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2012 Trek 3900 Disc -> Gravel Bike

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Cyclocross and Gravelbiking (Recreational) This has to be the most physically intense sport ever invented. It's high speed bicycle racing on a short off road course or riding the off pavement rides on gravel like : "Unbound Gravel". We also have a dedicated Racing forum for the Cyclocross Hard Core Racers.

2012 Trek 3900 Disc -> Gravel Bike

Old 06-17-20, 01:12 PM
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nipperkin92
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2012 Trek 3900 Disc -> Gravel Bike

Recently picked up a 2012 Trek 3900 Disc for a pretty good price. I've been wanting to get into "bike building" as a hobby, particularly making a gravel bike. Frame is 57 cm. I'm 6'0" with relatively long arms/legs and a short torso. When riding the bike, I felt like it was big compared to other mountain bikes I've rode, but similar a road bike. I decided to purchase it with the understanding that gravel bikes should be sized similarly to road bikes. So a few questions:
  1. Is this a good starting point for a gravel bike build for me, or should I try selling the bike again?
  2. If viable, what should I start doing to modify the bike? Or are there any good resources online to learn about bike tinkering?
Thanks!
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Old 06-18-20, 09:19 AM
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gequiroga
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Hello! when one converts from flat bars to drop bars, the distance from your saddle to where your hands will be increases, so if the bike feels already big, it will be worse after the conversion, so I wouldn't recommend that bike for the conversion.

When one tries to make this type of conversions you have to consider three main things (this is my personal opinion and only a rough description)

1) Frame Geometry
2) Braking
3) Gear shifting

1) You need to start a conversion with a suitable frame that will accommodate your idea for the bike after. For a gravel bike, you cannot pick a road bike because then you wouldn't have clearance for bigger wheels, and you cannot pick a modern MTB because the top tubes are too long and the bike will end too stretched. I found that hybrid bikes are the most suitable kind to start conversions on because they are generally cheaper, often have some road components, and their initial comfort geometry translates into a nice riding position once you change flat bars for drop bars.
the web 99spokes (can't post links) has a database of geometries that you can check when in doubt, you can go ahead and compare for example, a merida speeder (hybrid) with a canyon grail AL (gravel) and you'll find them to be really similar.

2) Braking: road levers are not compatible with MTB brakes. Mechanical road bike levers pull about half the length of cable than MTB levers, and for this reason you cannot use MTB calipers or V-brakes with road levers, you can use cantilever brakes, mini v-brakes and road calipers like the avid bb5 and bb7 road, tektro spyre and any other specifically designed for road levers. You could also choose to use a pull ratio adjuster to make them compatible.

3) Gear shifting: you have to check if the derailleurs on your bike are compatible with the road lever you intend to put into the bike, because road and MTB groupsets are not always compatible, Shimano, for example, has compatibility between road and MTB up to 9 speeds, from 10 up, they are not compatible. there are also pull ratio adapters and you can look up "mullet drivetrains" for more examples.

Also, road levers are quite expensive, so I think you may want to secure some cheap ones before starting a project like this.

Examples of a possible conversion, out of my mind so you can see more or less the process:
A used trex FX shows up, it's a 3x8, v-brake bike. I check the geometry on 99spokes and a L frame is similar to a M frame on a gravel bike. I buy it.
I get a pair of used Sora levers, 2x9 speeds.
I need a new 9 speed cassette on the back, this is no problem as 8 speed hubs are compatible up to 10 speed cassettes, and 11 if they are MTB hubs.
I need to replace the V-brakes for either cantilevers or mini v-brakes, or buy a pull ratio adapter if the brakes are worth it.
I have to check if I can make the front derailleur work with the 2x lever to shift between the 2 bigger chainrings. I manage to do that and end up with a 48/38 gearing up front
I decide I need a MTB cassette in the back to get a low enough gearing so I buy a 11-40 sunrace cassette and a alivio 9-speed MTB derailleur which is compatible with the 9-speed sora lever because it's less than 10 speeds.
I fit a drop bar on the bike and then find out it's a bit stretched so I replace the 110mm stem with a 80mm one.

I hope that helps a bit.
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Old 06-18-20, 01:43 PM
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People build gravel bikes out of all sorts of other discipline bikes. It all depends on how much compromise you want and how willing you are to spend money.
I wouldnt want an aluminum frame 3x8 mtb with suspension fork as a place to start, but everyone has different goals and likes.
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Old 06-19-20, 05:51 PM
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Originally Posted by gequiroga
Hello! when one converts from flat bars to drop bars, the distance from your saddle to where your hands will be increases, so if the bike feels already big, it will be worse after the conversion, so I wouldn't recommend that bike for the conversion.

