower and lower to higher cogs.
Are Freewheel Hubs Good For Heavy Weight Riders?
It is said that freewheels are even better than the freehubs, if you constantly replace them after one year, the bad thing about the freewheel hubs is that these are made up of metal and steel, which can easily bend and deform after riding through harsh terrains, so for heavier riders if you are still using freewheels then replace or change them after every year to increase their working performance.
Check the bearings of the freewheels on both right and left sides and also check the axles after every month, if the axle started bending from one side or from the center line, then you have to change or replace the bearings of the exact side with new bearings to avoid risks on the trails.
Is It Possible To Stretch 126mm Dropouts To 130mm?
Yes, it is only possible if the frame of the bike is steel, but it is not possible for aluminum and carbon frames because these will break as compared to steel frames. both the left and right tube of the front fork and rear seat tube is stretched to increase 4mm of inner spacing.