Any formula or best sizing setup for a dirt drop?

[jimi911]

Couple of the guys (CX racers) were talking about dirt drops with me last week and got on the topic of of retro dirt drops.

Is there a best sizing setup for a dirt drop? They were of the opinion that you really sized down for a dirt drop to get a shorter toptube - thus limiting the stretch of the drop bars. (Like Cunningham??).

What works best? I have only mocked up a '85 stumpy team and I did find it was quite a stretch with the drop bars despite being "my size".

 

[hamster]

Having played around with drops on MTB frames for 20 years:
1. You do need a short top tube frame - anything after around 1995 is out.
2. A really short and tall stem is necessary - simply a stumpy stem is not enough due to the drop of the bars. If ahead system then you need another 30-50mm of stackers under it.
3. As a starting point, aim for an imaginary flat handlebar to pass through the body of the drop levers.

Enjoy!

 

[jimi911]

Having played around with drops on MTB frames for 20 years:
1. You do need a short top tube frame - anything after around 1995 is out.
2. A really short and tall stem is necessary - simply a stumpy stem is not enough due to the drop of the bars. If ahead system then you need another 30-50mm of stackers under it.
3. As a starting point, aim for an imaginary flat handlebar to pass through the body of the drop levers.

Enjoy!

Is it just a top tube thing or a total size down? Or is the size down just a result trying to get a shorter top tube and not build a custom frame.

I usually ride a 18" frame with a 22.25" toptube. So I was advised to dip to a 15-16" frame with a 20-21" toptube. Does that sound right?

 

[hamster]

There is no need to reduce the length of the seat tube in itself - after all road bikes tend to have larger frame sizes for the same rider.
The reason is to get a shorter top tube. I think what you have been advised sounds sense. Charlie Cunningham liked short seat tubes, which tends to mean that most dirt drop bike photos have tiddly frame sizes.

Try using a lower-end MTB frame as they tend to be shorter in the top tube than race-orientated / high end stuff. Early 90s Marins seem to work out well. My tourer is based on a 19" 1990 Marin Palisades with 90mm quill stem. I tried a stump-neck stem on a 18" 1999 Cindercone and it was almost unrideable on the drops as the reach was so long.

Remember also that adding to the bar height has the same effect as shortening the top tube. I reckon bar tops should be less than 50mm lower than the saddle, unless you have an extremely short top tube.

 

[Ductape]

The ultimate goal is to get the drops in the same place you would desire your standard bars to be.

 

[jimi911]

Good info. My 85 stumpy team was never meant to be a serious rider so it will be fine for what little time I spend on it. I'll keep an eye out for a suitable rider frame