When one tries to make this type of conversions you have to consider three main things (this is my personal opinion and only a rough description)

1) Frame Geometry
2) Braking
3) Gear shifting

1) You need to start a conversion with a suitable frame that will accommodate your idea for the bike after. For a gravel bike, you cannot pick a road bike because then you wouldn't have clearance for bigger wheels, and you cannot pick a modern MTB because the top tubes are too long and the bike will end too stretched. I found that hybrid bikes are the most suitable kind to start conversions on because they are generally cheaper, often have some road components, and their initial comfort geometry translates into a nice riding position once you change flat bars for drop bars.
the web 99spokes (can't post links) has a database of geometries that you can check when in doubt, you can go ahead and compare for example, a merida speeder (hybrid) with a canyon grail AL (gravel) and you'll find them to be really similar.

2) Braking: road levers are not compatible with MTB brakes. Mechanical road bike levers pull about half the length of cable than MTB levers, and for this reason you cannot use MTB calipers or V-brakes with road levers, you can use cantilever brakes, mini v-brakes and road calipers like the avid bb5 and bb7 road, tektro spyre and any other specifically designed for road levers. You could also choose to use a pull ratio adjuster to make them compatible.

3) Gear shifting: you have to check if the derailleurs on your bike are compatible with the road lever you intend to put into the bike, because road and MTB groupsets are not always compatible, Shimano, for example, has compatibility between road and MTB up to 9 speeds, from 10 up, they are not compatible. there are also pull ratio adapters and you can look up "mullet drivetrains" for more examples.

Also, road levers are quite expensive, so I think you may want to secure some cheap ones before starting a project like this.

Examples of a possible conversion, out of my mind so you can see more or less the process:
A used trex FX shows up, it's a 3x8, v-brake bike. I check the geometry on 99spokes and a L frame is similar to a M frame on a gravel bike. I buy it.
I get a pair of used Sora levers, 2x9 speeds.
I need a new 9 speed cassette on the back, this is no problem as 8 speed hubs are compatible up to 10 speed cassettes, and 11 if they are MTB hubs.
I need to replace the V-brakes for either cantilevers or mini v-brakes, or buy a pull ratio adapter if the brakes are worth it.
I have to check if I can make the front derailleur work with the 2x lever to shift between the 2 bigger chainrings. I manage to do that and end up with a 48/38 gearing up front
I decide I need a MTB cassette in the back to get a low enough gearing so I buy a 11-40 sunrace cassette and a alivio 9-speed MTB derailleur which is compatible with the 9-speed sora lever because it's less than 10 speeds.
I fit a drop bar on the bike and then find out it's a bit stretched so I replace the 110mm stem with a 80mm one.

I hope that helps a bit.
Originally Posted by mstateglfr
People build gravel bikes out of all sorts of other discipline bikes. It all depends on how much compromise you want and how willing you are to spend money.
I wouldnt want an aluminum frame 3x8 mtb with suspension fork as a place to start, but everyone has different goals and likes.
Thank you both for your advice. Was able to pick up a nice hybrid on the cheap. Any thoughts on how much I should sell the Trek for? Thanks.
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Old 06-22-20, 02:37 AM
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gequiroga
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to decide the cost of a bike you should scout your local marketplace for a bit, clean and take some nice photos to get a good price on the bike. Which bike did you get?
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Old 07-02-20, 09:04 PM
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nipperkin
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Originally Posted by gequiroga
Hello! when one converts from flat bars to drop bars, the distance from your saddle to where your hands will be increases, so if the bike feels already big, it will be worse after the conversion, so I wouldn't recommend that bike for the conversion.

When one tries to make this type of conversions you have to consider three main things (this is my personal opinion and only a rough description)

1) Frame Geometry
2) Braking
3) Gear shifting

1) You need to start a conversion with a suitable frame that will accommodate your idea for the bike after. For a gravel bike, you cannot pick a road bike because then you wouldn't have clearance for bigger wheels, and you cannot pick a modern MTB because the top tubes are too long and the bike will end too stretched. I found that hybrid bikes are the most suitable kind to start conversions on because they are generally cheaper, often have some road components, and their initial comfort geometry translates into a nice riding position once you change flat bars for drop bars.
the web 99spokes (can't post links) has a database of geometries that you can check when in doubt, you can go ahead and compare for example, a merida speeder (hybrid) with a canyon grail AL (gravel) and you'll find them to be really similar.

