I ride a Hero Sprint, MTB, with 6 gears in rear and 3 in the front. I am thinking about upgrading my 6 gears to something like 7 or 8. Is it possible, if yes, how much does it cost, and what are the plus and minus points i might face if i did that?
Yes, you can upgrade your mountain bike. However its a slippery slope of knock-on upgrades that can cost more than a nice, lightly-used bike.
For starters, you can't generally slap on a cassette or freewheel with more gears, because they're wider.
So the first thing to do is measure the "Over-Locknut Dimension", or OLD. This is the horizontal distance between the locknuts on the hub, and is equal to the gap between the inside surfaces of the rear dropouts.
If you have a steel bike frame, is slightly more options than an aluminium or carbon frame, because it can be cold-set (aka slightly widened at the rear dropouts) to fit a wider hub. Can't do that with aluminium or carbon frames,
If you have a steel bike you can increase about one size, so 126mm to 130mm or 130mm to 135mm. Then you have to align the rear dropouts again, using alignment tools like these half-axles showing minor tweak required.
If you don't realign the dropouts, the wheel will wander easily, even hard pedal presses can pull the wheel out of alignment.
Once you have a wider hub, you can fit a wider cassette (don't bother buying a hub that takes a freewheel - that way lies bent axles and wallet-tears)
So you need a cassette, which will require a new chain. Generally speaking 6/7/8 speed chains all use close to the same width, but you want an unworn chain to help prolong the life of your cassette.
Then check out the front chainrings for wear. They wear slower than the cassette, but if the chain is making bad contact it could be time for new chainrings.
You will very likely need a new right-hand shifter too. Your six-speed unit will have 6 indents/positions, and it won't handle the extra gears. If you have a friction shifter it might work, or it might not.
While changing all this stuff you may as well install new cable inner wires and outer tubes too. A cheap set costs about a happy meal.
Once that's all done, you need to reindex your gears, and might find that the 6 speed rear derailleur doesn't have enough range. First try backing out the limit screws, but if you still can't get the chain to reach the biggest and smallest cogs then you're up for a new rear mech.
- Entire wheel
- Rear derailleur mech
So yes its possible, but can be financially bad idea. If you have spare parts from another bike, it becomes more cost-effective.
Well, I tell you: you should think before upgrade your gearing. Ask yourself: Do much declines and inclines? In my experience, you cannot run out of a 3*6 combination on flats or even the meager gradients of elevated corridors in cities. As for if it's possible or not, yes it is. You'll need to change your freewheel, change your derailleur and change your shifters and this will cost you a great deal if you consider how much you actually spent on buying that bike. (Well I did some research, turns out that the upgrades alone will cost you a considerable fraction of the cost of your bike, especially since the country you're living in doesn't have an abundance of upgrade parts.) Also, you might need to get a new chain along with all the other things but it won't be required if you're lucky and you get a similarly spaced cassette.
A generic 7 speed cassette- INR 450 Generic 7-speed cassette on Amazon.in
Shimano's Tourney TZ derailleur- INR 1800 Shimano Tourney TZ on eBay.in
Shimano TX-50 7s shifter set- INR 1000 Shimano Revoshift 7-speed on bikeinn.com
Hero Sprint costs INR 6800 Hero Sprint on Amazon.in
As can be seen, the upgrade costs make about 47% of the bike's market price, which makes the upgrades highly absurd, also considering that with this money, the person can probably sell this bike and upgrade to a bike of a nicer build and his desired gear set-up. I hope this updated answer is better.