# Compact chainring-fr. der ?? for Chas re: 595



## Local399 (Jan 5, 2006)

Chas (or anyone else who may know this),

What is the smallest big ring (compact) you can use on a 595? Can you go down to a 46t? Will the front derauiller braze on hanger allow (go low enough) for this? If not, will a 48t work? Will be a Dura Ace 7800 front derauiller (if that matters).

As always, thank you all in advance,

Steve


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## Local399 (Jan 5, 2006)

Anyone? Anyone?

I called Look in San Jose and they were clueless...Funny, you would think they would know this...

Thank you (again) in advance,

Steve


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## chas (Aug 10, 2006)

Local399 said:


> I called Look in San Jose and they were clueless...Funny, you would think they would know this...


Hi Steve,

I didn't speak with you when you called and sorry we couldn't help, but the reason we don't have the answer (and I'm guessing the reason no one else has answered your question yet) is because none of us know anyone who has used a 46t outer chainring on their 595. You gotta admit, it's not exactly a common practice. 

As you were told when you called, the 595 will work with a standard compact set-up (50-34) assuming you are using the correct front derailleur. Additionally, some companies (SRAM for example) offer their braze-on front derailleur with two different mounting holes that allow you extra room to lower the derailleur. There may be other after-market options for lowering the FD as well. I'll do some poking around and let you know if I find anything. 

Another option would to try to have a custom cassette built that will give you lower gearing - perhaps a 30 or 32t large cog.

*[email protected]*


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## Local399 (Jan 5, 2006)

Chas,

I didnt have a clue that you worked in San Jose, for some reason I thought NV...

I would have asked for you by name had I known.

I dont think thats a really impossible question to answer. As a matter of fact, I called Excelsports.com and a nice gentleman measured his cyclo-cross bike with its 48t big ring-Dura Ace front derauiler came up with a measurement that I could reference...

I took his measurement of 9.8 cm (from center of BB to top of outer plate, Oh, he said it was really tight and could move up a few mm...to maybe 10.0 or so) and then I installed a Dura Ace front derauiler on the 595 and came up with 10.5 cm. So, I am still not sure (without buying all the parts and trying it for myself) if that 1/2 cm will be make a big enough difference for a 48t...And knowing that a SRAM has a way of lowering the front der....well, I dont own one of those either.

As for a bigger rear cassette, that means that you have to get into MTB rear derauilers....Not something I'd want to do.

And, why wouldnt a 46-34 set up be something that isnt common practice? The rings exist to do this, so people must be doing it. 

I am a spinner, not a masher and find that I hardly ever use my 53 ring at all...Having a 50 would be the same (pretty much) as the 53...Thats why I thought a 46-34 front chainring combo would be perfect for riding, given my terrain and style of riding.

Chas, I would love it of you could do some "poking around" and see what you come up with! That would be terrific...

Thank you and next time I will ask for you,

Steve


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## chas (Aug 10, 2006)

Local399 said:


> I took his measurement of 9.8 cm (from center of BB to top of outer plate, Oh, he said it was really tight and could move up a few mm...to maybe 10.0 or so) and then I installed a Dura Ace front derauiler on the 595 and came up with 10.5 cm. So, I am still not sure (without buying all the parts and trying it for myself) if that 1/2 cm will be make a big enough difference for a 48t..
> 
> And, why wouldnt a 46-34 set up be something that isnt common practice? The rings exist to do this, so people must be doing it.


I'd guess it isn't common practice because most people want bigger gearing than that. People generally want really low gearing on the low end rather than really low gearing on the high end. 46t and 48t are common on cross bikes but they usually either have a clamp on FD, or the braze on hanger is mounted lower than on a road bike. 

At any rate, after doing some quick math (read: may not be 100% accurate, but should be close enough), I don't think you'll be able to use a 46t or 48t chainring with your current set up. 

There's about 13mm between teeth on a standard chainring, making the circumference of a 46t chainring: 13mm x 46 = 598mm

diameter = circumference / 3.14, so D = 190mm, making the radius = 9.5 cm.

Using the same math, the 48t chainring will have a radius of 9.9 cm, while a 50T chainring would be 10.3 cm.

If, in it's lowest position, your Dura-Ace front derailleur sits 10.5 cm above the center of the B/B, that leaves 1cm gap between the chainring and der for the 46t, and 6mm for the 48t. For reference, the gap would be 2mm for a standard compact 50t chainring.

The recommended gap is between 2 and 3mm. 

Hopefully that gives you all the information you need to make your decision.

*[email protected]*


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## C-40 (Feb 4, 2004)

*rule of thumb...*

A FD must be lowered 2mm for each tooth smaller, so a 48 would required a 4mm additional drop and a 46 would require an 8mm additional drop, compared to a 50T.

I have to agree that using a 50T and a larger first cog is a better way to lower the gearing, although Shimano doesn't support 13T top cogs like Campy, who offers a 13-26 or a 13-29. A 50/13 is the same as a 46/12.


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## chas (Aug 10, 2006)

C-40 said:


> A FD must be lowed 2mm for each tooth smaller, so a 48 would required a 4mm additional drop and a 46 would require an 8mm additional drop, compared to a 50T.


Thanks for the tip C-40. It's nice to see that I actually retained something from my 8th grade geometry class. 

Local399, 

Have you been able to find a set up that will work for what you're trying to do?

*[email protected]*


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