2) Braking: road levers are not compatible with MTB brakes. Mechanical road bike levers pull about half the length of cable than MTB levers, and for this reason you cannot use MTB calipers or V-brakes with road levers, you can use cantilever brakes, mini v-brakes and road calipers like the avid bb5 and bb7 road, tektro spyre and any other specifically designed for road levers. You could also choose to use a pull ratio adjuster to make them compatible.

3) Gear shifting: you have to check if the derailleurs on your bike are compatible with the road lever you intend to put into the bike, because road and MTB groupsets are not always compatible, Shimano, for example, has compatibility between road and MTB up to 9 speeds, from 10 up, they are not compatible. there are also pull ratio adapters and you can look up "mullet drivetrains" for more examples.

Also, road levers are quite expensive, so I think you may want to secure some cheap ones before starting a project like this.

Examples of a possible conversion, out of my mind so you can see more or less the process:
A used trex FX shows up, it's a 3x8, v-brake bike. I check the geometry on 99spokes and a L frame is similar to a M frame on a gravel bike. I buy it.
I get a pair of used Sora levers, 2x9 speeds.
I need a new 9 speed cassette on the back, this is no problem as 8 speed hubs are compatible up to 10 speed cassettes, and 11 if they are MTB hubs.
I need to replace the V-brakes for either cantilevers or mini v-brakes, or buy a pull ratio adapter if the brakes are worth it.
I have to check if I can make the front derailleur work with the 2x lever to shift between the 2 bigger chainrings. I manage to do that and end up with a 48/38 gearing up front
I decide I need a MTB cassette in the back to get a low enough gearing so I buy a 11-40 sunrace cassette and a alivio 9-speed MTB derailleur which is compatible with the 9-speed sora lever because it's less than 10 speeds.
I fit a drop bar on the bike and then find out it's a bit stretched so I replace the 110mm stem with a 80mm one.

I hope that helps a bit.
Originally Posted by gequiroga
to decide the cost of a bike you should scout your local marketplace for a bit, clean and take some nice photos to get a good price on the bike. Which bike did you get?
Ended up finding a 54cm 90's Fuji Del Rey CX. I found a pair of used 9 speed Shimano Sora shifters and also ordered a handlebar and stem adapter, not sure what stem length to get though.

As far as the drivetrain, I found a used short-cage Shimano Sora derailleur for about $10. It has a gear capacity of 37T, so I'm currently deciding how to configure the drivetrain.

The bike currently has a 3x crankset...I would like to find a 2x, but don't want to spend too much, especially if the 3x is compatible with the shifters. I can, however, purchase a 52/39T crankset for $20, would appreciate recommendations on this. As far as the rear cassette, I have a 11-24T (or something like that) cassette, but I feel like it will be too "road bike-y" for a gravel bike...again, recommendations on this would be great.

I also would need to invest in the tools to change everything...cable cutter, chain remover, cassette chain whip, torque wrench, etc. etc....so I'm considering just installing the handlebars/brifters on my own and going to a shop to install everything else, if this is more cost effective. Any other advice is appreciated.

Thanks
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Old 07-06-20, 03:12 PM
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If the crankset allows, you could just buy new chainrings and a chainguard or just use two chainrings, I have a hybrid and I went from 48-38-28 to 42-26 and a chainguard (all from ebay) with the same deraileur and it works nicely and suits my needs. Also, you could check the maximun allowed sprocket size for the deraileur and maybe upgrade to have a easier gearing. Remember also that 9 speed shimano MTB derailers are also compatible with the brifters if you need to replace to get more capacity and/or bigger casette.

In case you decide the purchase of a new crankset, you have to check the compatibility, as road and MTB cranksets are not always interchangeable and a hybrid may have either.
About the stem length, you have to consider that using drops instead of flat bars, increases the reach of the bike, but is a matter of trying. If you were too upright with flat bars, you can use the same stem length, if not you should go shorter.

About the tools, as you said you wanted to start a hobby, I would recommend you acquiring a cheap set of bike tools, this is likely to have all the basis for less than the cost of the shop visit, and you will be more entertained and gain new skills also.
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