Mongoose Terrex Questions

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Re: Mongoose Terrex Questions

Postby dddd » Sun Jan 29, 2017 12:19 pm

Ok, I finally found the bottom bracket that I removed from my Mongoose Hondo/Terrex.

Good thing that I measured it, it's a 130mm overall length (has no threaded studs, is threaded inside for bolts).

It also is 6mm longer on the drive side than on the left side, so that means that the entire spindle is offset 3mm to the right with respect to the bearing races that are machined onto it.

I ended up using a Shimano mtb crank that was designed with a 110mm spindle in mind, and with no offset. I used a 118mm bottom bracket though, which positioned the chainrings where they should be as far as the front derailer and freewheel position requires.
So it looks like any standard mtb crankset can work, but that the spindle used should be about 4mm longer on each end that what is normally spec'd for the particular crankset.
And if you're sticking with the stock (and extremely heavy) steel crankset, look for the longest Shimano cartridge bottom bracket which will be 127.5mm.
The "shell width" and threading are "68mm English" which is another part of the description of what you'll need regardless of whatever crankset and spindle length that you decide on.
A "fixed cup spacer" can be added to the right end of the bottom bracket upon installation if the one you buy turns out not to be long enough. A 2.5mm spacer moves the chainrings out the same amount that a 5mm-longer bottom bracket spindle will.

There is probably a lighter, aluminum 8s- or 9s-spaced crankset available for $30-40, so might be a good time to decide on that option before choosing the bottom bracket spindle length. I went with used parts that I had (an LX 8s crankset with stripped left-pedal threads that I got for free and repaired), together with a good used Shimano 118mm cartridge bb. As you can see, I also changed the shifters, levers, chain and rear derailer as well, and used a suspension seatpost that I shimmed to fit the frame's 28.6mm seat tube.

Image
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Re: Mongoose Terrex Questions

Postby 2TrakMind » Sun Jan 29, 2017 5:00 pm

At this point I'm not really thinking of replacing the crankset, unless absolutely necessary. If I understand you correctly then, the original BB was 130mm, correct?
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Re: Mongoose Terrex Questions

Postby dddd » Sun Jan 29, 2017 5:01 pm

Yeah, 130mm, but Shimano doesn't offer them quite that long.

The bottom bracket, cranks, shifters and brake levers are the ridiculously cheezy parts on these bikes in my opinion.

The stock rear derailer is also a candidate for upgrade since the design of the wheels puts the spokes in a more purely vertical orientation, so there is less than good clearance between the derailer cage and the spokes. The cage on the stock derailer has a snagging edge on it that better derailers avoid by giving the inner cage plate a more featured shape that acts like a ramp to prevent snagging on the spokes. Many Terrex/Hondo owners have had their derailer "explode" or get more or less torn off when the derailer contacted the spokes!

I also identified the issue of the fork not being able to seal out dirt in a much earlier post, one more thing about the componentry of these bikes that is more or less sub-standard and eventually inadequate for real off-road use. Still, I managed to put in 500 (GOOD) hard miles on mine with an on-going sequence of selected upgrades (eventually replaced everything except the wheels, tires, forks, front derailer, headset, cables and brakes). I replaced parts mostly on the basis of need as the miles wore on, but the handlebar, stem and saddle were replaced just because I had better parts laying around and because I didn't trust the cheap-looking freewheel to hold up to much hard riding.

No reason that someone can't ride their bike as-is though(!), ...until harder riding on rougher trails reveals the sort of issues that I dealt with. I still like the bike, and even after buying and upgrading my lighter Huffy TR-745, I plan to go on to "stage two" of my upgrades to my Hondo/Terrex, with better tires and with the Huffy's original Suntour XCT fork and the Mongoose's original wide-rim wheelset all getting installed (or re-installed) on the Mongoose! I am also seriously considering also doing a "ghetto tubeless" conversion to my Hondo (as an experiment to see if this is practical with these rims) so that I can use the lighter tires at low pressures that would otherwise cause more pinch-flats on rocky terrain (I have sure had enough of those with lightweight 2.3" tires on these super-wide Hondo/Terrex rims as installed on my aluminum Huffy).

