Sloping top tube query for sizing

[Aaron76]

Hi all,

I wonder if someone can help offer some guidance on frame sizing when it comes to sloping top tubes. I really like Marin's, in particular the horizontal top tubes of their earlier 1990 era zolatone bike frames. I normally find that a 19" (centre to centre) seat tube frame puts me nicely on a 22.5" sized top tube, and its a great fit for me. I would normally aim for a 22.5" top tube generally on road bikes, so its all happy days.

However, I've started to get interested in later Marin's circa 1992 onwards towards 1996 but noticed the frames now incorporate a sloping top tube. The question is, would you advise I stick with my usual 19" centre-centre seat tube size, but noting that the Marin frame geometries now have a longer actual top tube...23". I've done some reading online about actual vs effective top tube length, and why there was a push towards sloping tubes to cut down on the number of frame sizes. I'm just wondering if a sloping tube will feel too long, and whether I should drop down to a 17.5" seat tube, which lists the top tube as 22.5".

Can you offer any advice, would you normally go down a size for a sloping top tube vs a more horizontal geometry? Would an extra 0.5" make much difference anyway?

Thanks in advance for any help or guidance.

Aaron

 

[FluffyChicken]

Effective top tube length is the horizontal measure of a traditional measurement.
The later one would also come with a shorter stem and the front end would be higher up too, due to fork length increase.
Which in general cancels out any effective top tube increase.

Or buy and try, have fun and see if you like the position. You can always move the seat forward and back too, as that angle may alter as well.

Or sell swap for a smaller size.

There are people with better early 90s Marin knowledge, though Marin did really slope the top tube, not like others did.

I think length also changes with model, higher end usually a it longer?
@marc two tone

 

[2manyoranges]

Just to add to the FC:

1 sort out exactly the point where the centre point is for the tope turn and the head tube
2 hold the measure perfectly horizontally from that point
3 then use the point at which this line intersects with the middle line of the seat tube

That's your effective (or VIRTUAL) top tube length.
See 3 in this Stanton geometry diagram


switch9er-2022

 

[Prodigal Son]

The longer thr suspension fork travel, the shorter the stem must be therefore you compensate w/ longer front center/eff top tube and wider bars. It´s complicated.

 

[marc two tone]

Yep, the 1992+ team issue and team marin had a longer top tube regardless of 'frame size' even the 15.5".

I would, go no less than a 19" for any 92-97 marin. As they have a racey head -down geometry. And, have a front suspension fork issue (very picky). Manitou 2, 3,4 look good, work erm....questionable.
Judy etc work well, bugger the geometry up.

 

[2manyoranges]

argh apologies - guilty of rushing through original post - didn't really respond to your ACTUAL question - apologies...ie with the later sloping tt models should you size down?

You haven't supplied your height or any weird morphology (long arms, short trunk, short legs etc) and I think these things are important. Mostly, I was riding bikes which now seem too small when I get on them - I am 5-7 normal proportions and rode a 15.5 Marin in the late 80 and 90s Team versions with long tt. 55cms actual, if I recall. Geometry is a weird 'ecological' thing, where all dimensions in a bike combine to work, or not work. I rode many undersized bikes then, too short in the TT, to get good standover, for which we compensated with putting layback seat posts in, which horribly slackened the seat tube angle (VIRTUAL) and had headtubes which were too short, giving a heads down position. My current modern bikes have around 59cms TT length, but with radically shorter stems, which works of course with the longer travel forks - but it's true that even BITD I was trying to get frames with stretched TT and would put on short (at the time) stems - i.e. 90mms. I certainly wouldn't go down from the 19 inch frame size.

 

[madjh]

Marins of mid 90s have a long top tube, specially if you go to the top of the range. You have to check the table, insted of C-C if you check the C-T, it will match better, with slooping the C-C is not the best way. C-C in marins are quite low and cand be a bit cofusing.

In Marin I use 1" less than in another 20s bicycles, I own a C-C 15.5" frame (Team Issue 1996) and my hight is 1,7m and it fits me, it is not small. C-T is around 16,7" so close to my size, 17" or 17.5" in modern stuff, but it is long. Check all the measures because Marin didn´t did the same as others, it is a bit more complicated.

 

[2manyoranges]

I have chart with the following Team Issues Ti of 1996 re TT

15.5 TT 22.5
17.5 TT 23.0
19.5 TT 23.5

I am 5-7 or 170cm tall

My current bikes which I regard as a very good fit:

Cotic BfE Max Small TT 24.0
Cotic Simple Med 23.3
Stanton Switch9er Small 23.4
Ragley mmmBop Large 23.8

Note the huge variation in designed size - I own small, medium and large bikes and they all are a good fit.

This invokes the vexed issue of REACH
Since a hardtail rotates around the rear contact patch, the geometry changes dramatically with compression in the forks. The mmmBop has 150 forks - as a LARGE it has a SHORTER reach than the SMALL BfEMax - so they actually feel very similar in fir when dynamic. Which is good.

 

[Prodigal Son]

I believe what is most important is the front center number: bb to front hub axle. Top tube, well, your reach to bars can be changed w/ stems but you can´t change the front center: too long or too short and the bike handles badly.