So anyway, stay tuned here for everybody's solutions and enjoy riding your new bike! Don't be put off by parts costs, because like I've said you can play "opportunist" by using cast-off parts from older and/or better bikes to make great upgrades (like derailers, cranks, forks, shifters, levers, you-name-it).
Last edited by dddd on Sun Jan 29, 2017 5:38 pm, edited 2 times in total.
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Re: Mongoose Terrex Questions

Postby 2TrakMind » Sun Jan 29, 2017 5:05 pm

I see that. Thanks!
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Re: Mongoose Terrex Questions

Postby dddd » Sun Jan 29, 2017 8:38 pm

I called the stock crankset "cheezy" but that's not really a fair description. It weighs "a ton" but I never had a problem with it. The better crank I installed does shift faster and the rings resist bending better, for what that's worth, but steel chainrings have a longer wear life and that really counts if you're piling up miles riding off road.
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Re: Mongoose Terrex Questions

Postby 2TrakMind » Mon Jan 30, 2017 5:34 am

I am replacing a 25 year old Giant Attraction, which weighs even more than this bike does, so I'm not really concerned about the weight right now. I just want components that won't break as soon as I hit the trails. I'll probably look at replacing the crank later on, but for now I'm sticking with things like the shifters, derailleurs, and BB. I'm going to replace the chain too while I'm at it, since I'll have it off anyway. At this point, that's about all my budget will allow.
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Re: Mongoose Terrex Questions

Postby 2TrakMind » Fri Feb 03, 2017 4:12 pm

dddd, I just received an email from Pacific Bikes (which was a surprise) telling me that the bottom bracket spindle is 124.5mm, so according to them, it's 5.5mm shorter than what you measured. I have a 127.5 on its way, so it will be interesting to see how things actually line up.
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Re: Mongoose Terrex Questions

Postby dddd » Sat Feb 04, 2017 10:29 pm

The 127.5mm bottom bracket is symmetrical, so it has no offset toward the right side. Altogether then, it will be effectively shorter on the right end. You will probably only need to adjust the front derailer cable and limit screws, but if that won't make it shift well even down to the smallest ring, then a cup spacer can be added if needed to move the bb and crankset outward a few millimeters.
I'm hoping that you'll measure your bike's spindle when you get the chance and post your measurement here.
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Re: Mongoose Terrex Questions

Postby 2TrakMind » Sun Feb 05, 2017 6:05 am

I didn't realize that. So it seems that if the original spindle is truly 130mm offset, a 127.5mm symmetrical will likely impact the driveline fairly significantly then. It should be here tomorrow, so I'll know soon enough, I guess. I'll let you know how it goes.
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Re: Mongoose Terrex Questions

Postby dddd » Sun Feb 05, 2017 5:42 pm

Fear not, since there are reasons why I usually prefer to install a shorter bottom bracket than what most inexpensive bikes come with.

There are a few reasons why many bikes come with a longer than optimum bottom bracket.
It used to be because beginning riders tended to ride the bike with both "friction" shifters moved fully to put slack in the cables, which meant the chain was running "cross-chained" on the smallest front and rear sprockets. The bike maker didn't want the chain to rub against the side of the biggest sprocket when these riders rode their bikes in this manner with both cables slack.
On newer bikes with indexed shifters, a longer spindle still allows the bike to be more rideable when the cable isn't adjusted to index accurately or especially when one or both shifters are slipping or broken, and/or when either or both cables are stretched, broken or just way out of adjustment.

With the Hondo/Terrex models, there is also the likelihood that the buyer would purchase a tire in the popular 2.8" width, since that is what the bike comes with. But, the stock tires are special in that they only measure near that wide when installed on the weird, super-wide rims these bikes come with. The "aftermarket" 2.8" tire you might buy will be considerably wider than the stock tires, so a longer bb spindle would add needed chain clearance along the side of the tire.

The longer bb spindles also help protect against the chain possibly slipping off the large chainring toward the right if the chainring has a few bent teeth or is bent out of plane, which is super-common on cheaper bikes with thin, stamped chainrings and chainring "spider" that they bolt to. A chain slipping off to the "outside" like that is more dangerous because the rider's legs suddenly lose all pedaling resistance which can throw the rider.

So again, I usually prefer to at least test-fit a shorter-than-stock bb spindle in order to better center the rear sprocket stack with the chainrings, which makes for quitter running and less friction.

Having to re-install the crankset and bb in order to install a fixed-cup-spacer is a hassle, but is sure easier now that today's cartridge bottom brackets are pre-adjusted and come in just two pieces. So for me it's no big deal.
If after installing the new bb and also loosening the front derailer cable and lo-limit adjusting screw, if it won't shift reliably down to the smallest chainring, only then would you need to add a spacer to the right side of the frame's bottom bracket in order to move the crankset over to the right side.
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